Audio system measurements
Encyclopedia
Audio system measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are within limits considered acceptable. Some aspects of measurement and specification relate only to intended usage. Audio system measurements often accommodate psychoacoustic principles to measure the system in a way that relates to human hearing.
tape, dbx
and Dolby noise reduction techniques revealed the unsatisfactory nature of many basic engineering measurements. The specification of weighted CCIR-468 quasi-peak noise
, and weighted quasi-peak wow and flutter became particularly widely used and attempts were made to find more valid methods for distortion measurement.
Measurements based on psychoacoustics, such as the measurement of noise, often use a weighting filter
. It is well established that human hearing is more sensitive to some frequencies than others, as demonstrated by equal-loudness contours, but it is not well appreciated that these contours vary depending on the type of sound. The measured curves for pure tones, for instance, are different from those for random noise. The ear also responds less well to short bursts, below 100 to 200 ms, than to continuous sounds such that a quasi-peak
detector has been found to give the most representative results when noise contains click or bursts, as is often the case for noise in digital systems. For these reasons, a set of subjectively valid measurement techniques have been devised and incorporated into BS, IEC
, EBU
and ITU
standards. These methods of audio quality measurement
are used by broadcast engineers throughout most of the world, as well as by some audio professionals, though the older A-weighting
standard for continuous tones is still commonly used by others.
No single measurement can assess audio quality. Instead, engineers use a series of measurements to analyze various types of degradation that can reduce fidelity. Thus, when testing an analogue tape machine it is necessary to test for wow and flutter
and tape speed variations over longer periods, as well as for distortion and noise. When testing a digital system, testing for speed variations is normally considered unnecessary because of the accuracy of clocks in digital circuitry, but testing for aliasing
and timing jitter
is often desirable, as these have caused audible degradation in many systems.
Once subjectively valid methods have been shown to correlate well with listening tests over a wide range of conditions, then such methods are generally adopted as preferred. Standard engineering methods are not always sufficient when comparing like with like. One CD player, for example, might have higher measured noise than another CD player when measured with a RMS method, or even an A-weighted RMS method, yet sound quieter and measure lower when 468-weighting is used. This could be because it has more noise at high frequencies, or even at frequencies beyond 20 kHz, both of which are less important since human ears are less sensitive to them. (See noise shaping
.) This effect is how Dolby B works and why it was introduced. Cassette noise, which was predominately high frequency and unavoidable given the small size and speed of the recorded track could be made subjectively much less important. The noise sounded 10 dB quieter, but failed to measure much better unless 468-weighting was used rather than A-weighting.
(Fs) : This measurement tells you over what frequency range output level for an audio component will remain reasonably constant (either within a specified decibel
range, or no more than a certain number of dB
from the amplitude at 1kHz
). Some audio components such as tone controls are designed to adjust the loudness of signal content at particular frequencies, e.g., a bass control allows the attenuation or accentuation of low frequency signal content, in which case the specification may specify the frequency response is taken with tone controls "flat" or disabled. Preamplifiers may also contain equalizer
s, filters for example to play LP
s requiring RIAA frequency response correction, in which case the specification may describe how closely the response matches the standard. By comparison, Frequency range is a term sometimes used of loudspeakers and other transducers to indicate the frequencies that are usable, without normally specifying a decibel range. Power bandwidth
is also related to frequency response - indicating the range of frequencies usable at high power (since frequency response measurements are normally taken at low signal levels, where slew rate
limitations or transformer
saturation would not be a problem.
Total harmonic distortion
(THD) : Music material contains distinct tones, and some kinds of distortion involve spurious tones at double or triple the frequencies of those tones. Such harmonically related distortion is called harmonic distortion. For high fidelity
, this is usually expected to be < 1% for electronic devices; mechanical elements such as loudspeakers usually have inescapable higher levels. Low distortion is relatively easy to achieve in electronics with use of negative feedback
, but the use of high levels of feedback in this manner has been the topic of much controversy among audiophile
s. Essentially all loudspeakers produce more distortion than electronics, and 1–5% distortion is not unheard of at moderately loud listening levels. Human ears are less sensitive to distortion in the bass frequencies, and levels are usually expected to be under 10% at loud playback. Distortion that creates only even-order harmonics for a sine wave input is sometimes considered less bothersome than odd-order distortion.
Output power
Intermodulation distortion (IMD) : Distortion that is not harmonically related to the signal being amplified is intermodulation distortion. It is a measure of the level of spurious signals resulting from unwanted combination of different frequency input signals. This effect results from non-linearities in the system. Sufficiently high levels of negative feedback can reduce this effect in an amplifier. Many believe it is better to design electronics in a way to minimize feedback levels, though this is difficult to achieve while meeting other high accuracy requirements. Intermodulation in loudspeaker drivers is, as with harmonic distortion, almost always larger than in most electronics. IMD increases with cone excursion. Reducing a driver's bandwidth directly reduces IMD. This is achieved by splitting the desired frequency range into separate bands and employing separate drivers for each band of frequencies, and feeding them through a crossover filter network
. Steep slope crossover filters are most effective at IMD reduction, but may be too expensive to implement using high-current components and may introduce ringing distortion.
