A peak programme meter (PPM) is an instrument for indicating the level, loudness or volume of an electronic audio signal.
It was originally developed in the 1930s, independently in both Britain and Germany, as a superior alternative to earlier types of meter which were not much use for monitoring peak audio levels. Despite the name though, the PPM is actually a quasi-peak meter with quite a slow integration time of around 5-10 ms. This was deliberate, the intention being that while the PPM could be used to avoid overload caused by peaks, it would ignore fast peaks because the ear is tolerant of distortion lasting a few milliseconds.
There are nowadays many types of peak and quasi-peak indicating audio level meters, but this article confines itself to the British and German designs (as specified in IEC 60268-10) and their derivatives. Differences in national practices are listed under National variants below.
In common with many other types of audio level meter, PPMs originally took the form of dedicated hardware but can now be implemented in software as well. The latter 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.
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History
United Kingdom
The BBC used a number of methods of measuring programme volume in its early years, including the 'volume indicator' and 'slide-back voltmeter'.
By 1932, when the BBC moved to purpose-built facilities in Broadcasting House, 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 logarithmic 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 attack and slow decay time constants, and a simple scale calibrated from 1 to 7.
The time constants 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.[1][2]
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: "Sound system equipment. Methods for specifying and measuring the characteristics of peak programme level meters".[3]
The BBC PPM was adopted by other organisations around the world, including the EBU, CBC and ABC. They all used the same dynamics but with slightly different scales. The EBU variant is formalised as the Type IIb PPM in IEC 60268-10.[3]
Germany
In about 1936 and 1937, German broadcasters also developed a peak programme meter, independently of work in the UK. Like the BBC PPM, this was a quasi-peak design with a fast rise and slow decay. It became standardised as DIN 45406. This standard was subsequently replaced by the Type I meter in IEC 60268-10, but the meter is still known colloquially as a DIN PPM. It continues to be used in much of northern Europe, and in Scandinavia in a variant known as 'Nordic'. Compared to the Type II designs it has faster attack and decay times, a wider dynamic range and the scale is calibrated in dB relative to maximum level rather than alignment level.[3]
USA
PPMs were considered for use in the USA, but rejected in favour of a 'Standard Volume Indicator' (VU meter) on grounds of cost. Joint research by CBS, NBC and Bell Labs found that using an experimental design of PPM (with an 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.[4]
Design characteristics
- A short integration time constant, in order to register quasi-peaks longer than a few milliseconds in duration. Overloads due to shorter peaks were considered unimportant on the grounds that the human ear cannot detect distortion due to momentary clipping. On typical, real-world audio signals, a PPM under-reads the true peak by 6 to 8 dB.[5]
- A long decay time constant, to give the operator longer to see the peaks and reduce eye strain.
- A meter movement with divisions marked 1 to 7 using a white legend and needle against a black background, again to reduce eye strain.
- A logarithmic scale giving 4 decibels per division, except for some early designs which did not follow this rule below PPM 2.
- A standard reference level of PPM 4 corresponding to 0 dBu (BBC PPM).
National variants
IEC 60268-10 specifies three variants. The BBC and EBU types differ only in the way the seven marks are labelled. VU meter parameters are included for comparison.
| Specification | a.k.a. | Scale marks | Alignment level[6] | Digital ref. level[7] | Integration time | Fallback time |
|---|---|---|---|---|---|---|
| IEC 60268-10 / I | DIN | −50, −40, −30, −20, −10, −5, 0, +5, | −9 | −9 = −18 dBFS | −2 dB in 5 ms | 20 dB in 1.5 s |
| Nordic | −36, −30, −24, −18, −12, −6, TEST, +6, +9 | TEST | TEST = −18 dBFS | −2 dB in 5 ms | 20 dB in 1.7 s | |
| IEC 60268-10 / IIa | BBC | 1, 2, 3, 4, 5, 6, 7 | 4 | 4 = −18 dBFS | −2 dB in 10 ms | 24 dB in 2.8 s |
| IEC 60268-10 / IIb | EBU | −12, −8, −4, TEST, +4, +8, +12 | TEST | TEST = −18 dBFS | −2 dB in 10 ms | 24 dB in 2.8 s |
| ANSI C 16.5 IEC 60268-17 |
VU meter | −20 to +3 dB | 0 VU | 0VU = −20 dBFS | 99% in 300 ms | 99% in 300 ms |
The integration time for PPMs is defined by the IEC 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".[8]
Stereo PPMs
Twin-needle PPMs are sometimes used for stereo. Red and green needles are used for left and right (A and B in BBC parlance), and white and yellow needles 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.
