Dither And Noise-Shaping - Prism Sound Titan Operation Manual

Prism Sound Titan Operation Manual Revision 1.00
clocks, because these units often have other analogue problems whose severity might obscure
jitter-related effects.
In general, some of the widely-noted effects of sampling jitter are not surprising – for example the
muddying of brass, strings and high-frequency percussion and the loss of stereo (or multi-channel)
imaging. These are well explained by the worse distortions which result in the lab at loud, high
frequencies, and the way that sampling jitter produces quiet, aharmonic components, perhaps only
subliminally perceptible, which blur our impression of the ambience which creates a soundstage.
Other effects are harder to explain – for example there is wide observation that large amounts of
sampling jitter can take the edge off extreme bass rendition. Such reports are probably too
widespread to be ignored, but defy explanation within current theory.
Titan and CleverClox
Titan is designed to source clocks which are as stable and accurate as possible, and also with the
aim of being insensitive to the quality of incoming clocks. It is designed to remove jitter from any
selected reference sync source before it is used as a conversion timebase, so as to eliminate any
audible effects of sampling jitter, whatever sync source is used.
Titan does this with the help of Prism Sound's unique CleverClox clock technology, which removes
the jitter from any selected clock source down to sub-sonic frequencies, without the need for a
narrow-band quartz VCO. CleverClox can adapt to any reference, irrespective of frequency, and
regardless of how much jitter it has, derives an ultra-stable conversion timebase.
7.3

Dither and noise-shaping

Titan can dither or noise-shape its digital output to produce high-quality 16 bit output (for, say, a CD
master) from 20 bit or 24 bit recordings. This section discusses the principles and choices involved in
word-length reduction.
Truncation and dithering
There are many points in a digital audio signal path where precision can be lost. For example, in a
digital transfer from 24-bits to 16-bits, or in an analogue to digital conversion. In this situation it is not
sufficient just to discard low-order bits – this causes truncation distortion, characterised by aharmonic
frequency components and unnatural, harsh decays.
Instead, it is preferable to use some sort of 'dithering' process, whereby the truncation process is
linearized by modulating the signal prior to the truncation, usually by the addition of a small amount of
noise. By adding a random element to the truncation decision, small components as far as 30dB
below the noise floor can be accurately represented, and an analogue-like low-signal performance
can be realised. This is achieved at the expense of slightly raising of the noise floor, although with
some dithering schemes such as noise-shaping, linearization can be achieved with no noticeable
increase in noise.
How can dithering allow information to be preserved below the least-significant bit? It seems
impossible. Consider a simple example where the audio samples are numbers between one and six,
and we are going to 'truncate' them (i.e. reduce their resolution) so that numbers from one to three
become zero, and those from four to six become one. Clearly much information will be lost, and all
excursions of the signal between one and three and between four and six will not affect the output at
all. But if we throw a die for each sample, add the number of spots to that sample, and translate
totals of six and below to zero and totals of seven and above to one, we have a simple dithering
scheme. Input samples of three will be more likely to result in outputs of one than will inputs of one.
The throw of the die is our dither noise. Since all the faces of the die have an equal chance of
occurring, this is known as 'rectangular probability distribution function' (RPDF) dither, which in fact
does not produce perfect linearization. We actually use 'triangular probability distribution function'
© 2013 Prism Media Products Ltd 1.45