Peak Programme Meter - PPMs and Digital Audio Levels

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 defining Alignment Level as −18 dBFS. Thus a peak to the Permitted Maximum Level as indicated on a quasi-PPM corresponds to −9 or −10 dBFS. This 9-10 dB margin allows for operator error, the true peak typically being several dB higher than the PPM indication, and that subsequent signal processing (e.g. sample rate conversion) may increase the amplitude.

SMPTE RP 0155 recommends a different alignment level, corresponding 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.

The SMPTE and the EBU agree that regardless of whether −18 or −20 dBFS is used as the Alignment Level, that level should be declared and that in both cases programme should peak to a Permitted Maximum Level of −9 dBFS when measured on an IEC 60268-10 quasi-PPM with an integration time of 10 milliseconds.