Peak programme meter
Encyclopedia
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
used in professional audio
Professional audio
Professional audio, also 'pro audio', refers to both an activity and a type of audio equipment. Typically it encompasses the production or reproduction of sound for an audience, by individuals who do such work as an occupation like live event support, using sound reinforcement systems designed for...
for indicating the level of an audio
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...
signal.
There are many different kinds of PPM. They fall into broad categories:
- True peak programme meter. This shows the peak level of the waveform no matter how brief its duration.
- Quasi peak programme meter (QPPM). This only shows the true level of the peak if it exceeds a certain duration, typically a few milliseconds. On peaks of shorter duration, it will indicate less than the true peak level. The extent of the shortfall is determined by the 'integration time'.
- Sample peak programme meter (SPPM). This is a PPM for digital audio which shows only peak sample values, not the true waveform peaks (which may fall between samples and be up to 3 dB higher in amplitude). It may have either a 'true' or a 'quasi' integration characteristic.
- Over-sampling peak programme meter. This is a sample PPM in which the signal has first been over-sampled, typically by a factor of four, to alleviate the problem with a basic sample PPM.
In professional usage, where consistent level measurements are needed across an industry, audio level meters often comply with a detailed formal standard. This ensures that all meters that comply with the standard will give the same indication on a given audio signal. The principal standard for PPMs is 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"...
60268-10. It describes two different quasi-PPM designs which have their roots in meters originally developed in the 1930s for the AM radio broadcasting
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
networks of Germany (Type I) and the United Kingdom (Type II).
The term Peak Programme Meter usually refers to these IEC-specified types and similar designs.
PPMs were originally designed for monitoring analogue audio signals but are also now used with digital audio.
PPMs do not provide effective loudness monitoring
Loudness monitoring
Loudness monitoring of programme levels is needed in radio and television broadcasting, as well as in audio post production. Traditional methods of measuring signal levels such as the Peak programme meter, and VU meter do not give the subjectively valid measure of loudness which many would argue is...
. Newer types of meter do, and there is now a push within the broadcasting industry to move away from traditional level meters such as those featured in this article to two new types: loudness meters based on EBU Tech. 3341 and oversampling
Oversampling
In signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than twice the bandwidth or highest frequency of the signal being sampled...
true PPMs. The former would be used to standardise broadcast loudness to −23 LUFS and the latter to prevent digital clipping.
Design characteristics
Display technologies
In common with many other types of audio level meter, PPMs originally used electro-mechanical displays. These took the form of moving-coil panel meters or mirror galvanometerMirror galvanometer
thumb|right|200px|A mirror galvanometerA mirror galvanometer is a mechanical meter that senses electric current, except that instead of moving a needle, it moves a mirror. The mirror reflects a beam of light, which projects onto a meter, and acts as a long, weightless, massless pointer...
s with demanding 'ballistics': the key requirement being that the indicated level should rise as quickly as possible with negligible overshoot
Overshoot (signal)
In signal processing, control theory, electronics, and mathematics, overshoot is when a signal or function exceeds its target. It arises especially in the step response of bandlimited systems such as low-pass filters...
. These displays require active driver electronics.
Nowadays PPMs are often implemented as 'bargraph' incremental displays using solid-state illuminated segments in a vertical or horizontal array. For these, IEC 60268-10 requires a minimum of 100 segments and a resolution better than 0.5 dB at the higher levels.
Many operators prefer the moving-coil meter type of display in which a needle moves in an arc, because an angular movement is easier for the human eye to monitor than the linear movement of a bargraph.
PPMs can also be implemented in software: this can be done on a general-purpose computer or by a dedicated device which can 'burn in' a PPM image onto a video waveform for display on a picture monitor.
Scales and scale marks
PPMs often use white-on-black displays, to minimise eyestrain especially with extended periods of use.PPMs are usually calibrated in one of these ways:
- In decibels relative to Alignment Level (e.g. Nordic, EBU)
- In decibels relative to Maximum Permitted Level (e.g. DIN, ABC, SABC)
- In decibels relative to 0 dBu (e.g. CBC)
- In decibels relative to 0 dBFS (e.g. IEC 60268-18)
- In simple numerical marks which can be correlated with any of the above (e.g. British)
Whichever scheme is used, usually there is a scale mark corresponding to Alignment Level.
