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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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Digital Drive Upsamplers, DACs, jitter, shakes and analogue withdrawals, this is it. |
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In Reply to: Remember - KISS is best posted by Christine Tham on April 18, 2007 at 14:52:29:
Most of the pro audio interfaces, eg. Digidesign Pro Tools, Lynx, RME, uses a voltage controlled oscillator (VCO) as a clock, rather than fixed frequency crystal oscillators (XO).The reason for doing this is so that sampling rates can be easily changed "on the fly" - a pretty essential requirement for pro audio.
However, VCOs are inherently more susceptible to jitter than XOs. They are very sensitive to logic induced modulation, and "drift" over time unless controlled using a PLL.
A device that accepts word clock in is effectively using the clock input to drive a VCO, and using PLL to compare the output with the input. This is less effective than syncing a VCO to an XO (which is what the "internal" clock does).
But using XO driving a DAC directly delivers better results than VCO.
Another technique is to use a "clock divider". This is common with DVD players, which generate a 44.1 or 48 audio clock based on the 27MHz master video clock. This is also not as effective as using fixed frequency XO.
XOs are also unstable (can drift based on temperature) but less so than VCOs. The main disadvantage is that they are fixed frequency, which means to change between 44.1 to 48 for example requires switching to a different XO.
This is not as easy as it sounds, because the DAC effectively needs to be "rebooted" when the XO is switched. This is why most devices that uses switchable XO requires a power cycle to change sampling rates (eg. my Edirol SD-90).
The E-MU 1820M is one of very few devices out there that uses switchable XOs but allow sampling rates to be switched on the fly. E-MU does this through very clever design, but even so it's rather cumbersome (effectively, they do a reboot of the DSP - they also mute audio outputs during the process so that you don't get a "screech" as the DAC resets).
So I should probably clarify, I'm really looking for a DAC with switchable XO rather than VCO. Unfortunately, that eliminates most of the DACs on the market.
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Follow Ups
- just a bit more information on clocks to help you understand - Christine Tham 18:13:39 04/18/07 (1)
- Thanks Christine. You're a walking wiki. That's a keeper. :o) nt - Presto 13:46:55 04/19/07 (0)
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