Technical Background; Synchronization And Jitter; Background; Why Are Good Clocks So Rare - Prism Sound ADA-8 Operation Manual

Prism Sound ADA-8 Multi-channel A/D D/A Converter Operation Manual - Revision 1.00

7 Technical Background

This section provides background information concerning various technical concepts and
proprietary Prism Sound processes relevant to the ADA-8.

7.1 Synchronization and jitter

7.1.1 Background

Good clock stability is probably the most important issue separating good-quality A/D and D/A
converters from the mass. With the linearity of modern converter devices beginning to rival
and exceed the performance of the best analogue circuits, digital recordings would already be
'beyond reproach' if clock stability did not so often marr potential quality.
Why is good clock stability so unusual? Probably because most conversion equipment has to
compromise between clock stability, operational requirements and cost. The ideal clock
system in an A/D or D/A converter would be ultimately stable, i.e. would exhibit no sampling
jitter at the point of conversion, whether operating from an internal clock or from an external
synchronization reference of any format and at any sample rate. But this is a very tall order
for the circuit designer, especially one on a budget.

7.1.2 Why are good clocks so rare?

Most converters on the market can provide workmanlike performance when internally
clocked, since this is only a matter of providing a stable clock oscillator (or range of
oscillators) at a fixed frequency (or frequencies) – although even this is not always well-
executed. The real problem is that in most installations the data converters can almost never
operate from their own internal clocks since they must be slaved to a central master reference
or, in the case of D/A converters, to their incoming data.
The externally-clocked design challenge is, by necessity, a trade-off since the more stable a
clock oscillator is, the less is its 'pull-range' of frequency adjustment: but we would ideally like
an oscillator which can operate over a wide range of sample rates, perhaps from <32kHz to
>48kHz, plus multiples thereof. But such an oscillator would inevitably have poor stability – at
least in terms of the stringent requirements for high-quality audio conversion. On the other
hand, if we limit the ranges of rates at which the oscillator needs to operate to small 'islands'
around the standard sample rates we could use a bank of oscillators, selecting the
appropriate oscillator according to our desired sample rate. But this is expensive and, in any
case, the pull-range of an ordinary quartz crystal oscillator is still generally insufficient to meet
the tolerance demands of the digital audio interfacing standards. As well as a very stable
clock oscillator, a good sounding converter must have a PLL (phase-locked loop) with a loop-
filter which steeply attenuates incoming reference jitter towards higher frequencies.
Unfortunately, even if sourcing equipment provides a carrier with low jitter, cabling always
adds unacceptable amounts, especially poor quality or high-capacitance cable, which results
directly in sampling jitter in the receiving converter if filtering is inadequate. But why are these
things so important?

7.1.3 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
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