I recently came across an issue that previously I was unaware of. Apparently, RF emissions from motherboards are sometimes an issue with compliance body certification. A solution employed by the MB mfr is to adjust the clock frequency (typically in a +/- 5% range) which changes the emissions frequency, and can lead to compliance certification. The setting can be adjusted in BIOS. If the MB was shipped with the adjusted clock, it makes clock sync for audio problematic, and adds jitter (or at least that is the claim).
There is a Wikipedia page on the subject (not particularly helpful, but some). I ran across it at the AQXOX website (manufacturer of audio components, Germany)
" ...
Optimize PC´s BIOS for AUDIO-Streaming - switch off the CLOCK-spreading / clock spread option / spread spectrum - less Jitter
If you enter the PC´s BIOS you will find somewhere in the voltage/clock options the CLOCK Spreading or Clock Spread Spectrum or SPREAD Spectrum option. This function is actually meant for EMI (electromagnetic radiation) test during the CE-test of the Mainboards. If the Motherboards during the CE - certification-tests for high radiations in a certain frequency range produce to high distortions, caused by possible overlay of frequencies (also harmonic waves) and thus a reinforcement (constructional interference) of the radiated signal. This radiation behavior can be changed by letting the systemclock not longer precisely work on a specific frequency but changing its frequency very fast. The clock-frequency fluctuates and thus flattens the spikes because SPREADING it on a broader frequency band. The BIOS adjustment possibilities can look like: 0.25%; 0.5%; 1.5%; Enabled and Disabled. Disabled means the Spreading is switched off. This is the best option, however you should restart it, if radio or TV are disturbed. Disabled improves also the PC performance. Sonically effects: What the CLOCK Spreading now to do with the sound? Very simple, an instable/varying system clock produces JITTERS, all the same whether the audio data are transmitted via USB or over an internal or external sound-port or soundcard by SPdif or TOSlink.
See what wikipedia says about one of the effects of this spreading - the clock skew
http://en.wikipedia.org/wiki/Clock_skew
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From:
http://www.aqvox.de/tech.html
If present, this would be a model-by-model variation, so would require a check to determine if it's an issue for your specific hardware.
Note: First Post
I didn't know of this forum previously, was pointed to it by a UK reseller, Keith at purite audio (~co.uk)