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Neil Hastie
2004-11-24, 16:38
There seems to be some confusion on this thread regarding the
format and use of the SPDIF. The following link may help
explain the operation.

http://www.epanorama.net/documents/audio/spdif.html

The basic problem is that Sony and Philips decided to pass both
data and clock in the same signal stream. To do this a system called
biphase-mark coding (or Manchester coding) is used.
Data is represeted by the logic level of the signal and is trivial to
extract.
The clock is represented by the edges of the signal stream and is a lot
more complex to reconstruct to the accuracy required by high definition
audio. Timing error in the recovered clock is known as jitter. Modifying the
edge speed of the signal pulses in the spdif stream modifies the jitter
seen by the receiver. There are measurable differences in jitter between
optical and wired connections in most systems due to this.

A lot of time and effort has been spent trying to remove jitter from the
recovered clock. As mentioned above it largely comes down to the amount
of money you are willing to pay. PLLs are good, Multilevel PLLs are better,
very large data buffers seem to be best.

One other point := If you have a transport powered by a cheap switching
power supply and containing a Ghz radio transceiver you may find an optical
conection is the best way to minimise the amount of RF noise injected into
your audio system.

Neil

(Using an optical link to feed a DAC with a 4 second data buffer).