Dithering - Wells JASON Owner's Manual

OWNERS MANUAL FOR WEISS JASON CD TRANSPORT

Dithering

You have probably not heard the term dithering in conjunction with audio. Actually it is a term
widely used in the professional audio realm but not so much in the High-End Hi-Fi market.
What is dithering? Suppose a digital recording has been made with a 24 bit A/D converter and a 24
bit recorder. Now this recording should be transferred to a CD which has just 16 bits per sample,
as you know. What to do with those 8 bits which are too many? The simplest way is to cut them
off, truncate them. This, unfortunately, generates harmonic distortions at low levels, but which
nonethless cause the audio to sound harsh and unpleasant. The harmonic distortion is generated
because the eight bits which are cut off from the 24 bits are correlated with the audio signal, hence
the resulting error is also correlated and thus there are distortions and not just noise (noise would
be uncorrelated). The dithering technique now is used to de-correlate the error from the signal.
This can be achieved by adding a very low level noise to the original 24 bit signal before
truncation. After truncation the signal does not show any distortion components but a slightly
increased noise floor. This works like magic..... the distortion is replaced by a small noise – much
more pleasant.
I have given the example of a 24 bit recording which has to be truncated to 16 bits. Where is the
application in High-End Hi-Fi audio? More and more signal processing is implemented in the digital
domain. Think of digital equalizers, digital volume controls, upsamplers, digital pre-amplifiers,
decoders for encoded signals on DVD etc. All those applications perform some mathematical
operations on the digital audio signal. This in turn causes the wordlength of the signal to be
increased. E.g. an input signal to an upsampler may have a wordlength of 16 bits (off a CD), but
the output signal of the upsampler may have 24 bits or even more. This comes from the fact that
the mathematical operations employed in such devices increase the word length. E.g. a
multiplication of two 2 digit numbers results in a four digit number. So after the upsampler the
word length may be higher than the subsequent processor may be able to accept. In this example,
after the upsampler there may be a D/A converter with a 24 bit input word length capability. So if
the upsampler generates a word length of more than 24 bits it should be dithered to 24 bits for
maximum signal fidelity.
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Page: Date: 10/04 /dw