Most PPMs have an approximately logarithmic scale, i.e. roughly linear in decibels, to provide useful indications over a wide 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...
.
Integration time
| Tone burst duration (ms) | | Under-indication | |
|---|---|---|
| Type I | Type II | |
| 100 | 0 dB | |
| 10 | −1 dB | −2 dB |
| 5 | −2 dB | −4 dB |
| 3 | −4 dB | |
| 1.5 | −7 dB | |
| 0.5 | −17 dB | |
| 0.4 | −15 dB |
Quasi-PPMs have a short integration
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
time in order to register peaks longer than a few milliseconds
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
in duration. In the original context of AM radio broadcasting in the 1930s, overloads due to shorter peaks were considered unimportant on the grounds that the human ear could not detect 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...
due to momentary clipping. Ignoring momentary clipping made it possible to increase average modulation levels. In modern digital audio practice, where quality standards are hopefully much higher than AM radio in the 1930s, clipping of even short peaks is usually regarded as something to avoid.
On typical, real-world audio signals, a quasi-PPM under-reads the true peak by 6 to 8 dB. Nevertheless, quasi-PPMs are still widely used in the digital age because of their usefulness in achieving programme balance. Overloads are avoided by allowing, typically, 9 dB of 'headroom' when controlling digital levels with a quasi-PPM.
The extent to which quasi-PPMs show less than the true amplitude of momentary peaks is determined by the 'integration time'. This is defined by IEC 60268-10 as "the duration of a burst of sinusoidal voltage of 5000 Hz at reference level which results in an indication 2 dB below reference indication". This standard also contains tables showing the difference between indicated and true peaks for tone bursts of other durations. The longer the integration time, the greater the difference between the true and indicated peaks.
In earlier standards, different methods of measurement and criteria were used, such as 0.2 Neper
Neper
The neper is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. It has the unit symbol Np. The unit's name is derived from the name of John Napier, the inventor of logarithms...
or 80% voltage instead of 2 dB, but the practical difference between them was small.
A Type I PPM has an integration time of 5 milliseconds and a Type II PPM has an integration time of 10 milliseconds.
Return time
All PPMs have a return time which is much longer than the integration time, to give the operator more time to see the peaks and reduce eye strain. Type I PPMs fall back 20 dB in 1.7 seconds. Type II PPMs fall back 24 dB in 2.8 seconds.History and national variants
The PPM was originally developed for use in AM radio broadcastingAM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
networks in the 1930s, independently in both Germany and the United Kingdom, as a superior alternative to earlier types of meter which were not much use for monitoring peak audio levels. These were both 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...
meters with some features in common but otherwise substantially different.
Germany
In about 1936 and 1937, German broadcasters developed a peak programme meter which used a mirror galvanometer known as a 'lichtzeiger' (light pointer) for the display. The system consisted of a drive amplifier (e.g. ARDARD (broadcaster)
ARD is a joint organization of Germany's regional public-service broadcasters...
types U21 and U71) and a separate display unit (e.g. ARD types J47 and J48). A stereo version, known as a 'doppel lichtzeiger' contained two mirror galvanometer displays in a single housing. Such displays were still used until the 1970s, when solid-state bargraph displays became the norm.
The design became standardised as DIN 45406. It evolved into the Type I meter in IEC 60268-10 and it is still known colloquially as a DIN PPM. Compared to the Type II designs it has faster integration and return times, a much wider dynamic range and a scale which is semi-logarithmic and calibrated in dB relative to maximum level. It continues to be used in much of northern Europe.
In German broadcasting, the nominal analogue signal corresponding to Maximum Permitted Level was standardised by ARD at 1.55 volts (+6 dBu), and this is the usual sensitivity of a DIN-type PPM for an indication of 0 dB. Alignment Level (−3 dBu) is shown on the meter by a scale mark at −9.
