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 60268-10. 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 meter, 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 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 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
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 60268-10 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.
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