A peak programme meter (PPM) is an instrument used in professional audio that indicates the level of an audio signal.
Different kinds of PPM 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 indicates 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. It shows only peak sample values, not true waveform peaks (which may fall between samples and may be higher in amplitude). It may have either a 'true' or a 'quasi' integration characteristic. Over-sampling peak programme meter. This is a sample PPM that first oversamples the signal, typically by a factor of four, to alleviate the problems of a basic sample PPM.
In professional use, which requires consistent level measurements across an industry, audio level meters often comply with a formal standard. This ensures that all compliant meters indicate the same level for a given audio signal. The principal standard for PPMs is IEC . It describes two different quasi-PPM designs that have roots in meters originally developed in the 1930s for the AM radio broadcasting 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. Though originally designed for monitoring analogue audio signals, these PPMs are now also used with digital audio.
PPMs do not provide effective loudness monitoring. Newer types of meters do, and there is now a push within the broadcasting industry to move away from the traditional level meters described in this article to two new types: loudness meters based on EBU Tech. 3341 and oversampling 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 meters, PPMs originally used electro-mechanical displays. 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. 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 they feel the angular movement is easier for the human eye to monitor than the linear movement of a bar graph.
PPMs can also be implemented in software, in a general-purpose computer or by a dedicated device that inserts a PPM image into a picture signal for display on a picture monitor.
Level definitions
A variety of terms, such as line-up level and operating level exist, and their meaning may vary from place to place. In an attempt to 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. BS.645 defined three reference levels: Measurement Level (ML), Alignment Level (AL) and Permitted Maximum Level (PML). This document shows the readings corresponding to these levels for several types of meters. Alignment Level is the level of a steady sine-wave alignment tone. Permitted Maximum Level refers to the permitted maximum meter indication that operators should aim for on speech, music, etc., not tone.
Scales and scale marks PPMs often use white-on-black displays to minimise eye strain, 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 Permitted Maximum 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 that 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.
Integration time Tone burstduration (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 −9 dB 0.5 −17 dB 0.4 −15 dB
Quasi-PPMs use a short integration time so they can register peaks longer than a few milliseconds 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 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 as, "...the duration of a burst of sinusoidal voltage of 5000 Hz at reference level that 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 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 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 independently in both Germany and the United Kingdom, for use in AM radio broadcasting networks in the 1930s. These were quasi-peak meters with some features in common but otherwise substantially different. They are superior to earlier types of meters that were not good for monitoring peak audio levels.
IEC 60268-10 Type I PPM
Germany
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. The system consisted of a drive amplifier (e.g., ARD types U21 and U71) and a separate display unit (e.g., ARD types J47 and J48). A stereo version, known as a "Doppel-Lichtzeigerinstrument" 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 semi-logarithmic scale, and is calibrated in dB relative to Permitted Maximum Level. It remains in use in much of northern Europe.
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. 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 Permitted Maximum Level (+9 dBu). Compared to the DIN scale, the Nordic scale is more logarithmic and covers a somewhat smaller dynamic range.
IEC 60268-10 Type II PPM
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 called 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 integration and slow return times, and a simple scale calibrated from 1 to 7.
Mayo and others determined the integration and return times by a series of experiments. At first, they intended to create a true peak meter to prevent transmitters from exceeding 100% modulation. They created a prototype meter with an integration time of about 1 ms. They found that the ear tolerates distortion of only a few ms, and that a registration time of 4 ms is sufficient. They made the return time a compromise between a rapid return, which was tiring to the eye, and a slow return, which made control difficult. Engineers decided that the meter should take between 2 and 3 seconds 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 Permitted Maximum 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, CBC and ABC 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 electromechanical, 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 was ending production of the Type 74 dual-needle meter movement. In 2010, Sifam ended all PPM meter movement manufacturing. 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