Noise : The level of unwanted noise generated by the system itself, or by interference from external sources added to the signal. Hum usually refers to noise only at power line frequencies (as opposed to broadband white noise
), which is introduced through induction of power line signals into the inputs of gain stages. Or from inadequately regulated power supplies.
Crosstalk : The introduction of noise (from another signal channel) caused by ground currents, stray inductance or capacitance between components or lines. Crosstalk reduces, sometimes noticeably, separation between channels (e.g., in a stereo system). A crosstalk measurement
yields a figure in dB relative to a nominal level of signal in the path receiving interference. Crosstalk is normally only a problem in equipment that processes multiple audio channels in the same chassis.
Common-mode rejection ratio
(CMRR) : In balanced audio
systems, there are equal and opposite signals (difference-mode) in inputs, and any interference imposed on both leads will be subtracted, canceling out that interference (i.e., the common-mode). CMRR is a measure of a system's ability to ignore such interference, and especially hum at its input. It is generally only significant with long lines on an input, or when some kinds of ground loop
problems exist. Unbalanced inputs do not have common mode resistance; induced noise on their inputs appears directly as noise or hum.
Dynamic range
and Signal-to-noise ratio
(SNR) : The difference between the maximum level a component can accommodate and the noise level it produces. Input noise is not counted in this measurement. It is measured in dB.
Phase distortion
, Group delay
, and Phase delay : A perfect audio component will maintain the phase
coherency of a signal over the full range of frequencies. Phase distortion can be extremely difficult to reduce or eliminate. The human ear is largely insensitive to phase distortion, though it is exquisitely sensitive to relative phase relationships within heard sounds. The complex nature of our sensitivity to phase errors, coupled with the lack of a convenient test that delivers an easily understood quality rating, is the reason that it is not a part of conventional audio specifications. Multi-driver loudspeaker systems may have complex phase distortions, caused or corrected by crossovers, driver placement, and the phase behaviour of the specific driver.
Transient response
: A system may have low distortion for a steady-state signal, but not on sudden transients. In amplifiers, this problem can be traced to power supplies in some instances, to insufficient high frequency performance or to excessive negative feedback. Related measurements are slew rate
and rise time
. Distortion in transient response can be hard to measure. Many otherwise good power amplifier designs have been found to have inadequate slew rates, by modern standards. In loudspeakers, transient response performance is affected by the mass and resonances of drivers and enclosures and by group delay and phase delay
introduced by crossover filtering or inadequate time alignment of the loudspeaker's drivers. Most loudspeaker
s generate significant amounts of transient distortion, though some designs are less prone to this (e.g. electrostatic loudspeaker
s, plasma arc tweeters, ribbon tweeters and horn enclosures with multiple entry points).
Damping factor
: A higher number is generally believed to be better. This is a measure of how well a power amplifier
controls the undesired motion of a loudspeaker
driver. An amplifier must be able to suppress resonance
s caused by mechanical motion (e.g., inertia
) of a speaker cone, especially a low frequency driver with greater mass. For conventional loudspeaker drivers, this essentially involves ensuring that the output impedance
of the amplifier is close to zero and that the speaker wires are sufficiently short and have sufficiently large diameter. Damping factor is the ratio of the output impedance of an amplifier and connecting cables to the DC resistance of a voice coil
, which means that long, high resistance speaker wires will reduce the damping factor. A damping factor of 20 or greater is considered adequate for live sound reinforcement system
s, as the SPL of inertia-related driver movement is 26 dB less than signal level and won't be heard. Negative feedback in an amplifier lowers its effective output impedance and thus increases its damping factor.
and flutter : These measurements are related to physical motion in a component, largely the drive mechanism of analogue
media, such as vinyl records and magnetic tape
. "Wow" is slow speed (a few Hz) variation, caused by longer term drift of the drive motor speed, whereas "flutter" is faster speed (a few tens of Hz) variations, usually caused by mechanical defects such as out-of-roundness of the capstan of a tape transport mechanism. The measurement is given in % and a lower number is better.
Rumble : The measure of the low frequency (many tens of Hz) noise contributed by the turntable
of an analogue playback system. It is caused by imperfect bearings, uneven motor windings, vibrations in driving bands in some turntables, room vibrations (e.g., from traffic) that is transmitted by the turntable mounting and so to the phono cartridge. A lower number is better.
techniques) the data itself is perfectly maintained. The data is typically buffered in a memory, and is clocked
out by a very precise crystal oscillator
. The data usually does not degenerate as it passes through many stages, because each stage regenerates new symbols for transmission.
