Peak metering
Peak Programme Meters (PPM) are also classic analogue meter designs, almost as iconic as the VU. They react much faster than VU meters (though still not instantly), making them useful in broadcast applications where peak levels need to be kept within strict limits; but they tell you little about the loudness of the signal.
Pixels on a display have no ‘inertia’ to speak of, so the peak meters your DAW during recording and playback react instantly, revealing exactly what the sample values are and thus making it always clear how close you are to clipping (ie, exceeding 0dBFS).
This doesn’t quite tell the whole story, however. When you zoom in on an audio waveform to look at individual sample values, most editors simply join the sample points with straight lines – but that’s not what really happens. When converted to analogue, the smooth curves of the signal can go higher in level than the samples on either side at any given point, potentially exceeding 0dBFS. Acon Digital’s Acoustica audio editor is unusual in that it displays the smooth analogue waveform when zoomed in: the highlighted section in the screenshot is clearly higher in level than the samples on either side.
These digital-to-analogue conversion anomalies are known as Inter-Sample Peaks (ISPs), and you need an oversampling ‘True Peak’ meter to detect them. 2x oversampling would put a new sample point halfway through the highlighted section in the screenshot, giving a much more accurate reading of the true waveform level. However, meters need to use 4x oversampling or higher to confirm to BS.1770 True Peak specifications.