Apple

On Dec 24, 2005, at 12:46 AM, Markus Medau wrote:

Hi Markus,

Hi Philippe,

Beside the use of a "feedback endpoint", the standard points on the
possibility to use "implicit feedback" provided that the device
complies to some "constraints" on its endpoints (satisfied by my
device). If I have correctly understood, the principle is to deduce
the data rate to use on the OUT stream from the data rate measured on
the IN stream.

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?

and an asyncronous out endpoint providing feedback about if you currently need a higher, same or lower samplerate to not run into buffer underruns or overruns on firmware side.

I 'll give a try to the feedback endpoint solution.

Hence my question: does the AppleUSBAudio driver
implements this "implicit feedback" algorithm?

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.

If yes, would it work also with "old" Apple computers which are
"only" 1.1 USB host (e.g. G4 MDD)?

Yes, of course.

Cheers

Markus

References:
	>Re: Synchronizing with the actual USB device audio clock (From: Markus Medau <email@hidden>)