Digital systems have their own problems. Digitizing adds noise, which is measurable and depends on the resolution ('number of bits") of the system, regardless of other quality issues. Timing errors in sampling clocks (jitter
) result in non-linear distortion (FM modulation) of the signal. One quality measurement for a digital system (Bit Error Rate) relates to the probability of an error in transmission or reception. Other metrics on the quality of the system are defined by the sample rate and bit depth
. In general, digital systems are much less prone to error than analog systems; However, nearly all digital systems have analog inputs and/or outputs, and certainly all of those that interact with the analog world do so. These analog components of the digital system can suffer analog effects and potentially compromise the integrity of a well designed digital system.
Jitter
: A measurement of the variation in period (periodic jitter) and absolute timing (random jitter) between measured clock timing versus an ideal clock. Less jitter is generally better for sampling systems.
Sample rate : A specification of the rate at which measurements are taken of the analog signal. This is measured in samples per second, or hertz
. A higher sampling rate allows a greater total bandwidth or pass-band frequency response and allows less-steep anti-aliasing/anti-imaging filters to be used in the stop-band, which can in turn improve overall phase linearity in the pass-band.
Bit depth
: A specification of the precision of each measurement. For example, a 3-bit system would be able to measure 23 = 8 different levels, so it would round the actual level at each point to the nearest representable. Typical values for audio are 8-bit, 16-bit, 24-bit, and 32-bit. The bit depth determines the theoretical maximum signal-to-noise ratio or dynamic range for the system. It is common for devices to create more noise than the minimum possible noise floor, however. Sometimes this is done intentionally; dither
noise is added to decrease the negative effects of quantization noise by converting it into a higher level of uncorrelated noise.
Sample accuracy/synchronization : Not as much a specification as an ability. Since independent digital audio devices are each run by their own crystal oscillator
, and no two crystals are exactly the same, the sample rate will be slightly different. This will cause the devices to drift apart over time. The effects of this can vary. If one digital device is used to monitor another digital device, this will cause dropouts or distortion in the audio, as one device will be producing more or less data than the other per unit time. If two independent devices record at the same time, one will lag the other more and more over time. This effect can be circumvented with a wordclock synchronization. It can also be corrected in the digital domain using a drift correction algorithm. Such an algorithm compares the relative rates of two or more devices and drops or adds samples from the streams of any devices that drift too far from the master device. Sample rate will also vary slightly over time, as crystals change in temperature, etc. See also clock recovery
Linearity : Differential non-linearity and integral non-linearity are two measurements of the accuracy of an analog-to-digital converter
. Basically, they measure how close the threshold levels for each bit are to the theoretical equally-spaced levels.
) tests.
Some argue that because human hearing and perception are not fully understood, listener experience should be valued above everything else. This tactic is often encountered in the "high-end audio" world, where it is used to sell amplifiers with poor specifications. The usefulness of blind listening tests and common objective performance measurements, e.g., THD, are questioned. For instance, crossover distortion at a given THD is much more audible than clipping distortion at the same THD, since the harmonics produced are at higher frequencies. This does not imply that the defect is somehow unquantifiable or unmeasurable; just that a single THD number is inadequate to specify it and must be interpreted with care. Taking THD measurements at different output levels would expose whether the distortion is clipping (which increases with level) or crossover (which decreases with level).
Whichever the view, it should be noted that some measurements have been traditionally used, despite having no objective value. For example, THD is an average of a number of harmonics equally weighted, even though research performed decades ago identifies that lower order harmonics are harder to hear at the same level, compared with higher order ones. In addition, even order harmonics are said to be generally harder to hear than odd order. A number of formulas that attempt to correlate THD with actual audibility have been published, however none have gained mainstream use.
It is claimed that subtle changes in sound quality are easier to hear in non-blind tests than blind tests. Objective performance measurements are said not to fit in with ordinary listener experience. Writing in Stereophile
magazine, John Atkinson recalls his experience of an amplifier that performed well objectively and in blind listening tests, but did not sound good in actual use.
Subjectivity and frequency weighting
Subjectively valid methods came to prominence in consumer audio in the UK and Europe in the 1970s, when the introduction of compact cassetteCompact Cassette
The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
tape, dbx
Dbx (noise reduction)
dbx is a family of noise reduction systems developed by the company of the same name. The most common implementations are dbx Type I and dbx Type II for analog tape recording and, less commonly, vinyl LPs. A separate implementation, known as dbx-TV, is part of the MTS system used to provide stereo...
and Dolby noise reduction techniques revealed the unsatisfactory nature of many basic engineering measurements. The specification of weighted CCIR-468 quasi-peak noise
ITU-R 468 noise weighting
ITU-R 468 is a standard relating to noise measurement, widely used when measuring noise in audio systems. The standard defines a weighting filter curve, together with a quasi-peak rectifier having special characteristics as defined by specified tone-burst tests...
