Clocking And Jitter - Prism Sound Titan Operation Manual

Prism Sound Titan Operation Manual Revision 1.00
In the case of Titan, problems of latency and stability are improved by a couple of useful features:
First of all, the operator can control the buffer delays within the Mac and Windows drivers directly,
irrespective of what buffering is employed by the user's particular audio software. It is generally
recommended that these buffer delays are set long, in order to provide best stability. However, for
the user with a powerful and tightly-optimized setup, who has contained audio processing tasks and
needs low latency, the buffer delay can be minimized. For more information, see the Unit settings
section.
For foldback and over-dubbing situations, all of Titan's outputs (analogue 1-4, S/PDIF, ADAT, DO,
and headphone outputs) have a comprehensive mixer capability which can mix any of the unit's
inputs with each output's computer feed in order to build a dedicated monitor mix with extremely low
latency. Incoming audio to the mix doesn't have to go in and out of the computer at all - the mix is
handled within the Titan hardware itself. For more information, see the Outputs tab and Mixer tabs
sections.
7.2

Clocking and jitter

Good clock stability is probably the single most important issue separating good-quality analogue
interfaces from the rest. With the linearity of modern A/D and D/A converter chips 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 degrade their potential quality.
Why is good clock stability so rare? 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 jitter (frequency variations) 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 circuit designers,
especially if they are on a budget.
Why are good clocks so rare?
Most analogue interfaces 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 many
installations the analogue interfaces can almost never operate from their own internal clocks since
they must be slaved to an external reference sync, or maybe to a clock from a host computer.
The externally-clocked design challenge has traditionally been a trade-off since the more stable a
clock oscillator is, the less is its range of frequency adjustment: but we would ideally like an oscillator
which can operate over a wide range of sample rates, perhaps from <44.1kHz 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 range 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 reference clock with low jitter, cabling always
adds unacceptable amounts, especially poor quality or high-capacitance cable, which results directly
in sampling jitter in the analogue interface if jitter-filtering is inadequate.
Prism Sound's unique CleverClox technology breaks these traditional constraints, allowing a low jitter
clock to be re-created from any reference sync, no matter how much jitter it has and no matter what
its frequency.
© 2013 Prism Media Products Ltd 1.43