How important is the transport when using a DAC?

Newbie chip in here: normally when using standalone DAC, I would think people have a pure digital system, with some sort of transport like a computer or squeezebox to feed into the DAC to play the ripped lossless files. Alternative path would be a decent CD player with DAC built in. What the advantage of having a standalone DAC but still use CD player instead of lossless files ?

02-14-13: Edorr
Very much a function of the overall architecture. With reclocking and asynchronous DACs, the difference are definitely a lot smaller then with a traditional synchronous architecture. It is still a bit of a mystery to me why a transport would make any difference if the bits are fed into a buffer and then completely reclocked, but I guess they still do.

Good comment; good question. I suspect that one reason a transport can still make a difference if the data is completely reclocked is that digital noise associated with the low-to-high and high-to-low transitions (i.e., the risetimes and falltimes) of the signal that is received from the transport can to some degree couple past the buffer circuitry and contribute to jitter at the point where D/A conversion is performed. The coupling occurring via grounds, stray capacitances, and other possible paths through the circuitry.

The magnitude and character of that kind of effect figures to be dependent on unspecified and/or unspecifiable design characteristics of both components, and also the interconnect cable, and to not have a great deal of predictability.

Regards,
-- Al

Almarg is correct. What he is referring to is sometimes called a glitch. it can create noise modulation and time smearing that is not corrected for by resampling. Also since the signal from transport to dac is an analog signal (not digital as most people think) the cable can introduce noise that is not totally removed by reclocking
Alan

Alan, thank you for seconding my comment. But I would question the reference to the transport's output signal as being analog. Both S/PDIF and AES/EBU signals combine clock, data, and other information into a biphase mark encoded signal. The data that is embedded in that signal corresponds to the 1's and 0's which digitally represent the original analog waveform.

See, for example, page 2 of this paper, starting at the middle of the page.

Regards,
-- Al

Is anyone familiar with the Arcam rdac? It looks pretty basic with coax, toslink and usb inputs. Outputs are by way of a single pair of RCAs. It is advertised by Crutchfield as being an improvement over your sound card and it sells for $479 delivered. The Sony DVD player currently being used as a transport is murky sounding at best. Unfortunately Arcam does not make a transport. They decided to make ipod docks instead. It is interesting reading all of the technical information, but does anybody have a recommendation for a transport to be used with the Arcam rdac?