, and weighted quasi-peak wow and flutter became particularly widely used and attempts were made to find more valid methods for distortion measurement.
Measurements based on psychoacoustics, such as the measurement of noise, often use a weighting filter
Weighting filter
A weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.- Audio applications :...
. It is well established that human hearing is more sensitive to some frequencies than others, as demonstrated by equal-loudness contours, but it is not well appreciated that these contours vary depending on the type of sound. The measured curves for pure tones, for instance, are different from those for random noise. The ear also responds less well to short bursts, below 100 to 200 ms, than to continuous sounds such that a quasi-peak
Quasi-peak
Quasi-peak means 'not quite peak', or 'aiming towards peak but not actually peak'. The term is commonly used when referring to electronic detectors or rectifiers. Despite the above definition, the term quasi-peak should not be interpreted as vague in any way...
detector has been found to give the most representative results when noise contains click or bursts, as is often the case for noise in digital systems. For these reasons, a set of subjectively valid measurement techniques have been devised and incorporated into BS, IEC
International Electrotechnical Commission
The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...
, EBU
European Broadcasting Union
The European Broadcasting Union is a confederation of 74 broadcasting organisations from 56 countries, and 49 associate broadcasters from a further 25...
and ITU
Itu
Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
standards. These methods of audio quality measurement
Audio quality measurement
Audio quality measurement seeks to quantify the various forms of corruption present in an audio system or device. The results of such measurement are used to maintain standards in broadcasting, to compile specifications, and to compare pieces of equipment....
are used by broadcast engineers throughout most of the world, as well as by some audio professionals, though the older A-weighting
A-weighting
A Weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly known example is frequency weighting in sound level measurement where a specific set of weighting curves known as A,...
standard for continuous tones is still commonly used by others.
No single measurement can assess audio quality. Instead, engineers use a series of measurements to analyze various types of degradation that can reduce fidelity. Thus, when testing an analogue tape machine it is necessary to test for wow and flutter
Wow and Flutter
Wow and Flutter is an EP by the post-rock band Stereolab, released in 1994. A limited edition of 3000 7" copies was released with hand-painted covers. Two of the tracks appear on Mars Audiac Quintet, which was released the same year.-Track listing:...
and tape speed variations over longer periods, as well as for distortion and noise. When testing a digital system, testing for speed variations is normally considered unnecessary because of the accuracy of clocks in digital circuitry, but testing for aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
and timing jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
is often desirable, as these have caused audible degradation in many systems.
Once subjectively valid methods have been shown to correlate well with listening tests over a wide range of conditions, then such methods are generally adopted as preferred. Standard engineering methods are not always sufficient when comparing like with like. One CD player, for example, might have higher measured noise than another CD player when measured with a RMS method, or even an A-weighted RMS method, yet sound quieter and measure lower when 468-weighting is used. This could be because it has more noise at high frequencies, or even at frequencies beyond 20 kHz, both of which are less important since human ears are less sensitive to them. (See noise shaping
Noise shaping
Noise shaping is a technique typically used in digital audio, image, and video processing, usually in combination with dithering, as part of the process of quantization or bit-depth reduction of a digital signal...
.) This effect is how Dolby B works and why it was introduced. Cassette noise, which was predominately high frequency and unavoidable given the small size and speed of the recorded track could be made subjectively much less important. The noise sounded 10 dB quieter, but failed to measure much better unless 468-weighting was used rather than A-weighting.
Analog electrical
Frequency responseFrequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
(Fs) : This measurement tells you over what frequency range output level for an audio component will remain reasonably constant (either within a specified decibel
DB
DB may refer to:In science and technology:*Decibel , a logarithmic unit of measurement in acoustics and electronics*Dubnium , a chemical element*DB connector, a size of D-subminiature electrical connector...
range, or no more than a certain number of dB
DB
DB may refer to:In science and technology:*Decibel , a logarithmic unit of measurement in acoustics and electronics*Dubnium , a chemical element*DB connector, a size of D-subminiature electrical connector...
from the amplitude at 1kHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
). Some audio components such as tone controls are designed to adjust the loudness of signal content at particular frequencies, e.g., a bass control allows the attenuation or accentuation of low frequency signal content, in which case the specification may specify the frequency response is taken with tone controls "flat" or disabled. Preamplifiers may also contain equalizer
Equalization (audio)
Equalization is the process commonly used in sound recording and reproduction to alter the frequency response of an audio system using linear filters. Most hi-fi equipment uses relatively simple filters to make bass and treble adjustments. Graphic and parametric equalizers have much more...
s, filters for example to play LP
LP album
The LP, or long-playing microgroove record, is a format for phonograph records, an analog sound storage medium. Introduced by Columbia Records in 1948, it was soon adopted as a new standard by the entire record industry...
s requiring RIAA frequency response correction, in which case the specification may describe how closely the response matches the standard. By comparison, Frequency range is a term sometimes used of loudspeakers and other transducers to indicate the frequencies that are usable, without normally specifying a decibel range. Power bandwidth
Power bandwidth
The power bandwidth of an amplifier is sometimes taken as the frequency range for which the rated power output of an amplifier can be maintained to at least half of the full rated power...
is also related to frequency response - indicating the range of frequencies usable at high power (since frequency response measurements are normally taken at low signal levels, where slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
limitations or transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
saturation would not be a problem.
