Prism Sound Titan Operation Manual page 57

Prism Sound Titan Operation Manual Revision 1.00
once, and without the possibility of error correction. Although there is a possibility that an error can be
detected, this is of little use since no correction or retransmission is possible. So, unlike a computer
interconnection, a mission-critical digital audio connection must ensure that no bit errors can EVER
occur in the data stream EVER! This can be hard to guarantee in the real world, especially when the
system sample rate is high.
This was not really much of a problem when these interfaces were first standardised, since the
bandwidth requirement was quite modest when the maximum sample rate was only 48kHz.
Unfortunately, back then, the use of analogue audio cables for digital audio transmission was actively
encouraged by the choice of XLR and RCA/phono connectors for AES3 and S/PDIF respectively,
even though they typically have poor bandwidth. But for AES3 and S/PDIF, the bandwidth
requirement is directly proportional to the sample rate, since a fixed number of audio and status bits
are transmitted per stereo sample (note that for ADAT/SMUX connections the bandwidth requirement
does NOT rise with sample rate since the number of channels carried is reduced as the sample rate
is increased instead).
Many modern digital audio devices can operate at sample rates as high as 192kHz, and (sad to say)
many digital audio cabling setups don't have the bandwidth to support this reliably. Actually, it's worse
than that - much of the 192kHz-capable equipment has digital audio ports which (either admittedly or
otherwise) don't support reliable operation at 192kHz whatever cable is used. This is particularly true
of TOSLINK ports (the optical variant of S/PDIF).
Conversion quality issues
But surely the sound quality of a digital audio setup can't depend on the choice of digital audio
cabling, so long as all the data bits get through? Sadly, and familiarly, though - it can. Because in
may cases the audio data stream is used to pass the sampling clock as well as the audio data
between equipment. If the receiving equipment gets a clock which has been degraded by a low-
bandwidth interface, and if it uses this clock for A/D or D/A conversion, then the sound quality of that
box will be degraded. This effect is known as 'sampling jitter'. Unfortunately the biphase coding
scheme used in AES3 and S/PDIF is very effective at converting low cable bandwidth into clock jitter.
It should be pointed out that this is an entirely avoidable problem, since any box which relies on
deriving a jitter-free clock for A/D or D/A conversion (or for sample-rate conversion) can take steps to
eliminate incoming jitter - but many don't. The Prism Sound CleverClox technology in Titan does
exactly this, as explained in the Clocking and jitter section. This problem isn't really a cabling issue,
but an equipment design issue. However, in most cases we can't change the design of poor-quality
converters, but we can cover up their problems to some extent with good cabling!
Even though Titan is insensitive to incoming clock jitter, and even though it transmits very low jitter at
its digital audio and clock outputs, the question of cable quality may still be relevant if Titan is
transmitting to equipment which itself has poor jitter rejection capabilities. Note that audio quality
degradation by cable-induced jitter is just as much a problem at low sample rates as at high sample
rates.
Interference issues
A properly designed copper AES3 or S/PDIF interface will not cause audio-frequency ground
continuity between the connected equipments, so hum loops should not occur. However, high-
frequency ground continuity is essential if EMC legislation is to be met. This means that high-
frequency interference such as from poor-quality switch-mode power supplies (see the Analogue
interconnections section) can equally well be passed through copper digital audio interconnections. If
this is a problem in your system, consider using a TOSLINK connection instead.
Maximising cable performance
In general, the best copper cable for digital audio is the cable with the lowest capacitance, since that
will cause the least loss of bandwidth. For that reason, prefer cables specifically designed for digital
audio, or for analogue video; don't use analogue audio cables - they don't have the bandwidth for
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