Scandinavia
In Scandinavia a variant of the DIN PPM known as 'Nordic' is used. It has the same integration and return times but a different scale, with 'TEST' corresponding to Alignment Level (0 dBu) and +9 corresponding to Maximum Permitted Level (+9 dBu). Compared to the DIN scale, the Nordic scale is more logarithmic and covers a somewhat smaller dynamic range.United Kingdom
By 1932, when the BBC moved to purpose-built facilities in Broadcasting House
Broadcasting House
Broadcasting House is the headquarters and registered office of the BBC in Portland Place and Langham Place, London.The building includes the BBC Radio Theatre from where music and speech programmes are recorded in front of a studio audience...
, the first audio meter to be known as a 'programme meter' was introduced. It was developed by Charles Holt-Smith of the Research Department and became known as the 'Smith meter'. This was the first meter with white markings on a black background. It was driven by a circuit that gave a roughly logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...
ic transfer characteristic, so it could be calibrated in decibels. The overall characteristics were the product of the driver circuit and the movement's ballistics.
The first of the PPMs was designed by C. G. Mayo, also of the BBC's Research Department. It came into service in 1938. It kept the Smith meter's logarithmic, white-on-black display, and included all the key design features that are still used to this day with only slight modification: full-wave rectification, fast integration and slow return times, and a simple scale calibrated from 1 to 7.
The integration and return times were determined after a series of experiments. At first it was intended to create a true peak meter to prevent transmitters from exceeding 100% modulation. A prototype meter was created with an integration time of about 1 ms. It was found that the ear is tolerant of distortion lasting a few ms, and that a 'registration time' of 4 ms would suffice. The return time had to be a compromise between a rapid return, which was tiring to the eye, and a slow return, which made control difficult. It was decided that the meter should take between 2s and 3s to drop back 26 dB.
The BBC PPM became the subject of several formal standards: BS 4297:1968 (superseded); BS5428:Part 9:1981 (superseded) and then BS 6840-10:1991. The text of the latter is identical to the Type IIa PPM in IEC 60268-10:1991.
Alignment level (0 dBu) and Maximum Permitted Level (+8 dBu) correspond to scale marks '4' and '6' respectively.
The BBC PPM was adopted by commercial broadcasters in the UK. Other organisations around the world, including the 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...
, CBC
Canadian Broadcasting Corporation
The Canadian Broadcasting Corporation, commonly known as CBC and officially as CBC/Radio-Canada, is a Canadian crown corporation that serves as the national public radio and television broadcaster...
and ABC
American Broadcasting Company
The American Broadcasting Company is an American commercial broadcasting television network. Created in 1943 from the former NBC Blue radio network, ABC is owned by The Walt Disney Company and is part of Disney-ABC Television Group. Its first broadcast on television was in 1948...
used the same dynamics but with slightly different scales.
Modern British PPMs have a 4 dB spacing between the scale marks. Older designs had a 6 dB spacing between '1' and '2'. This discrepancy can sometimes also be found at the equivalent position on the derived CBC and ABC scales.
From its inception in 1939 until 2009, the PPM display was available in the form of an electro-mechanical, moving-coil meter movement with a demanding ballistic specification. For many years these were manufactured by Ernest Turner and Company, and in later years by Sifam, based in Torquay. In 2009 Sifam announced it would end production of the Type 74 dual-needle meter movement. In 2010 this was followed by the end of all PPM meter movement manufacturing by Sifam. Three major users - Bryant Unlimited, Canford Audio and TSL - placed final orders with Sifam for large stocks of the meters to supply manufacturing and maintenance activities for several years.
Stereo British PPMs
In the UK, twin-needle PPMs are sometimes used for stereo. Red and green needles are used for left and right. This colour convention comes from port and starboard
Port and starboard
Port and starboard are nautical terms which refer to the left and right sides, respectively, of a ship or aircraft as perceived by a person on board facing the bow . At night, the port side of a vessel is indicated with a red navigation light and the starboard side with a green one.The starboard...
navigation lights, in accordance with an ancient BBC in-joke about Broadcasting House resembling a battleship. White and yellow needles are used for sum and difference (M and S). A more recent variation is to use a black needle with a dayglo orange tip for S instead of yellow.
The sensitivity of the S indication can be increased on some meter installations by 20 dB; this is to aid line-up procedures, e.g. of stereo mic pairs, or the azimuth of analogue tape machine heads, which rely on cancellation of the S signal.