- A component having a 'flat' frequency response will not change the weighting (i.e., intensity) of signal content across the specified frequency range. The frequency range often specified for audio components is between 20 HzHertzThe hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to 20 kHz, which broadly reflects the human hearing range (the highest audible frequency for most people is less than 20 kHz, with 16 kHz being more typical ). Components with 'flat' frequency responses are often described as being linear. Most audio components are designed to be linear across their entire operating range. Well-designed solid-state amplifiers and CD players may have a frequency response that varies by only 0.2 dB between 20 Hz to 20 kHz. Loudspeakers tend to have a considerably less flat frequency responses than this.
Total harmonic distortion
Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
(THD) : Music material contains distinct tones, and some kinds of distortion involve spurious tones at double or triple the frequencies of those tones. Such harmonically related distortion is called harmonic distortion. For high fidelity
High fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
, this is usually expected to be < 1% for electronic devices; mechanical elements such as loudspeakers usually have inescapable higher levels. Low distortion is relatively easy to achieve in electronics with use of negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
, but the use of high levels of feedback in this manner has been the topic of much controversy among audiophile
Audiophile
An audiophile is a person who enjoys listening to recorded music, usually in a home. Some audiophiles are more interested in collecting and listening to music, while others are more interested in collecting and listening to audio components, whose "sound quality" they consider as important as the...
s. Essentially all loudspeakers produce more distortion than electronics, and 1–5% distortion is not unheard of at moderately loud listening levels. Human ears are less sensitive to distortion in the bass frequencies, and levels are usually expected to be under 10% at loud playback. Distortion that creates only even-order harmonics for a sine wave input is sometimes considered less bothersome than odd-order distortion.
Output power
Audio power
Audio power is the electrical power transferred from an audio amplifier to a loudspeaker, measured in watts. The electrical power delivered to the loudspeaker, together with its sensitivity, determines the sound power level generated .Amplifiers are limited in the electrical energy they can...
- Output power for amplifiers is ideally measured and quoted as maximum Root Mean Square (RMSRoot mean squareIn mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
) powerPower (physics)In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
output per channel, at a specified distortion level at a particular load, which, by convention and government regulation, is considered the most meaningful measure of power available on music signals, though real, non-clippingClipping (audio)Clipping is a form of waveform distortion that occurs when an amplifier is overdriven and attempts to deliver an output voltage or current beyond its maximum capability...
music has a high peak-to-average ratio, and usually averages well below the maximum possible. The commonly given measurement of PMPO (peak music power out) is largely meaningless and often used in marketing literature; in the late 1960s there was much controversy over this point and the US Government (FTA) required that RMS figures be quoted for all high fidelity equipment. Music power has been making a comeback in recent years. See also Audio powerAudio powerAudio power is the electrical power transferred from an audio amplifier to a loudspeaker, measured in watts. The electrical power delivered to the loudspeaker, together with its sensitivity, determines the sound power level generated .Amplifiers are limited in the electrical energy they can...
. - Power specifications require the load impedance to be specified, and in some cases two figures will be given (for instance, a power amplifier for loudspeakers will be typically measured at 4 and 8 ohmOhmThe ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s). Any amplifier will drive more current to a lower impedance load. For example, it will deliver more power into a 4-ohm load, as compared to 8-ohm, but it must not be assumed that it is capable of sustaining the extra current unless it is specified so. Power supply limitations may limit high current performance.
Intermodulation distortion (IMD) : Distortion that is not harmonically related to the signal being amplified is intermodulation distortion. It is a measure of the level of spurious signals resulting from unwanted combination of different frequency input signals. This effect results from non-linearities in the system. Sufficiently high levels of negative feedback can reduce this effect in an amplifier. Many believe it is better to design electronics in a way to minimize feedback levels, though this is difficult to achieve while meeting other high accuracy requirements. Intermodulation in loudspeaker drivers is, as with harmonic distortion, almost always larger than in most electronics. IMD increases with cone excursion. Reducing a driver's bandwidth directly reduces IMD. This is achieved by splitting the desired frequency range into separate bands and employing separate drivers for each band of frequencies, and feeding them through a crossover filter network
Audio crossover
Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi-fi speaker systems use a...
. Steep slope crossover filters are most effective at IMD reduction, but may be too expensive to implement using high-current components and may introduce ringing distortion.
Noise : The level of unwanted noise generated by the system itself, or by interference from external sources added to the signal. Hum usually refers to noise only at power line frequencies (as opposed to broadband white noise
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
), which is introduced through induction of power line signals into the inputs of gain stages. Or from inadequately regulated power supplies.
