Programme level
Encyclopedia
Programme level refers to the signal level that an audio
source is transmitted or recorded at, and is important in audio if listeners of Compact Disc
s (CDs), radio
and television
are to get the best experience, without excessive noise
in quiet periods or distortion of loud sounds. Programme level is often measured using a peak programme meter
or a VU meter
.
The level of an audio signal is among the most basic of measurements, and yet widespread misunderstanding and disagreement about programme levels has become arguably the greatest single obstacle to high quality sound reproduction.
. Sound level meters respond quite slowly, even on a "fast" setting: they use a root mean square
(RMS) rectifier which by definition must take a slow running average of the square of the input voltage. Music is complex, and constantly varying, with brief peaks originating from many sources including the initial impact of sticks on cymbal
s and drum
s. A loud band
might measure 100 dB SPL on a sound level meter, yet have peaks reaching 130 dB SPL or higher.
A recording system must handle these peaks; they can be measured using a peak responding meter with an integration time of 0.5 ms or less (not a standard IEC type PPM which has a longer integration time).
The sound level meter
is useless for properly assessing noise levels, since the commonly used A-weighting
is based on equal-loudness contours for pure tones, and is not valid for the random noise.
The subjective loudness of noise is best measured using a noise-meter to the ITU-R 468 noise weighting
standard. The chart below shows, on this basis, the real range of live music, and then the level capabilities of various stages in the audio chain, from microphone
to loudspeaker
.
(AL) corresponding to 100 dB SPL has been assumed throughout. Any lower level would imply severe clipping at the first stage; the master recording. Top quality microphone
s do not present a problem; most will handle 130 dB SPL without severe distortion
, and a few manage more than 140 dB SPL.
The master recording
process, using current 24-bit techniques, offers around 99 dB of "true" dynamic range (based on the ITU-R 468 noise weighting standard); identical to the dynamic range of a good studio microphone, though very few recordings will use just one microphone, and so the noise on most recordings is likely to be the sum of several microphones after mixing, and probably at least 6 dB worse than shown.
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
source is transmitted or recorded at, and is important in audio if listeners of Compact Disc
Compact Disc
The 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 ,...
s (CDs), radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
and television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
are to get the best experience, without excessive noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
in quiet periods or distortion of loud sounds. Programme level is often measured using a peak programme meter
Peak programme meter
A peak programme meter is an instrument used in professional audio for indicating the level of an audio signal.There are many different kinds of PPM. They fall into broad categories:...
or a VU meter
VU meter
A VU meter is often included in audio equipment to display a signal level in Volume Units; the device is sometimes also called volume indicator ....
.
The level of an audio signal is among the most basic of measurements, and yet widespread misunderstanding and disagreement about programme levels has become arguably the greatest single obstacle to high quality sound reproduction.
How it works
Live sound covers an enormous range of levels, but this is not something that can be demonstrated with a conventional sound level meterSound level meter
Sound level meters measure sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise. However, the reading given by a sound level meter does not correlate well to...
. Sound level meters respond quite slowly, even on a "fast" setting: they use a root mean square
Root mean square
In 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...
(RMS) rectifier which by definition must take a slow running average of the square of the input voltage. Music is complex, and constantly varying, with brief peaks originating from many sources including the initial impact of sticks on cymbal
Cymbal
Cymbals are a common percussion instrument. Cymbals consist of thin, normally round plates of various alloys; see cymbal making for a discussion of their manufacture. The greater majority of cymbals are of indefinite pitch, although small disc-shaped cymbals based on ancient designs sound a...
s and drum
Drum
The drum is a member of the percussion group of musical instruments, which is technically classified as the membranophones. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a shell and struck, either directly with the player's hands, or with a...
s. A loud band
Band (music)
In music, a musical ensemble or band is a group of musicians that works together to perform music. The following articles concern types of musical bands:* All-female band* Big band* Boy band* Christian band* Church band* Concert band* Cover band...
might measure 100 dB SPL on a sound level meter, yet have peaks reaching 130 dB SPL or higher.
A recording system must handle these peaks; they can be measured using a peak responding meter with an integration time of 0.5 ms or less (not a standard IEC type PPM which has a longer integration time).
The sound level meter
Sound level meter
Sound level meters measure sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise. However, the reading given by a sound level meter does not correlate well to...
is useless for properly assessing noise levels, since the commonly used 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,...
is based on equal-loudness contours for pure tones, and is not valid for the random noise.
The subjective loudness of noise is best measured using a noise-meter to the ITU-R 468 noise weighting
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...
standard. The chart below shows, on this basis, the real range of live music, and then the level capabilities of various stages in the audio chain, from microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
to 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...
.
Analysing programme levels
This chart is based on the assumption that what goes in should come out—true high-fidelity—and so an Alignment LevelAlignment level
The alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level...
(AL) corresponding to 100 dB SPL has been assumed throughout. Any lower level would imply severe clipping at the first stage; the master recording. Top quality microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s do not present a problem; most will handle 130 dB SPL without severe distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
, and a few manage more than 140 dB SPL.
The master recording
Master recording
A multitrack recording master tape, disk or computer files on which productions are developed for later mixing, is known as the multi-track master, while the tape, disk or computer files holding a mix is called a mixed master.It is standard practice to make a copy of a master recording, known as...
process, using current 24-bit techniques, offers around 99 dB of "true" dynamic range (based on the ITU-R 468 noise weighting standard); identical to the dynamic range of a good studio microphone, though very few recordings will use just one microphone, and so the noise on most recordings is likely to be the sum of several microphones after mixing, and probably at least 6 dB worse than shown.
See also
- 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....
- Noise measurementNoise 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....
- Weighting filterWeighting filterA weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.- Audio applications :...
- Equal-loudness contourEqual-loudness contourAn equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...
- Fletcher-Munson curves
External links
- EBU Recommendation R68-2000
- AES Preprint 4828 - Levels in Digital Audio Broadcasting by Neil Gilchrist (not free)
- EBU Recommendation R117-2006 (against loudness warLoudness warThe loudness war or loudness race is a pejorative term for the apparent competition to digitally master and release recordings with increasing loudness.The phenomenon was first reported with respect to mastering practices for 7" singles...
) - AES Convention Paper 5538 On Levelling and Loudness Problems at Broadcast Studios
- EBU R89-1997 on CD-R levels