If you have a "kick booty" DAC, does the transport

Keep in mind that the actual electrical signal carrying the data from the transport to the DAC is an analog signal that is sampled at intervals by the DAC. Any corruption of this analog waveform will alter what the DAC "sees" on the link, especially the signal timing, the width of the pulses and the voltage during the sampling windows. Any variations in these can produce jitter (timing variations) in the recovered signal. This amounts to a transformation of amplitude variations in the analog signal to frequency changes in the digital signal. A lot of work has gone into minimizing these effects, but we're still not all the way there yet. For instance, the quality of the transport's power supply will have an effect on the accuracy of that analog signal carrying the encoded bitstream. Think of the output stage of the transport as the output stage of an ultra-high frequency preamp, and you'll start to get the picture. There's a lot more involved than the 1's and 0's.

I use a kick-ass DAC by anybody's measure, an Audio Note 4.1x Balanced Signature worth over $20K, and it makes the sonic defferences between transports and digital interconnects all the more more obvious. I recently upgraded my transport and interconnect from a a $5K combo to a $10K combo, and the improvement was striking.

Svhoang,

It's actually pretty complicated. At every stage in both the transport and the DAC, digital information (conceptually consisting of 1's and 0's) is superimposed (or "rides") on a modulated analog carrier signal. There is no such thing as a "pure" digital signal in electronics - a digital signal is really just an information-processing abstraction.

The process of digital to analog conversion you describe is better thought of as taking a very high frequency modulated analog signal coming in from the transport, extracting the digital information from the analog carrier, processing this information through the necessary filters and producing a reconstruction of the original sound pressure waves (the "analog").

The point is, at every stage in the process, from the pits on the CD throught the transport, the cabling, and all the circuitry out to the output jacks on the DAC, the signals are analog. The digital information rides along on these analog signals as a passenger. Anything that can corrupt an analog signal can affect the digital information it carries.

The idea that there are somehow 0's and 1's flowing around inside a "digital" component is unfortunately a massive oversimplification, one that leads to notions like "bits are bits" and "the quality of the transport doesn't matter because it's not analog".

It's all analog. Everything matters.

Kthomas,

Much (most?) of the problem is that the SPDIF interface doesn't carry timing information, certainly not bit-level timing info. The time base has to be recovered from the signal, and that is inherently problematic on a high frequency link. This is why digital cable quality is so much more important that you'd think it ought to be. It's also why quasi-proprietary interfaces like IIS were developed, and why there is curently so much interest in USB - they pass explicit timing information as well as the data, and so should minimize this particular problem.

Kthomas,

One further note - the reason you don't have this problem in a computer is because of the "system clock". This high frequency clock signal is fed to all parts of the system, and synchronizes all data transfers, even between boards, at the bit level.

In a system like the standard CD transport/DAC, each box has its own clock and the two are not synchronized. This is a theoretical argument in favour of one-box players. Of course it only addresses one issue (timing), and there are plenty of other ways to muck up a high-frequency signal.