Crosstalk : The introduction of noise (from another signal channel) caused by ground currents, stray inductance or capacitance between components or lines. Crosstalk reduces, sometimes noticeably, separation between channels (e.g., in a stereo system). A crosstalk measurement
Crosstalk measurement
Crosstalk measurement is made on audio systems to determine the amount of signal leaking across from one channel to another.Interchannel crosstalk applies between the two channels of a stereo system, and is usually not very important on modern systems, though it was hard to keep below the desired...
yields a figure in dB relative to a nominal level of signal in the path receiving interference. Crosstalk is normally only a problem in equipment that processes multiple audio channels in the same chassis.
Common-mode rejection ratio
Common-mode rejection ratio
The common-mode rejection ratio of a differential amplifier is the tendency of the devices to reject the input signals common to both input leads...
(CMRR) : In balanced audio
Balanced audio
Balanced audio is a method of interconnecting audio equipment using impedance-balanced lines. This type of connection is very important in sound recording and production because it allows for the use of long cables while reducing susceptibility to external noise.Balanced connections use...
systems, there are equal and opposite signals (difference-mode) in inputs, and any interference imposed on both leads will be subtracted, canceling out that interference (i.e., the common-mode). CMRR is a measure of a system's ability to ignore such interference, and especially hum at its input. It is generally only significant with long lines on an input, or when some kinds of ground loop
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
problems exist. Unbalanced inputs do not have common mode resistance; induced noise on their inputs appears directly as noise or hum.
Dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
and Signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(SNR) : The difference between the maximum level a component can accommodate and the noise level it produces. Input noise is not counted in this measurement. It is measured in dB.
- Dynamic range refers to the ratio of maximum to minimum loudness in a given signal source (eg, music or programme material), and this measurement also quantifies the maximum dynamic range an audio system can carry. This is the ratio (usually expressed in dBDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
) between the noise floor of the device with no signal and the maximum signal (usually a sine waveSine waveThe sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
) that can be output at a specified (low) distortion level. - Since the early 1990s it has been recommended by several authorities including the Audio Engineering SocietyAudio Engineering SocietyEstablished in 1948, the Audio Engineering Society draws its membership from amongst engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products for audio, and persons working...
that measurements of dynamic range be made with an audio signal present. This avoids questionable measurements based on the use of blank media, or muting circuits.
- Signal-to-noise ratio (SNR), however, is the ratio between the noise floor and an arbitrary reference level or alignment levelAlignment levelThe alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level...
. In "professional" recording equipment, this reference level is usually +4 dBu (IEC 60268-17), though sometimes 0 dBu (UK and Europe - EBU standard Alignment level). 'Test level', 'measurement level' and 'line-up level' mean different things, often leading to confusion. In "consumer" equipment, no standard exists, though −10 dBV and −6 dBu are common. - Different media characteristically exhibit different amounts of noiseNoise measurementNoise measurement is carried out in various fields.In acoustics, it can be for the purpose of measuring environmental noise, or part of a test procedure using white noise, or some other specialised form of test signal....
and headroom. Though the values vary widely between units, a typical analogue cassetteCompact CassetteThe Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...
might give 60 dBDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
, a CDCompact DiscThe Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
almost 100 dB. Most modern quality amplifiers have >110 dB dynamic range, which approaches that of the human earEarThe ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
, usually taken as around 130 dB. See Programme levels.
Phase distortion
Phase distortion
In signal processing, phase distortion or phase-frequency distortion is distortion that occurs when a filter's phase response is not linear over the frequency range of interest, that is, the phase shift introduced by a circuit or device is not directly proportional to frequency, or the...
, Group delay
Group delay
Group delay is a measure of the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component...
, and Phase delay : A perfect audio component will maintain the phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
coherency of a signal over the full range of frequencies. Phase distortion can be extremely difficult to reduce or eliminate. The human ear is largely insensitive to phase distortion, though it is exquisitely sensitive to relative phase relationships within heard sounds. The complex nature of our sensitivity to phase errors, coupled with the lack of a convenient test that delivers an easily understood quality rating, is the reason that it is not a part of conventional audio specifications. Multi-driver loudspeaker systems may have complex phase distortions, caused or corrected by crossovers, driver placement, and the phase behaviour of the specific driver.
Transient response
Transient response
In electrical engineering and mechanical engineering, a transient response or natural response is the response of a system to a change from equilibrium. The transient response is not necessarily tied to "on/off" events but to any event that affects the equilibrium of the system...
: A system may have low distortion for a steady-state signal, but not on sudden transients. In amplifiers, this problem can be traced to power supplies in some instances, to insufficient high frequency performance or to excessive negative feedback. Related measurements are slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
and rise time
Rise time
In electronics, when describing a voltage or current step function, rise time refers to the time required for a signal to change from a specified low value to a specified high value...
