Re: HAL timing issues
Re: HAL timing issues
- Subject: Re: HAL timing issues
- From: Jeff Moore <email@hidden>
- Date: Tue, 5 Oct 2004 12:07:11 -0700
The HAL's internal clock runs off of the time stamps your driver
provides. The clock has to filter these time stamps to derive the true
rate of the device. This filter code has a certain amount of delay
built into it to help manage the jitter. When the clock starts up, it
primes itself with the nominal rate. It takes a few time stamps for the
clock to work the actual time stamp data through the filter and lock on
to the true rate.
The behavior you are seeing is what I would expect to see when the true
rate of the device is slower than than the nominal rate. The question
is, is this really the rate your device is running at, or are your time
stamps off?
On Oct 5, 2004, at 10:01 AM, nick wrote:
Hi
I'm writing a kernel mode CoreAudio driver for a USB device at the
moment, it's working well so far but I'm having fun (sic) trying to
figure out what's going on with the HAL during the first second or so
of audio (input) streaming.
What I'm seeing, from a little user-space test app, is that at first
the HAL seems to be getting audio pretty quick, only to then be
'throttled' before settling on a constant rate. At the moment this is
all pretty unscientific - i'm just seeing when my IOProc is being
called.
When I perform the same test with the Apple built-in audio device my
IOProc is called at constant intervals from the outset, and I don't
see this 'throttling' effect.
It's almost as if my device is running at a slower sample rate than
HAL expects... But this can't be the case - audio i record through it
when played back is at the correct pitch, for example.
Any ideas what the problem could be?
Thanks,
Nick
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--
Jeff Moore
Core Audio
Apple
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