I think we're getting pretty close to off-topic here. It is well-established
that the output from CoreAudio is bit-accurate compared to any lossless file
played. It's doubtful that you're going to discover anything at all by writing
an output device driver that saves output data to a file. What would be more
useful is to record optical input data to a file, and connect your CD player to
the Mac mini.
I suspect that your CD player has oversampling and digital filtering which is
altering the data there. It may even be converting from 16-bit to something
higher like 24-bit, in order to resolve the filter.
Note, I wasn't suggesting a TOSLINK analyzer, but merely a digital recorder
with TOSLINK input. You're really just fine using the Mac mini input. Writing
some code to compare the recorded data to your original file will be far, far
easier than writing an output device which saves to a file. Do not
underestimate the diffilculty of determining whether the resulting file equals
the original file. TOSLINK has no file start or file end markers to bound your
saved files.
Regarding your questions about my experience, I'm not talking about technical
differences, but perceived differences in the audio. Yes, S/P-DIF is nearly
the same as AES/EBU, but I blame the clock more than the format. I am not
using optical for the comparison, yet, but you've inspired me to test that, too
(although I have used up my optical inputs for HDTV and PlayStation :-). I am
comparing coaxial digital to balanced digital.
Brian Willoughby
Sound Consulting
Begin forwarded message:
My DAC does reclocking, so I hope the clock will not be the main reason for
the difference.
It would be great to have a Toslink analyzer like Brian Willoughby is
suggesting, but quit a bit of work. I was thinking of an output device for the
macMini that is not sending the samples to the SPDIF port, but to a file. Or
better, to both the file and port. Then I could check the file data agains the
original wav file.
Can this be done? Or does it exists already? I do not know enough about the
Apple audio architecture (yet).
Using the optical input may be the easiest way to go. I agree it is a little
dangerous to use the macMini to test itselfs. But when the resulting file
equals the original file, I think it is must mean that the data on the optical
wires is the same too. Or not?
And Brian:
About the difference in AES/EBU versus S/P-DIF differences where you are
talking about, I'm a little confused. Isn't S/P-DIF (almost) the same as
AES/EBU? Or do you mean copper when saying AES/EBU and optical when saying
S/P-DIF? I know that for the serial protocol the clock is recoverred by
checking one particular signal edge in the protocol. Depending on the bandwith
of the connection and medium, jitter will be added because of the inter-symbol
interferention. For optical (lower bandwith) this will be worse. Maybe this
can explain the difference in your case?
Regards, Cor
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