M3 and M6
M and S needles are normally aligned so that M = L + R − 6 dB and S = L − R − 6 dB; this is known as the M6 standard. A signal, equal in amplitude and phase in the left and right channels will show the same meter deflection for M as it does for L and R.
M6 has largely replaced the earlier M3 standard in which the M and S needles indicated 3 dB higher. M3 was intended as a compromise to ensure mono compatibility when stereo programmes were listened to on mono receivers, which pick up only the M signal.
With M6, a widely-panned sound that peaked to its maximum 6 on, say, the left channel would peak only 4.5 on M. The same sound in the centre of the stereo image would peak 6 on M. This is a 6 dB variation for the mono listener.
With M0, i.e. simple M = L + R, a widely-panned sound peaking 6 would peak to 6 on M, but when panned centre would need to peak just 4.5 on each channel to keep M at 6. This is a 6 dB variation for the stereo listener.
With M3, any variations as sounds are panned are kept to 3 dB. Moreover, for most non-phase-coherent stereo sounds, the sum of the two channel voltages averages 3 dB (the full 6 dB sum is only achieved by exactly in-phase signals, i.e. a mono signal panned centre), so with M = L + R − 3 dB, most stereo sounds are a 'good fit' to the maximum permissible signal (PPM 6) on M, L and R.
M6, however, better reflects the absolute signal level and keeps the M needle at '4' for Alignment Level and '6' for peaks, without the operator having to remember to subtract 3 dB.
Commercial broadcasting in the UK initially used M3 but had switched to M6 by 1980. This was mandated by the IBA
Independent Broadcasting Authority
The Independent Broadcasting Authority was the regulatory body in the United Kingdom for commercial television - and commercial/independent radio broadcasts...
's Engineering Code of Practice. BBC installations used M3 until 1999. The BBC now uses M6, but this has not been rolled out universally - many parts of the corporation still use 'traditional' M3.
European Broadcasting Union
The EBU PPM is a variant of the British PPM designed for the control of programme levels in international programme exchange. It is formalised as the Type IIb PPM in IEC 60268-10. It is identical to the British PPM except for the scale plate, which is calibrated in dB relative to Alignment Level, which is marked 'TEST'. There are also ticks at 2 dB intervals and at +9 dB, corresponding to Maximum Permitted Level.USA
In the late 1930s PPMs were considered for use in the USA, but rejected in favour of a 'Standard Volume Indicator' (VU meterVU 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 ....
) on grounds of cost. Joint research by CBS
CBS
CBS Broadcasting Inc. is a major US commercial broadcasting television network, which started as a radio network. The name is derived from the initials of the network's former name, Columbia Broadcasting System. The network is sometimes referred to as the "Eye Network" in reference to the shape of...
, NBC
NBC
The National Broadcasting Company is an American commercial broadcasting television network and former radio network headquartered in the GE Building in New York City's Rockefeller Center with additional major offices near Los Angeles and in Chicago...
and Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
found that using an experimental design of PPM (with a relatively long integration time of 25 ms) in the control of programme levels gave only a 1 dB advantage over the VU meter, in terms of average output level for a given amount of distortion. It was felt that this was too small to justify the much greater expense. It was also found that VU meters gave more consistent readings than PPMs when comparing programme levels at the sending and receiving end of long line
Long line (telecommunications)
In telephone systems nomenclature, a long line is a transmission line in a long-distance communications network such as carrier systems, microwave radio relay links, geosynchronous satellite links, underground cables, aerial cables and open wire, and Submarine communications cables...
s subject to group 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...
, which altered the waveform. This finding has been disputed by others.
A widely-believed myth is that the PPM was developed as a superior alternative to the VU meter. In fact, the PPM came first, and if anything the VU meter was developed as an economical alternative to the PPM.
By 1980, ABC had about 100 PPMs in use in control rooms in New York and its Washington News Bureau, and was ordering new consoles with PPMs fitted. These were Type II PPMs with the seven marks labelled −22, −16, −12, −8, −4, 0 and +4. ABC found a modified version of the EBU meter based on the VU-meter 'A scale' to be the best since it let operators use their usual jargon such as 'zero level' etc. The appearance is similar to an EBU scale except that the numbers are 8 dB lower.
