Prism Sound Titan Operation Manual page 50

Prism Sound Titan Operation Manual Revision 1.00
But why is clock jitter so important?
Analysis of sampling jitter
Analysis of sampling jitter (small variations in the sampling intervals of an A/D or D/A converter)
shows that it produces a similar effect to phase modulation, where distortion components appear as '
sidebands' spaced away from the frequency of a converted tone by the frequency of the jitter itself.
These components get louder as the amount of jitter increases, but also as the frequency of the
converted tone increases. So sampling jitter produces distortions which should sound much worse
than conventional analogue harmonic distortions, since the spurious components appear at
aharmonic frequencies. High audio frequencies should suffer worse distortion than low frequencies.
For low-frequency jitter, the resulting distortion sidebands appear close in frequency to the audio
signals which produce them – this should mean that they are 'masked' from our hearing by the same
psycho-acoustic phenomenon upon which are based sub-band (perceptual) coding schemes such as
MPEG. This is fortunate, since it is quite difficult for a PLL to remove jitter to a good degree even at
moderate frequencies, but for very low frequencies it would be very difficult indeed.
The graph below shows the effects of 'JTEST', a special test stimulus to expose jitter susceptibility of
D/A converters. JTEST is basically an fs/4 tone (12kHz at fs=48kHz) which is specially coded to
cause an AES3 or S/PDIF carrier transmitted over a lossy cable to become very jittery by the time it
reaches the receiving D/A converter. The jitter produced has regular frequency components fs/96
apart (500Hz at fs=48kHz). The quality of the D/A converter's jitter rejection is shown by the degree
to which it suppresses the resulting 500Hz-spaced side-tones. In the example below, the upper trace
shows the poor jitter rejection of 'conventional' D/A converter design, where the conversion clock is
derived directly from the AES3 or S/PDIF receiving chip, without any further jitter filtering. Remember
that none of these side-tones is present in the digital audio signal - they are caused only by jitter. The
lower trace shows almost complete jitter rejection across the band by the CleverClox process in Titan.
Listening experience
In practice, it seems that the benefits of careful clock design are very apparent in listening tests. On
the other hand, it can sometimes be difficult to expose the shortcomings of converters with poor
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