. Distortion in transient response can be hard to measure. Many otherwise good power amplifier designs have been found to have inadequate slew rates, by modern standards. In loudspeakers, transient response performance is affected by the mass and resonances of drivers and enclosures and by group delay and phase delay
Group delay and phase delay
Group delay is a measure of the time delay of the amplitude envelopes of the various sinusoidal components of a signal through a device under test, and is a function of frequency for each component...
introduced by crossover filtering or inadequate time alignment of the loudspeaker's drivers. Most loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s generate significant amounts of transient distortion, though some designs are less prone to this (e.g. electrostatic loudspeaker
Electrostatic loudspeaker
An electrostatic loudspeaker is a loudspeaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field.-Design and functionality:...
s, plasma arc tweeters, ribbon tweeters and horn enclosures with multiple entry points).
Damping factor
Damping factor
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
: A higher number is generally believed to be better. This is a measure of how well a power amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
controls the undesired motion of a loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
driver. An amplifier must be able to suppress resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
s caused by mechanical motion (e.g., inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
) of a speaker cone, especially a low frequency driver with greater mass. For conventional loudspeaker drivers, this essentially involves ensuring that the output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
of the amplifier is close to zero and that the speaker wires are sufficiently short and have sufficiently large diameter. Damping factor is the ratio of the output impedance of an amplifier and connecting cables to the DC resistance of a voice coil
Voice coil
A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
, which means that long, high resistance speaker wires will reduce the damping factor. A damping factor of 20 or greater is considered adequate for live sound reinforcement system
Sound reinforcement system
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience...
s, as the SPL of inertia-related driver movement is 26 dB less than signal level and won't be heard. Negative feedback in an amplifier lowers its effective output impedance and thus increases its damping factor.
Mechanical
WowWow (recording)
Wow is a relatively slow form of flutter which can affect both gramophone records and tape recorders. In the latter, the collective expression wow and flutter is commonly used.-Gramophone records:...
and flutter : These measurements are related to physical motion in a component, largely the drive mechanism of analogue
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
media, such as vinyl records and magnetic tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
. "Wow" is slow speed (a few Hz) variation, caused by longer term drift of the drive motor speed, whereas "flutter" is faster speed (a few tens of Hz) variations, usually caused by mechanical defects such as out-of-roundness of the capstan of a tape transport mechanism. The measurement is given in % and a lower number is better.
Rumble : The measure of the low frequency (many tens of Hz) noise contributed by the turntable
Phonograph
The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
of an analogue playback system. It is caused by imperfect bearings, uneven motor windings, vibrations in driving bands in some turntables, room vibrations (e.g., from traffic) that is transmitted by the turntable mounting and so to the phono cartridge. A lower number is better.
Digital
Note that digital systems do not suffer from many of these effects at a signal level, though the same processes occur in the circuitry, since the data being handled is symbolic. As long as the symbol survives the transfer between components, and can be perfectly regenerated (e.g., by pulse shapingPulse shaping
In digital telecommunication, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way,...
techniques) the data itself is perfectly maintained. The data is typically buffered in a memory, and is clocked
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...
out by a very precise crystal oscillator
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
. The data usually does not degenerate as it passes through many stages, because each stage regenerates new symbols for transmission.
Digital systems have their own problems. Digitizing adds noise, which is measurable and depends on the resolution ('number of bits") of the system, regardless of other quality issues. Timing errors in sampling clocks (jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
) result in non-linear distortion (FM modulation) of the signal. One quality measurement for a digital system (Bit Error Rate) relates to the probability of an error in transmission or reception. Other metrics on the quality of the system are defined by the sample rate and bit depth
Audio bit depth
In digital audio, bit depth describes the number of bits of information recorded for each sample. Bit depth directly corresponds to the resolution of each sample in a set of digital audio data...
. In general, digital systems are much less prone to error than analog systems; However, nearly all digital systems have analog inputs and/or outputs, and certainly all of those that interact with the analog world do so. These analog components of the digital system can suffer analog effects and potentially compromise the integrity of a well designed digital system.
Jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
: A measurement of the variation in period (periodic jitter) and absolute timing (random jitter) between measured clock timing versus an ideal clock. Less jitter is generally better for sampling systems.
Sample rate : A specification of the rate at which measurements are taken of the analog signal. This is measured in samples per second, or hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. A higher sampling rate allows a greater total bandwidth or pass-band frequency response and allows less-steep anti-aliasing/anti-imaging filters to be used in the stop-band, which can in turn improve overall phase linearity in the pass-band.
Bit depth
Bit depth
Bit depth may refer to:* For bit depth in graphics, see color depth* For bit depth in audio, see audio bit depth* For bit depth in drilling for petroleum, see drilling rig...