To aid alignment on both VU meters and PPMS, ABC in New York used a special test signal known as ATS. A 440 Hz tone alternated between steady tone at +8 dBu (indicated at 0 VU and −8 PPM) and tone bursts at +16 dBu (indicated at 0 VU and 0 PPM).
Canada
By 1978 PPMs were in use at the Canadian Broadcasting Corporation's Vancouver plant. Some 30 or 40 PPMs were in use, with just one or two VU meters retained for settling telcoTelephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
disputes. These are Type II PPMs with the seven marks labelled −6, 0, +4, +8, +12, +16 and +20: this scaling shows absolute levels in dBu (or dBm into 600𝛀). The appearance is similar to the ABC PPM except that all the numbers are 16 dB higher.
South Africa
The South African Broadcasting CorporationSouth African Broadcasting Corporation
The South African Broadcasting Corporation is the state-owned broadcaster in South Africa and provides 18 radio stations as well as 3 television broadcasts to the general public.-Early years:Radio broadcasting began in South Africa in 1923...
(SABC) uses a Type II PPM modified with a black-on-white scale plate calibrated in percentage and dB relative to Maximum Permitted Level, which is +6 dBu. Alignment Level is 0 dBu or 50%.
IEC 60268-18 Digital PPM
IEC 60268-18 is a partial standard for a PPM designed for use with digital audio in both professional and consumer use, using "incremental dot or bar type displays or numerical displays". Such a display shows level relative to 0 dBFS. The integration time can have any value less than 5 ms − thus both true-peak and quasi-peak meters can comply, and different meters may indicate very different levels despite compliance with the standard. The return time has the same value as a Type I meter: 1.7±0.3 seconds for a 20 dB fall.Table of characteristics
IEC 60268-10 specifies three variants: Types I, IIa and IIb, known colloquially as the DIN, British and EBU types respectively. Types IIa and IIb differ only in the scale marks.The Nordic, ABC, CBC and SABC variants are not specified in IEC 60268-10. The Nordic PPM uses Type I ballistics with a different scale. The ABC, CBC and SABC variants use Type II ballistics with different scales.
For programme transmission from one country to another, where different technical practices may apply, ITU-R Rec. BS.645 defines three reference levels: Measurement Level (ML), Alignment Level (AL) and Permitted Maximum Level (PML), and shows the reading corresponding to Alignment Level for some types of meter.
Parameter
Parameter
Parameter from Ancient Greek παρά also “para” meaning “beside, subsidiary” and μέτρον also “metron” meaning “measure”, can be interpreted in mathematics, logic, linguistics, environmental science and other disciplines....
s for the 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 ....
and Nagra
Nagra
Nagra is the trademark referring to any of the series of mostly battery-operated portable professional audio recorders produced by Kudelski SA, based in Cheseaux-sur-Lausanne, Switzerland....
modulometer are included in the table below for comparison. Some information has been obtained from ITU-T Rec. J.15.
The Alignment Level meter indications shown here only apply with sinusoidal tone.
