Re: inOutputTime accuracy?
Re: inOutputTime accuracy?
- Subject: Re: inOutputTime accuracy?
- From: Jeff Moore <email@hidden>
- Date: Thu, 15 Aug 2002 12:37:07 -0700
You need to account for the latency of the hardware too. The output
time stamp passed in to your IOProc reflects the driver's safety
offset, but not the latency in the hardware.
One other thing I would do is to start the hardware and let it run for
a few cycles before taking your anchor point. The math in the HAL
really needs 2 time stamps from the driver before it can be locked on
to the true sample rate of the hardware. This makes the first go around
somewhat inaccurate by the difference between the nominal and the
actual sample rates.
On Thursday, August 15, 2002, at 07:08 AM, Jim Dolan wrote:
Hi -
My application needs to keep track of the sample position as it is
playing.
During the first output IOProc I take inOutputTime.mSampleTime and use
that
as an initialOffsetTime to the first sample that will be played.
Later when
I check for the sample position I ask the device for the current time
and if
the current time has passed the initialOffsetTime I know that we are
now
playing samples and can start incrementing the composition sample
position.
The sample position is roughly the currentTime - initialOffsetTime.
This
seems to work fine except that I do not think that the original
inOutputTime.mSampleTime is accurate. I am trying to sync audio to
video
and I notice that I need to substract 3k samples from that initial
offset to
get them to line up. As though the first sample is being played
earlier than
I was told in the IOProc. Once we begin, the relative accuracy from
IOProc
to IOProc is very good, but the start time seems wrong. I am using a
4096
sample frame size.
Is there a bug here or is there something that I am still not
accounting
for?
Thanks for your help...
Jimmy D.
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--
Jeff Moore
Core Audio
Apple
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