Audiophile USB to PCM

Going back to basics, there is a bitstream on the CD and the idea is to feed that to a DAC to convert it to analog. PCM just (I believe) means pulse code modulation, which basically means communicating bits between two devices by an agreed upon standard for interpreting voltage pulses on the wire between them. USB is a serial interface, which means among other things (I think) that its buffered, so its a bit more hardware to implement than something translating pulses into TTL signals for digital processing. So, because coax digital is PCM, and USB is specific serial protocol, there is a conversion from a serial protocol to a different electrical format. Hopefully doesn't mean that the actual datastream of 1's and 0's is changing, but when I've got a box that is USB on one side and coax digital on the other, I call that a "converter."

Of course, I could be dead wrong. Its happened before. ; )

AIFF/WAV/MP3 is just a means for storing the bitstream on media. There is all sorts of stuff added to store things--directory records, sector sequence data, etc--even if you aren't doing any compression. But, with compression, you also need software to recover the original bitstream (or something that looks pretty close, if it was lossy compression).

I wonder, based on what you said, whether your M-Audio somehow ran the bitstream through a low end DAC and, if you used the digital out, ran it through an analog to digital converter. Doesn't seem to make much sense to me, but might explain the problems.

Sample rate conversion sounds sketchy to me in terms of rewards... Can't get more samples per second out than there are samples per second in. But, that is what oversampling is and that seems well beyond my technical competence. How does the thing *sound*? I guess the benefits of creating a 24/96 stream out of a 16/44 stream is going to depend heavily on the quality of what is doing the conversion...

Its actually "Eric," not "Ed." I should probably stick a response in that "where does your userID come from" thread. Every time I buy or sell something on A'gon, I get an email addressed to "Ed." *sigh*

Eric -- thanks for the info. Maybe it is even worse -- maybe the M-Audio converted the signal from digital to analogue and back to digital. Whatever it was, the Apogee sounds way better, but I know the Proceed has good DAC's, so I am pretty certain something went wrong inside the M-Audio piece. At some point, I am going to take an SPDiF out of the Apogee into the Proceed to compare them in that capacity.

As I understand, the major advantage of USB connection is ability to take your SPDIF interface as far as possible from the computer that generates plenty of RFI and other noises. USB, being buffered solution isn't likely to introduce jitter, but you'll need a box (where jitter will inevitably appear) that accepts USB data flow, and converts it to SPDIF, i.e. performs quality reclocking, etc. Here, if this box is treated just as part of the audio system, and connected with short good SPDIF cable to the DAC, there should be less jitter then from a normal CD player. However, for low jitter this box must have audiophile-grade power supply, oscillator, and other parts, so its price level must be expected somewhere near any other audiophile DAC. Cheap solutions must introduce lots of jitter, making the whole idea nonsense.

This thread has been very useful. My thanks to Eric, Dmitrydr, Rsbeck, and Ultraviolet, and onhwy61!! I actually understand quite a bit (no pun intended!!) more now than I did before. The signal coming through the USB port is 1's and 0's just like PCM. As I understand it, however, the signal needs to be modified, NOT converted to the same format as the signal coming out of a source like a CD player. The USB protocol is different and the conversion devices we are discussing remove some sort of header on the signal. Most are felt to be bit accurate.

My problem with the Xitel is that its cheap chipset converts 44.1 to 48k and based in what I've heard and read I have some concern that this adding artifact above the noise floor.

While the Waveterminal can do sample rate conversion between 32k, 44.1k, and 48k, this feature is unnessary and will not be utilized. Sample rate conversion is not what I want here.

My Waveterminal U24 is arriving today. I'll be able to report on the sonics of it later. Hopefully, I will have found the holy grail of connecting my Powerbook G4 to my audiophile system!!!

harry