Bit Operation And Dither - RME Audio ADI-8 PRO User Manual

10. 16 bit Operation and Dither

Thanks to BIT SPLIT and COMBINE the ADI-8 PRO 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. Whenever copying 20-bit or 24-bit digital audio to a 16-
bit medium, the word length is reduced by discarding the lower bits. This truncation causes
distortion at the low-level components 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'.
Accomplishing dither in a totally FPGA-based device such as the ADI-8 PRO requires a
great deal of time and effort. And in most cases it 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 dithering 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.
A common misconception is an 'analog' way of thinking projected into the digital domain - that
by discarding the least significant bits, any low-level components of a signal would be lost. The
noise floor of a 24-bit signal at -112 dBFS would disappear completely when converted to 16-
bit, digital zero would be the result. This is wrong. All parts of the signal which were originally
below -96 dB will still be present in the 16-bit version (FFT proves this), but not at the original
levels. In the early days of the digital era, A/D converters were DC-ridden, and parts of the
signal really could be lost. However, the converter chips used in the ADI-8 PRO include
DC filters and automatic calibration, eliminating any DC offset.
Dither is used when reducing the word length from 24 to 20 or 16 bit. So the only case where
Dither makes sense in the ADI-8 PRO is recording directly onto a 16 bit medium. Apart
from the above notes, there are other good reasons why you can safely do without dither in the
ADI-8 PRO:
Transferring to 20-bit (such as ADAT XT or O2R) does not require dither, as the maximum
dynamic range of the ADI-8 PRO is 'only' 18.6 bit (or 112 dB), so there is no loss in a
20-bit (120 dB) system.
Tascam DA-38 or 98 owners can use the (often overlooked) built-in dither functions (please
refer to the respective manuals).
Transfering 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.
The dynamic range of the recorded signal source has to be far above 100 dB - but this is
seldom the case in real life situations, caused by the relatively large portion of noise your
sources suffer from.
To avoid any misconception: We are not saying that external dither is altogether pointless.
Even the ADI-8 PRO could benefit from sophisticated dither or noise-shaping when trans-
ferring data to 16-bit media in some cases. In reality however, DC-free converters and limita-
tions posed by real recording environments negate any advantages that dither might bring.
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 PRO © RME
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