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In Reply to: RE: Why are small PCI bus delays audible? posted by Tony Lauck on July 11, 2008 at 14:14:07:
A good programmer would argue that latency is a non-issue for playback. "Just set it to highest level as we get less context switches which is more efficient...". This is not correct for best SQ.
At a software, firmware and hardware level, PCI prefers small payloads. See here for more details.
"Latency jitter" as in variations in latency was thought to be the cause for why latency affects SQ. I've scrapped this idea.
From a Jitter viewpoint, when a soudcard's buffer is populated (whilst the other buffer is converted to SPDIF or whatever), there's a burst of electrical activity. The idea is to keep this burst as short as possible thereby reducing interference to soundcard's XO, i.e. reduce Jpp. We achieve this by setting latency to lowest possible level. Of course, using such a low latency would mean more frequent buffer loads. I called this the ASIO frequency (or ASIO Hz). At 32 samples latency for 96k output, ASIO Hz is 3kHz. This is now periodic in nature and is digitally induced. We now have Periodic Jitter - the worst kind which exists for all digital playback systems. ASIO gives us control over this.
I prefer higher ASIO Hz and you definitely want to avoid anything less than 1kHz. Why? Soundcard's PLL or PLLs down the chain will be able to further reduce this periodic jitter as the frequency is likely to be above PLL's cut-off.
cMP² designs for this. Here's an explanation of cMP's software. That's why I recommend ASIO together with Optimize set to Critical. cPlay offers an additional level of refinement by implementing an extremely efficient ASIO interface which is also the reason for not supporting other ASIO data types except for 32 bits.
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Follow Ups
- Why? To reduce digitally induced Periodic Jitter. Long answer follows... - cics 22:16:17 07/11/08 (1)
- RE: Why? To reduce digitally induced Periodic Jitter. Long answer follows... - Tony Lauck 09:19:33 07/12/08 (0)
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