Contents
History and Evolution of PPM Meters
PPM (Peak Program Meter) is designed to measure the peak value of a signal. Developed initially in 1932, the PPM meter as we know it today appeared in 1938. However, PPM does not indicate the actual peak voltage of the signal. The early devices used a 10ms detection time (Type II meter), and later this was reduced to 4ms (Type I meter). These detection times were chosen to ignore the quickest transient peaks, resulting in PPM often being called “Quasi PPM,” distinguishing it from true PPM. Very short transient signals can be displayed about 4dB lower. The reason for ignoring transient peaks is the assumption that momentary overloads in recording or transmission equipment are generally inaudible if they are less than 1ms analog overloads.
Comparison with VU Meters
PPM was developed before the VU (Volume Unit) meter, which was introduced in 1939. However, VU meters became more popular in audio equipment due to their capability to show averaged signal levels, thus aligning better with human perception. They were also simpler and relatively cheaper to implement. In the past, analog consoles were typically equipped with VU meters or dBu (RMS) meters that primarily displayed the average level of the signal. However, additional options had to be installed to check peak levels.
PPM Release Time and Scales
Unlike VU meters, PPMs reduce eye fatigue and make it easier to understand peak indications by having a very slow release time—returning to -20dB takes more than 1.5 seconds (Type I and II meter specs may slightly differ). The specifications for all types of PPMs are detailed in IEC 60268-10 (1991), and the BBC scale consists of white numbers from 1 to 7 displayed on a black background. Each mark is 4dB apart, and PPM 4 is the reference level (0dBu). PPMs from EBU, DIN, and Nordic have different scales.
The Concept of Quasi PPM
Although PPM was initially developed to accurately represent the peaks of a signal, in practice, the detection time was introduced to ignore short transient peaks. Consequently, PPM evolved into a metering system that shows a “Quasi-Peak,” making it less suitable for measuring exact transient peaks. For this reason, PPM is often called “Quasi PPM,” and other metering systems are used if one wishes to measure the actual peak level accurately. In essence, all analog PPMs can be considered Quasi PPMs.
Modern DAWs and Sample Peak Program Meters
Most modern DAWs (Digital Audio Workstations) offer Sample peak program meters as standard. These meters operate in a digital system and display the peak value of the signal accurately.