Dither - RME Audio ADI-8 DS User Manual

9.4 Dither

Thanks to BIT SPLIT and COMBINE the ADI-8 DS preserves full 24 bit resolution even when
working with 16 bit devices. It may happen that the actual recording situation does not allow a
usage of BIT SPLIT/COMBINE. When using Double Speed (88.2 and 96 kHz) BIT
SPLIT/COMBINE is not available. When transferring to a 16-bit medium, the word length is
reduced by discarding the lower bits. This truncation causes distortion at the low-level compo-
nents of the signal.
To combat this 'quantisation distortion', noise at a level corresponding to the least-significant bit
- or below - is added to the signal before truncation, randomly modulating the signal. This proc-
ess is called 'dithering'.
In most cases Dither can be dispensed with altogether. If you would like to know more about
this view (which some might consider provocative), please read the Tech Info 'Dither ADI-1 /
ADI-8 PRO: Remarks about the Need for Dither' on our website. To summarize: External dither-
ing is unnecessary if the sum of noise from the source as well as from the A/D converter is
above a certain threshold. And when using DC-free AD-converters truncating signals outside
the 16-bit range does not cause them to disappear altogether, but only changes their levels
slightly.
Dither is used when reducing the word length from 24 to 20 or 16 bit. Activating the function
PROCESS DITHER is helpful when recording the hi-resolution signal of the ADI-8 DS to any 16
bit destination, preventing distortion at very low levels caused by truncation. Apart from the
above notes, there are other good reasons why you can safely do without dither in the ADI-8
DS:
• Transferring to 20-bit (such as ADAT XT or O2R) does not require dither, as the maximum
dynamic range of the ADI-8 DS is 'only' 18.8 bit (or 113 dB), fitting completely into a 20-bit
(120 dB) system.
• Transferring data to a computer can be done in 20 or 24-bit word length. Dither then is
added at the very end of the chain, i.e. after all editing and mixing has been done.
Please note that Dither is most helpful at the end of the recording chain, at mastering down to 2
tracks and 16 bit.
User's Guide ADI-8 DS © RME
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