An earlier article "Getting Your Levels Right - PPMs, clipping and all that" explained that the normal analogue audio reference level in the UK radio industry is 0dBu (or PPM4), the peak level is +8dBu (or PPM 6) and the onset of clipping is at +12dBu (PPM 7). This article explains the how audio levels are usually measured in the digital world, and how the analogue and digital worlds can be related.
In the digital world, audio samples are represented by signed integer numbers. Analogue audio is converted to digital audio by an "Analogue to Digital Converter" (ADC) which converts 0V to the number 0, a positive voltage to a positive number, and a negative voltage to a negative number. These ADCs have defined minimum and maximum voltages that they can convert to digital. Voltages outside these limits get clipped to the limiting values.
The digital audio industry usually works in dBs, just like the analogue audio industry, but rather than using the reference of 0dBu (=0.775Vrms), they use the digital clipping level (aka Full Scale) as their reference. This is generally written as 0dBFS. In the analogue world, we'd call clipping +12dBu, so all we have to do to convert from dBu to dBFS is to add 12dB. The table below provides a comparison between the various scales:
To set the analogue input level on a digital recording device, simply send line-up tone at 0dBu (PPM 4) on your mixing desk and adjust the input gain on the recorder until the meter indicates -12dBFS.
On playback, adjust the playback level and/or input gain on your mixing desk until the line-up tone indicates 0dBu (PPM 4) on the desk's meters.
Note: The above is valid for the 16-bit digital devices (such as DAT, CD and MD) that are found in Hospital Radio stations at the moment. If the audio samples have more than 16-bit accuracy (the latest professional equipment usually offers at least 20 or 24 bits), then the European Broadcasting Union (EBU) recommends that reference level is set to -18dBFS to give more "headroom" (The Yanks, in the form of the AES and SMPTE, recommend -20dbFS just to be different.) This is not recommended with only 16-bit accuracy due to the poor signal-to-noise ratio that this would produce.
Nigel Dallard, Chief Engineer, Winchester Hospital Radio
9 October 1999
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