Peak programme meter
These took the form of moving-coil panel meters or mirror galvanometers with demanding 'ballistics': the key requirement being that the indicated level should rise as quickly as possible with negligible overshoot.
Nowadays PPMs are often implemented as 'bargraph' incremental displays using solid-state illuminated segments in a vertical or horizontal array.
In an attempt bring clarity to level definitions in the context of programme transmission from one country to another, where different technical practices may apply, ITU-R Rec.
Most PPMs have an approximately logarithmic scale, i.e., roughly linear in decibels, to provide useful indications over a wide dynamic range.
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.
"[6][7] This standard also contains tables showing the difference between indicated and true peaks for tone bursts of other durations.
The PPM was originally developed, independently in both Germany and the United Kingdom, for use in AM radio broadcasting networks in the 1930s.
In about 1936 and 1937, German broadcasters developed a peak programme meter with a mirror galvanometer known as a "Lichtzeigerinstrument" (light pointer) for the display.
Compared to the Type II designs it has faster integration and return times, a much wider dynamic range and a semi-logarithmic scale, and is calibrated in dB relative to Permitted Maximum Level.
[10] In German broadcasting, the nominal analogue signal corresponding to Permitted Maximum 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.
The BBC used a number of methods of measuring programme volume in its early years, including the 'volume indicator' and 'slide-back voltmeter'.
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.
Other organisations around the world, including the EBU, CBC and ABC used the same dynamics but with slightly different 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.
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.
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.
The M6 standard means that dual mono sources (e.g. a presenter panned to the centre of a stereo sound stage) can be peaked to 6 in both channels, with the M meter also showing 6.
This M3 format is designed to give a more accurate indication of the level of the summed mono signal when working with conventional stereo material.
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'.
In the late 1930s PPMs were considered for use in the US, 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 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 also found that VU meters gave more consistent readings than PPMs when comparing programme levels at the sending and receiving end of long lines subject to group delay, which altered the waveform.
ABC found that a modified version of the EBU meter based on the VU-meter 'A scale' was best, since it let operators use their usual jargon such as 'zero level' etc.
To aid alignment on both VU meters and PPMS, ABC in New York used a special test signal known as ATS.
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 600 Ω).
[19] The South African Broadcasting Corporation (SABC) uses a Type II PPM modified with a black-on-white scale plate calibrated in percentage and dB relative to Permitted Maximum Level, which is +6 dBu.
It has an integration time (−2 dB) of 7.5 ms,[21] and a semi-logarithmic scale with an appearance between that of a VU meter and a DIN-type PPM.
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.
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.
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.
