What is True Peak Measurement?
The PPM measurement contains a small amount of time averaging of the audio. Thus the actual instantaneous level may be higher than the PPM value. Additionally, as audio is converted from digital to analogue, a Reconstruction filter has to be used. This filter has to recreate values ‘between’ every pair of sample points, which could be higher or lower than the value of the actual samples. In fact, the true peak level of audio can be as much as 8dB higher than the maximum PPM value. For the audio not to clip when converted to analogue, there must be sufficient headroom available for the True Peak measurement not to exceed 0dB, which is why the maximum PPM value is always set less than the maximum.
EBU R128 states that the true peak level should not exceed -1dB. We measure True Peak using the algorithm specified in BS1770-3.
Processing and True Peak
A drawback of using digital processing of an analogue signal is the loss of the true peak of a sample due to sampling errors. If a peak falls between two samples values, the true value of this peak will not be detected. In order to ensure that the correct peak values are sampled the signal is oversampled.
Loudness is a newer measurement, and attempts to put a figure on ‘how loud’ the material appears to be to a listener. How a person ‘hears’ the loudness is based on many factors, such as type of programme material, the listeners mood and health, and whether they like the programme material, so it is quite subjective. The Loudness measurement attempts to put a number on how an average listener might respond to the programme material.
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