AppleUSBAudioDriver: Latency tuning
AppleUSBAudioDriver: Latency tuning
- Subject: AppleUSBAudioDriver: Latency tuning
- From: Chuck Carlson <email@hidden>
- Date: Sun, 1 Aug 2010 21:29:53 -0700
Hello,
I've been modding the AppleUSBAudioDriver 273.4.1 to work with our device.
The driver now works with our device, but the input to output passthru latency is huge, as expected. 20ms for 64 sample frames.
I fiddled with setOutputSampleLatency and setOutputSampleOffset but there are only a few milliseconds of adjustment room available.
The only way I know to reduce the latency a lot(8ms) is to set mLastPreparedBufferOffset to 350 in the output audio stream just before the stream runs. This makes PrepareWriteFrameList put the output samples 350 samples closer to the play head. I can go closer(450), but I don't want to push my luck till our testers sign off on this version.
I'm sure I'll have to do the same thing on the input side.
So why do I have to do such a kludgy thing to get latency reasonable? Is there some other adjustment or configuration i should be doing?
We've been shipping a version based on the the USBAudioDriver from 10.5.6 for over a year using this method and it has been stable under very heavy cpu loads.
Thanks,
--Chuck
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