| Type | | Standard | | Scale marks | | Typical gain structure | | Dynamic response | | Remarks | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| Alignment Level (AL) (tone) | | Maximum Permitted Level (MPL) | | Integration time (−2 dB) (ms) | | Return time | |||||||
| Physical level | Meter indication | w.r.t. AL (dB) | Physical level | Meter indication | ||||||
| DIN | IEC 60268-10 Type I | −50, −40, −30, −20, −10, −5, 0, +5, | −3 dBu | −9 | +9 | +6 dBu | 0 | 5 | 20 dB Decibel The 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... in 1.7±0.3 s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... |
|
| Nordic | ditto (variant) | −36, −30, −24, −18, −12, −6, TEST, +6, +9 | 0 dBu | TEST | +9 | +9 dBu | +9 | |||
| British | IEC 60268-10 Type IIa | 1, 2, 3, 4, 5, 6, 7 | 0 dBu | 4 | +8 | +8 dBu | 6 | 10 | 24 dB in 2.8±0.3 s | |
| EBU | IEC 60268-10 Type IIb | −12, −8, −4, TEST, +4, +8, +12 | 0 dBu | TEST | +9 | +9 dBu | +9 | |||
| ABC | ditto (variant) | −22, −16, −12, −8, −4, 0, +4 | +8 dBu | −8 | ||||||
| CBC | ditto (variant) | −6, 0, +4, +8, +12, +16, +20 | +8 dBu | +8 | ||||||
| SABC | ditto (variant) | −18, −12, −9, −6, 0, +6 | 0 dBu | −6 | +6 | +6 dBu | 0 | |||
| Digital | IEC 60268-18 | −60, −50, −40, −35, −30, −25, −20, −15, −10, −5, 0 | −18 dBFS | −18 | +9 | −9 dBFS | −9 | <5 | 20 dB in 1.7±0.3 s | EBU R.68 |
| −20 dBFS | −20 | +11 | −9 dBFS | −9 | SMPTE RP.0155 | |||||
| VU meter (N. America, Australia) | IEC 60268-17 | −20 to +3 dB | 0 or +4 or +8 dBu | 0 VU | 165 (approx.) | |||||
| VU meter (France) | −20 to +3 dB | +2 VU | 207±30 | |||||||
| Nagra modulometer (analogue) | proprietary | −30 to +5 dB | −8 | +8 | 0 | 7.5 | e.g. Nagra 4.2 | |||
Nagra modulometer
The 'modulometer' is a proprietary type of quasi-PPM found on NagraNagra
Nagra is the trademark referring to any of the series of mostly battery-operated portable professional audio recorders produced by Kudelski SA, based in Cheseaux-sur-Lausanne, Switzerland....
products. It has an integration time (−2 dB) of 7.5 ms, and a semi-logarithmic scale with an appearance between that of a VU meter and a DIN-type PPM. A stereo version ("double modulometer") uses a meter movement with two coaxial needles.
In typical practice for Nagra analogue tape recorders, Alignment Level is regarded as −8 and maximum level 0. Thus sound recordists using location mixers would typically send a tone at 0 VU or PPM 4 (British) and adjust the Nagra recorder's gain to read −8 on the modulometer.
Some newer digital recorders, e.g. the Nagra VI, have modulometers displayed as bargraphs calibrated in dBFS. For these, Alignment Level is as for any other digital PPM, i.e. −18 dBFS (EBU) or −20 dBFS (SMPTE).
Usage of meter by sound balancers
In order to use PPMs effectively in the control of sound levels it is necessary to understand the rationale and limitations of the design.Although the PPM is preferred by many engineers to the much slower VU meter used in the USA, it does require some interpretation in use. Although it gives a useful warning of overload it does not represent either true peak level or subjective loudness. The BBC have tables showing recommended settings for different types of programme, such as speech, classical music etc., which attempt to take account of the latter.
Regardless of the kind of programme, there is usually a nominal Maximum Permitted Level, as indicated on a PPM. Operators are expected to keep levels below it within reason. Practices vary between countries and organisations. In the UK the Maximum Permitted Level is 8 dB above Alignment Level, corresponding to '6' on the British PPM scale. ITU-T standards for international sound programme circuits specify a Maximum Permitted Level of 9 dB above Alignment Level. Accordingly, +9 dB is represented by a mark on the EBU PPM scale.
PPMs and digital audio levels
Because quasi-peak PPMs indicate neither loudness nor true peaks but something between the two, it is important to allow sufficient headroom when using them in the control of digital audio levels. The EBU convention (R68) provides for this by defining Alignment Level as −18 dBFS. Thus a peak to the Maximum Permitted Level as indicated on a quasi-PPM corresponds to −9 or −10 dBFS. This 9-10 dB margin allows for operator error, the true peak typically being several dB higher than the PPM indication, and that subsequent signal processing (e.g. sample rate conversion) may increase the amplitude.SMPTE RP 0155 recommends a different alignment level, corresponding to 0 VU, of −20 dBFS. The two conventions result in line-up tone levels that differ by 2 dB, but in practice the level of programme modulation tends to be very similar.
The SMPTE and the EBU agree that regardless of whether −18 or −20 dBFS is used as the Alignment Level, that level should be declared and that in both cases programme should peak to a Maximum Permitted Level of −9 dBFS when measured on an IEC 60268-10 quasi-PPM with an integration time of 10 milliseconds.