[ Somehow when I play music using my miniMac and iTunes, it does not sound the
[ same as when playing from my CD player.
[ Almost the same, but not exact the same.
[ In both cases I use an optical connection to the DAC and amplifier.
There is a very good reason why a DAC+amplifier could sound different when fed
exactly the same data from two difference source devices: The accuracy of the
clock on the S/P-DIF output can really make a difference.
I really hope the Mac mini sounds more correct. However, beware that
sometimes the human ear favors slight distortion and may prefer the less
accurate digital source. i.e. Don't always trust your ears, unless you've
trained them to distinguish different kinds of distortion.
[ So I was thinking I need to write my own audio unit.
[
[ How can I be sure the output is exactly the same? Is there a tool that will
[ dump audio samples to a file?
I still doubt you need to write an AudioUnit. However, you may need to write
some software before you're done.
If you are serious about this, then you should obtain some kind of digital
optical recording device that is bit accurate. You might be able to use the
Mac mini digital input, but I guess there might be some concern about using the
Mac mini to test itself. On a budget, I don't see any reason why that
wouldn't work, though. It will be difficult to compare the data between your
CD player and iTunes, simply because you won't be able to line up the
recordings very easily, especially not the CD player which has no external
sync. You could author a CD with a click followed by some quality music, and
then line up the click from each recording. You may need to write software to
compare the two recordings, although I have used Logic Pro to do this by using
the "invert" plugin that ships with Logic Pro and mixing both stereo recordings
at equal levels. They should completely cancel out if you've lined them up
precisely, and a good meter will tell you if there is any signal. Try this out
with the same recording on two tracks to make sure you understand what happens
when they do match, and then try it with your test recordings.
In my personal experience, feeding digital inputs from FireWire devices via
AES/EBU sounds much better than from USB devices via S/P-DIF. But that's just
with the specific brands of audio interfaces that I have in my test lab (2
different FW, 5 USB). It may or may not be directly related to the bus
technology. It's all about the quality of the clock signal. Of course, I have
different cables on each, and the input switching network on my DAC might not
treat AES/EBU inputs exactly the same as TOSLINK. However, the sound is
clearly different, even when playing from the same computer and switching the
default output device between FW and USB.
P.S. Now you've got me curious. I just might set up a test to compare
FireWire to USB with both going over S/P-DIF, just to eliminate the AES/EBU
variable. That's one combo I haven't tried, simply because the FireWire device
is question is rather new. It will be interesting to see if there is as much
of a difference.
Brian Willoughby
Sound Consulting
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