: A specification of the precision of each measurement. For example, a 3-bit system would be able to measure 23 = 8 different levels, so it would round the actual level at each point to the nearest representable. Typical values for audio are 8-bit, 16-bit, 24-bit, and 32-bit. The bit depth determines the theoretical maximum signal-to-noise ratio or dynamic range for the system. It is common for devices to create more noise than the minimum possible noise floor, however. Sometimes this is done intentionally; dither
Dither
Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images...
noise is added to decrease the negative effects of quantization noise by converting it into a higher level of uncorrelated noise.
- To calculate the maximum theoretical dynamic range of a digital system, find the total number of levels in the system. Dynamic Range = 20·log(# of different levels). Note: the log function has a base of 10. Example: An 8-bit system has 256 different possibilities, from 0 – 255. The smallest signal is 1 and the largest is 255. Dynamic Range = 20·log(255) = 48 dB.
Sample accuracy/synchronization : Not as much a specification as an ability. Since independent digital audio devices are each run by their own crystal oscillator
Crystal oscillator
A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency...
, and no two crystals are exactly the same, the sample rate will be slightly different. This will cause the devices to drift apart over time. The effects of this can vary. If one digital device is used to monitor another digital device, this will cause dropouts or distortion in the audio, as one device will be producing more or less data than the other per unit time. If two independent devices record at the same time, one will lag the other more and more over time. This effect can be circumvented with a wordclock synchronization. It can also be corrected in the digital domain using a drift correction algorithm. Such an algorithm compares the relative rates of two or more devices and drops or adds samples from the streams of any devices that drift too far from the master device. Sample rate will also vary slightly over time, as crystals change in temperature, etc. See also clock recovery
Clock recovery
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...
Linearity : Differential non-linearity and integral non-linearity are two measurements of the accuracy of an analog-to-digital converter
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
. Basically, they measure how close the threshold levels for each bit are to the theoretical equally-spaced levels.
Unquantifiable?
Many audio components are tested for performance using objective and quantifiable measurements, e.g., THD, dynamic range and frequency response. Some take the view that objective measurements are useful and often relate well to subjective performance, i.e., the sound quality as experienced by the listener. An example of this is the work of Toole on loudspeakers. He has shown that the performance of loudspeakers, as assessed in listening tests, are linked to objective measurements of loudspeaker performance. In Toole's work, listening tests were designed to eliminate any potential biases in results. Tests of this sort are called blind (or controlledScientific control
Scientific control allows for comparisons of concepts. It is a part of the scientific method. Scientific control is often used in discussion of natural experiments. For instance, during drug testing, scientists will try to control two groups to keep them as identical and normal as possible, then...
) tests.
Some argue that because human hearing and perception are not fully understood, listener experience should be valued above everything else. This tactic is often encountered in the "high-end audio" world, where it is used to sell amplifiers with poor specifications. The usefulness of blind listening tests and common objective performance measurements, e.g., THD, are questioned. For instance, crossover distortion at a given THD is much more audible than clipping distortion at the same THD, since the harmonics produced are at higher frequencies. This does not imply that the defect is somehow unquantifiable or unmeasurable; just that a single THD number is inadequate to specify it and must be interpreted with care. Taking THD measurements at different output levels would expose whether the distortion is clipping (which increases with level) or crossover (which decreases with level).
Whichever the view, it should be noted that some measurements have been traditionally used, despite having no objective value. For example, THD is an average of a number of harmonics equally weighted, even though research performed decades ago identifies that lower order harmonics are harder to hear at the same level, compared with higher order ones. In addition, even order harmonics are said to be generally harder to hear than odd order. A number of formulas that attempt to correlate THD with actual audibility have been published, however none have gained mainstream use.
It is claimed that subtle changes in sound quality are easier to hear in non-blind tests than blind tests. Objective performance measurements are said not to fit in with ordinary listener experience. Writing in Stereophile
Stereophile
Stereophile is a monthly magazine that focuses on high end audio equipment, such as loudspeakers and amplifiers, and audio-related news, such as online audio streaming. It was founded in 1962 by J. Gordon Holt....
magazine, John Atkinson recalls his experience of an amplifier that performed well objectively and in blind listening tests, but did not sound good in actual use.
See also
- ABX testABX testAn ABX test is a method of comparing two kinds of sensory stimuli to identify detectable differences. A subject is presented with two known samples , and one unknown sample X, for three samples total. X is randomly selected from A and B, and the subject identifies X as being either A or B...
- Audio quality measurementAudio quality measurementAudio quality measurement seeks to quantify the various forms of corruption present in an audio system or device. The results of such measurement are used to maintain standards in broadcasting, to compile specifications, and to compare pieces of equipment....
- High fidelityHigh fidelityHigh fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
- AudiophileAudiophileAn audiophile is a person who enjoys listening to recorded music, usually in a home. Some audiophiles are more interested in collecting and listening to music, while others are more interested in collecting and listening to audio components, whose "sound quality" they consider as important as the...
- Physics of music