RE: inOutputTime accuracy?
RE: inOutputTime accuracy?
- Subject: RE: inOutputTime accuracy?
- From: Jim Dolan <email@hidden>
- Date: Fri, 16 Aug 2002 14:10:46 -0400
Jeff -
Thanks for taking the extra time to help me figure this one out...
I think that the internal CoreAudio calculations are relatively correct from
callback to callback. Unfortunately that does not correspond to what I
hear out of the speakers. My guess is that just the initial values may be
wrong. I can show the descrepancy in one of two ways:
1) I loop one audio channel along with the svideo output of a Titanium Power
Book into the audio 1 and 2 inputs of a G4. I record the both signals with
an audio editor on the G4. I can now see the video and audio signals as
audio waveforms. I play a composition with a color bar frame and tone blip
once every second. Video updates are based on the sample position of the
audio and take a negligable amount of time to display. Once the sample
position reaches the point that I think audio is now playing (ie: the
initial offset), I issue the first video callback. The recorded waveforms
on the G4 shows the audio blip occurring 3k samples ahead of the video (you
can also see/hear the offset if you look closely). The sound comes out of
the speaker before I reach the offset.
2) I have a timeline indicator that also updates with the sample position.
I hear the sound before the indicator reaches the audio in the timeline.
This is just a visual test so it is hard to determine exactly how far off
things are, but the audio clearly occurs before the indicator reaches the
audio in the timeline.
Typically I see the initial inOutput.mSampleTime is ~8k. now.mSampleTime in
the first callback is ~4k. Substracting out the 3k samples leaves only 1k
sample positions to perform the callback and start playing the buffer. This
seems kinda short but not unrealistic?
Jimmy D.
-----Original Message-----
From: Jeff Moore [
mailto:email@hidden]
Sent: Friday, August 16, 2002 12:02 AM
To: CoreAudio (E-mail)
Subject: Re: inOutputTime accuracy?
That certainly sounds odd.
I have done tests where I have physically looped the output back to the
input. I can then put a pulse out of the output at a known time and see
where it comes up again in the input. This lets you see how the time
stamps line up with reality. When I do this, the error is no where near
3K samples, and it's definitely never early. I was using the built-in
hardware the last time I ran this test.
Can you tell me a bit more about what hardware you're using and what
system version? Also, can you describe exactly how you are determining
that things are 3K samples early?
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