I'm planning to use an "Echo Audio Indigo I/O" Cardbus card for recording sessions with my laptop.
However, the Indigo has no Digital Audio in/outs, i e no Toslink or COAX S/PDIF
I/O. So I need something for getting S/PDIF into and out of the laptop, t oo!

Now, I've seen the M-Audio Transit USB device, which is at a good price point for me.
However, I can't find out if this device is capable of bit-perfect digital audio INPUT.

Does anyone here know if this card can do bit-perfect S/PDIF input, or if there are
any alternative USB or Firewire or cardbus devices that can?

P.S. I know the Transit USB can do bit-perfect digital OUTPUT, which makes me happy, but
I also need bit-perfect INPUT.

Your best bet is probably to ask M-Audio directly. They should be able to tell you if the card does any bit-mangling on input.

Oh, I have asked M-Audio, but it is difficult to obtain unequivocal answers from mfgs.
M-Audio claim that with ASIO, the card will put a sound file on disk with the same sample rate
as the incoming stream. However, they haven't yet stated clearly if this means one can obtain
a bit-perfect copy of the input stream or not. I have asked for clarification, and that may
arrive sometime today.

A lot of S/PDIF inputs on simple sound cards resample even if the sample rates
are identical. They resample so as to avoid having to synchronise to the S/PDIF input stream.
This means, among other things, that any jitter in the incoming stream will be faithfully
carved in permanent rock..... And it could also mean overload! Resampling can in principle introduce
local excess peaks of 1.3 dBs. Mathematically this is a version of Gibb's phenomenon, i e it is not ringing
but the removal of part of what builds up a sharp edge.

Well, if M-Audio won't tell you then you need to test it yourself

Walk in to your local dealer with your laptop and ask if you can test the box with your machine. Then loop a cable between the out and in ports. Finally, play a sample/selection of samples with lots of dynamic range and record the SPDIF input. Finally, bit compare the audio data in the files and you have a close approximation of the answer.

While this wouldn't be and exhaustive test (in my experience these sorts of things vary with driver version, firmware version and other factors) it would go a long way to answering your question. I don't see any decent dealer having a problem with you doing this.

M-Audio came back today., saying:

Yes it is possible to achieve bit-perfect S/PDIF
input when using ASIO with the Transit USB.
The application that is being used must use the ASIO
protocol in order to do this.
Hope this help.
Thank you

SO
M-Audio Technical Support

Well, so I'm OK with this.