Is Audio from a SATA CD Drive via SPDIF Output Ever Converted?

Question

I assume your typical SATA CD drive in a PC is outputting a digital signal via the SATA cable when it's playing an audio CD. If you output that sound through your motherboards spdif output, ie Realtek ALC887 is the signal first converted to analog then back to digital or can you expect 44.1 KHz/16 bit unconverted sound data the whole way through?

I know there are dedicated audio CD players with optical outputs that you can connect to an integrated amp but I'm trying to figure out if you can accomplish the same thing with a typical PC.

Answer 1

It is possible for the SPDIF output to be "bit perfect" from the source during CD playback. For this to work it is necessary for the data on the CD to be of a sample rate and bit depth that your sound card supports for SPDIF output. Since just about any sound card should support 44.1 k samples/sec, stereo, 16-bit samples, this ought to not be a problem. SPDIF also supports 48k samples/sec. Theoretically it also supports up to 20 bits/sample, but few things implement that.

This can also be true during WAV playback from your hard drive, just as much as from CD - again, as long as the WAV data is compatible with a sample rate and bit depth that your sound card supports.

(Obviously, if SPDIF output is involved, the data has to be compatible with that too! If your WAV data is 96 ksamples/sec, that's going to get resampled to 44.1 or 48.)

But it's also possible that some additional processing is happening, even if sample rate or bit width conversion is not required by the output hardware. The Windows audio stack is known to resample everything to a different sample rate even if there was nothing going on that required it.

Here's an article by someone who is getting far better results by using a driver, ASIO4all, that allows an ASIO interface to any sound card - this should bypass all of the Windows resampling.