Re: Synchronizing with the actual USB device audio clock
Re: Synchronizing with the actual USB device audio clock
- Subject: Re: Synchronizing with the actual USB device audio clock
- From: Markus Medau <email@hidden>
- Date: Thu, 29 Dec 2005 13:21:12 +0100
Hi Philippe,
From
USB audio class specification input and output are basically independent
and not locked. If you want to lock to an external samplerate you need
an
adaptive in endpoint (so the computer follows)
Wouldn't it also work if the IN endpoint was asynchronous instead
of adaptive?
=> This would mean the computer gives feedback to the device about the
samplerate
it wants to see on input. This samplerate can be different from the
asynchronous output
rate (e.g. software samplerate converter used on host side). It never
made sense for
any of my applications so I never tried it and don't know if it's supported
at all.
It's
implemented but according to our experience it crackles from time to time.
It
crackles more often in Panther compared to Tiger, the number of crackles
seems
to be dependant on the feedback samplerate's delta. E.g. I had a
crackle
every ten minutes, half the delta gave me a crackle every twenty
minutes,
double the delta every five minutes (Tiger)....
For the time being, we have decided to rely on the generic Apple USB audio
driver. I hope that we won't have the "crackle" problem that you experienced
with your device. If it does happen we would still have the possibility
to write our own driver although I would prefer to avoid this extra work.
Yes, go for it and give Apple a hard time if you run into the same thing.
You might also
want to take a look at Apple's A26 (EMI 2|6) which reports to be asynchronous
but
feedbacks constants unless you set it to digital in / external sync.
Doing so you can watch
the story. As far as I remember they are using a high delta so you
don't need to wait
for ten minutes like I did here to hear it.
Cheers
Markus
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