It's Interesting that you mention this. From what I gather the problem with dac/transport combos was that the transport quality itself had an impact on the sound.
but some of the latest dacs that have been coming out have been using special methods to reduce jitter and reviewers claim that the Transport is no longer an issue, you can use any and the sound is not lessened.
How it should be imo.
< Long post ahead>
I heartily agree.
Transports are a problem indeed, and the interlink between transport and DAC is a major source of jitter. Jitter within a CD player is bad enough, owing to complex interactions between components. It has become popular to retrofit high stability clocks in players to try and reduce jitter. Reclocking a whole transport works up to a point, but a clean clock only goes so far in many players owing to power supply modulation, signal dependent switching artifacts and the multitude of logic gates interposed between clock injection at the transport and the recovered data stream at the DAC chip.
No DAC can every be completely free of transport "sound", but it is entirely possible to reduce the contribution to something negligable.
A technique gaining popularity is asynchronous resampling. In this scheme a rock solid clock is used to resample the incoming data using a high order polyphase oversampling filter, which removes short term jitter (down to the purity of the new clock) but cannot correct for frequency drifting and some other types of modulation in the source (nothing can!). This technology has matured in recent years with the need to glue all kinds of incompatible digital systems together. The complete severing of transport and DAC clock simplifies system integration hugley and gets rid of problems with clock coupling. The resampling process, however, does introduce a little bit of distortion and needs decent bit depth to maintain signal integrity.
Another common technique is to use a PLL (Phase-Locked Loop) which chases after the input clock (from the transport) steering a high-quality clock toward the frequency of the incoming transport clock, but interposes a low-pass filter to remove short term cycle to cycle clock phase noise. These are great, but have a few limitations, the main one being that they merely attenuate jitter (admirably) but can't remove all types and frequencies and certain designs can "motorboat" limiting the achieved jitter reduction. People seem more comfortable with the PLL idea because the data from the CD is not altered by resampling, but simply passed on bit-for-bit through the reclocking process.
A very expensive approach is to use a RAM player. Jitter is possible here too. A workable RAM player is expensive, but becoming a fiscal possibilty very quickly. But more on that some other time...
In short, there are several techniques which can attenuate jitter markedly. The typical jitter on the recovered clock for separate DACs is about 200pS RMS, even if the rtasnprot is better than this! There is empirical literature which shows that 100pS jitter is easily audible, and some quick mathematical approximations will show that less than 50pS is needed to realise the noise floor that CD is capable of. This would be fine were jitter just "noise", but jitter creates distortion. Since jitter is the source of some of the worst kinds of distortion audiophiles deserve to profit from the advances in jitter-reduction techniques. The AKSADAC is going to be a balls-and-all approach stopping short of cesium clocks and small computers normally used for measuring phase differences in relativity experiments, made from readily obtainable and time-tested components. How this is going to be done is being kept under wraps for now, but I can say it is related to a digital implmentation of a PLL.
I hope this long post opens up a can of worms, of sorts. I am convinced that CD player technology is the weak link in many systems that are otherwise extremely hi-end (e.g. AKSA). This is all the more irritating since a DAC/player is hard to build. Hence the AKSADAC, a kit to combat this shortcoming.
Adios for now.
T.
