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In the days of analogue tape, engineers were trained to record audio at high input levels. This is because analogue tape has a relatively loud ‘noise floor’: background hiss that’s inherent to the medium. The louder the signal onto tape, the lower the hiss becomes relative to the recorded signal. And at high amplitudes an effect called ‘tape saturation’ occurs; a gentle distortion that creates a perceived ‘warmth’ to analogue recordings.
Digital audio is different. The noise floor of a modern domestic digital recording system is very low (around -120dB) so avoiding hiss is easy. But there is an unavoidable maximum level in every system, referred to as 0dB. If you attempt to record a signal that exceeds this maximum, this information can’t be captured by the AD converters and is rounded off as 0dB. This error is known as ‘clipping’ and manifests as an unpleasant digital distortion.
Consequently we need to include plenty of ‘headroom’, the difference between the loudest level in a signal and the maximum level the system can capture. If your vocalist reaches -3dB on your input level meters, your headroom is said to be 3dB, the difference between -3dB and 0dB.
How much headroom should you leave? With 24-bit recording systems, the noise floor is theoretically up to 48dB lower than at 16-bit. Consequently, you can run all of your equipment at lower levels, avoiding the distortion inherent to amplifying weaker signals and thus have a cleaner and clearer signal path. It’s possible to leave headroom of around 18dB and have an extremely high quality recording.