Re: inOutputTime accuracy?
Re: inOutputTime accuracy?
- Subject: Re: inOutputTime accuracy?
- From: Jeff Moore <email@hidden>
- Date: Fri, 16 Aug 2002 11:41:56 -0700
Whoops, I see the hardware you are talking about =)
I have done time stamp tests with the built in hardware and the time
stamps are pretty much dead on. In 10.1.x and earlier there was a 43
sample mystery latency, but that's no where near the 3K you are dealing
with. Besides that, this was positive latency and you are describing
negative latency.
Is this a Jaguar or 10.1.x system?
On Friday, August 16, 2002, at 11:10 AM, Jim Dolan wrote:
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?
--
Jeff Moore
Core Audio
Apple
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