Audio levels on digital systems.
Causes endless confusion this..
On digital systems the level-indicators are usually referenced to "FS" ( Full Scale) which is the maximum level that the system can handle. That is why it often has "0" at the top of the scale. An indication of "- 20dB" means that the signal is 20dB below this maximum.
The temptation is to record as loud as possible in order to keep the signal out of the noise but this can be dangerous as you may run into clipping. Clipping in a digital system is sudden and disastrous, there's no way of making a digitally clipped signal sound good.
With a sixteen bit digital system (CD, DAT and most pro formats) you have more than 90dB of dynamic range, ie the smallest recordable sound can be 90dB (say, a thirtieth of the volume) less than the loudest. So you can afford to keep the level down a bit.
Most standards specify a peak level about 10dB less than FS. The EBU standard is exactly 10dB. It works like this:
Line-up or "zero level" is -18dBfs (18dB less than full scale) and programme material normally peaks to 8dB more than zero level, giving -10dBfs.
So why waste 10dB of headroom? Well, apart from field recording where the unexpected may always happen, it isn't immediatly obvious. The answer lies in the level measuring systems in use today. VU meters and even PPMs cannot record the very short peaks in signal level that can occur. If these peaks were too loud for the analog-to-digital convertor they would cause clicks and other distortion. These peaks can be as much as 6dB above the level indicated by even a modern analogue PPM.
So it is wise to set your peak level more than 6dB below FS, better make it 10 or 12dB just to be sure!
Note: In a purely digital environment with true digital level-monitoring you are quite safe recording right up to a dB or so below FS. Witness the huge levels on some CDs!
This subject also causes problems in the conversion between analogue and digital domains. Your correctly set-up analogue signal may be under or over recorded by your digital recorder if you don't have the input and output levels right. For broadcast gear, (EBU standard) "zero" line level test tone=0.775V rms (PPM 4 or VU -4dB) and should equate to -18fs on the digital recorder.
A lot of digital gear comes set-up to have "zero" level at -20dBfs, this is allowing even more headroom. Beware that analogue line levels can vary too, some gear operates at +4db and other at -10dB so some big errors can creep in here! The best rule is to operate your analogue gear at its normal level and set up the digital recorder so that displayed peaks on its own indicators don't go higher than -10dBfs.
A "don't quote me on this" production. (c) Dave Pick 2001.