mcbroomf wrote: I emailed Chris and he mentioned that I should be careful about paring the dAck with cheaper transports as it employs no jitter correction.
The dAck! actually does have a jitter filter in it. It uses a PLL to recover the clock, which is the same way that 99% of outboard DACs do it. This technique is just not very sophisticated, but it is the industry standard. Hantra is right in saying that non-OS is less sensitive to jitter than OS DACs in theory. Because of this, non-OS DACs rarely sound bad. However, the difference in performance between a good transport and a poor transport driving the dAck! is quite huge, and this is the cause for my warning.
When comparing between a monolithic CDP and CDP-driving-outboard-DAC, you have to be careful. The CDP has an internal master clock that is used to tell everything inside what to do - the transport needs to read samples, the filters inside need to process them, and the D/A to output samples. This is advantageous because the master clock is right inside, near the D/A chip, and is not subject to external fluctuations.
Contrast this to an external DAC, which must recover the clock from the S/PDIF stream. This stream is passed through several devices which can contribute jitter - the S/PDIF packager inside the CDP, the digital output buffer in the CDP, the cable, its terminations, and finally the PLL which recovers the clock. These components contribute jitter and contribute it in different ways (sometimes additive, sometimes subtractive (PLL), often nonlinear).
So when comparing a CDP to a DAC connected to the CDP, remember that the DAC is *always* disadvantaged in terms of the clock signal quality it has to work with. It may be the best DAC in the world, but if there is something very wrong along the chain, the quality will suffer. Most CDP manufacturers pay very little attention to the aftermarket expansion capabilities - they sell a feature, not performance - so the S/PDIF outs are very poor in most cases, especially with sub-$1K players. Dedicated transports almost always have much better jitter specifications because they had better or else they would be a useless product. It's kind of ironic - you're supposed to have expansion capabilities on your S/PDIF-capable redbook player but unfortunately, the manufacturer crippled it by slapping on a cheap output buffer. It's like selling a monster truck with 15-inch tires. It'll get you around, but you're not going to be mudding or crushing cars in something like that.
A nice article by Robert Harley about the practical consequences of jitter can be found here:
http://www.stereophile.com/reference/368/index.html-Chris
P.S. - There are some very sophisticated ways of recovering the clock nearly jitter-free, but they tend to be very expensive and very complicated.
