Manual

1.25

Operation Manual

Revision 1.01Prism Sound Callia

In summary

· Use good-quality cables with reputable connectors;

· Use balanced connections where possible; if you must use unbalanced connections, keep them

short;

· Ensure that signals passing between equipment do so at as high a level as is practical;

· If switching interference is heard, try to identify the source equipment by unplugging things one by

one. When you find the culprit, either re-plug it a long way from the audio equipment, or use a

power filter, or both.

6.4 Digital interconnections

It is understandable that little attention is usually paid to the quality of digital audio cabling. We are

used to interconnecting our computer equipment with low-cost cables without mishap, and with digital

audio it's rather logical to assume that no sound quality issues exist since we are simply moving

digital data around. But the choice of digital audio cabling can be important, because the problems of

transmitting digital audio data aren't really the same as for computer data at all.

It should be noted that the comments below apply to S/PDIF cabling only. There is no special cable

quality requirement when passing audio over a USB cable, since in that case the audio IS just data,

and Callia does not attempt to derive synchronisation or clocking information from the USB data

stream. There is no particular risk of audio data degradation, since the audio data is passed in

exactly the same way as any computer data over USB, and all USB cables must pass strict

compliance testing in order to bear the USB logo.

Data integrity issues

In general, digital audio interfacing problems are usually (but not always) the result of inadequate

interface bandwidth, which is most often due to the choice of cabling. In extreme cases this can

result in loss of data (and resulting dropouts in the audio) because (unlike computer interconnection

protocols) simple digital audio interfaces such as S/PDIF transmit the data only 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 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 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.

Many S/PDIF-connected 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).