M and S needles are normally aligned so that M = A + B – 6 dB and S = A – B – 6 dB; this is known as the M6 standard.
BBC installations used the M3 standard until 1999 in which the M and S needles indicated 3 dB higher. This was chosen as a compromise to ensure mono compatibility when stereo programmes were listened to on mono receivers, which pick up only the M signal. The BBC has now begun to use the M6 Standard, but this has not been rolled out universally - many parts of the corporation still use 'traditional' M3.
With the M6 standard, a widely-panned sound that peaked to its maximum 6 on, say, the A (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 = A + B, 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 = A + B – 3 dB, most stereo sounds are a 'good fit' to the maximum permissible signal (PPM 6) on M, A and B.
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.
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 precise meaningful indication of overload it cannot be assumed to represent 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 this fact.
Regardless of the kind of programme, 6 on the meter (BBC PPM) is nearly always considered the nominal maximum level, with anything more considered an overload.
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 specifying an alignment level (which corresponds to PPM '4') of -18 dBFS.[9] Thus a peak to the nominal maximum '6' (BBC PPM) corresponds to -10 dBFS. This 10 dB margin allows for the true peak typically being 6-8 dB higher than the PPM indication, and the fact that the true peak of the audio waveform can fall between PCM samples.
EBU R68 differs from SMPTE RP155 which specifies an alignment level (which corresponds 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.
IEC 60268-18 is a specification for a true-peak PPM designed for use with digital audio. It shows level relative to 0 dBFS.[3]
Trivia
The BBC's motto, 'Nation shall speak peace unto nation', has been adapted by engineers to reflect correct level as indicated on a PPM: 'Nation shall peak six unto nation'.
Notes and references
- ^ Pawley, Edward (1972). BBC Engineering 1922 - 1972. London: BBC. p. 118. ISBN 0-563-12127-0.
- ^ Ellis, H D M (1955). Godfrey, J W. ed. Studio Engineering for Sound Broadcasting. BBC Engineering Training Manuals. London: Iliffe & Sons. pp. 167-168.
- ^ a b c d Yonge, Mark (April/May 2008). "Audio Waveforms & Meters - Extra". Line Up (Institute of Broadcast Sound): 1. http://www.ibs.org.uk/public/lineuparchive/2008/115_Apr-May/10b_Audio_Waveforms_And_Meters_Extra.pdf.
- ^ Chinn, H A; Gannett, D K; Morris, R M (1940). "A New Standard Volume Indicator and Reference Level". Proc. IRE 28: 6. http://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf.
- ^ Yonge, Mark (April/May 2008). "Audio Waveforms & Meters". Line Up (Institute of Broadcast Sound): 34. http://www.ibs.org.uk/public/lineuparchive/2008/115_Apr-May/10a_Audio_Waveforms_and_Meters.pdf.
- ^ Defined by ITU-R Rec. BS.645 as the level of a sine wave 9dB below a sine wave at maximum permitted level.
- ^ From EBU R68 (for PPMs) and SMPTE RP155 (for VU meters)
- ^ The E.B.U. Standard Peak-Programme Meter for the Control of International Transmissions, European Broadcasting Union, http://www.ebu.ch/CMSimages/en/tec_doc_t3205_tcm6-10506.pdf
- ^ EBU Technical Recommendation R68-2000, European Broadcasting Union, http://www.ebu.ch/CMSimages/en/tec_text_r68-2000_tcm6-4669.pdf
See also
External links
- IBS Wiki: Metering
- VU and PPM meters: an elementary explanation
- Rane Pro Audio Reference definition for PPM
- Software to emulate a PPM (BBC Open source)
- PPM Emulation Software
- Highly configurable PPM software (commercial)
- PPMulator+ PPM software plugin/standalone (commercial)
- RTW Peak Programme Meters (commercial)
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