RE: inOutputTime accuracy?
RE: inOutputTime accuracy?
- Subject: RE: inOutputTime accuracy?
- From: Jim Dolan <email@hidden>
- Date: Fri, 16 Aug 2002 15:29:15 -0400
Jeff -
All of my Macs are at 10.1.4
Jimmy D.
-----Original Message-----
From: Jeff Moore [
mailto:email@hidden]
Sent: Friday, August 16, 2002 2:42 PM
To: CoreAudio-API
Subject: Re: inOutputTime accuracy?
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:
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Jeff -
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Thanks for taking the extra time to help me figure this one out...
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I think that the internal CoreAudio calculations are relatively
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correct from callback to callback. Unfortunately that does not
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correspond to what I hear out of the speakers. My guess is that just
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the initial values may be wrong. I can show the descrepancy in one of
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two ways:
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1) I loop one audio channel along with the svideo output of a Titanium
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Power Book into the audio 1 and 2 inputs of a G4. I record the both
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signals with an audio editor on the G4. I can now see the video and
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audio signals as audio waveforms. I play a composition with a color
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bar frame and tone blip once every second. Video updates are based on
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the sample position of the audio and take a negligable amount of time
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to display. Once the sample position reaches the point that I think
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audio is now playing (ie: the initial offset), I issue the first video
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callback. The recorded waveforms on the G4 shows the audio blip
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occurring 3k samples ahead of the video (you can also see/hear the
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offset if you look closely). The sound comes out of the speaker
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before I reach the offset.
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2) I have a timeline indicator that also updates with the sample
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position. I hear the sound before the indicator reaches the audio in
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the timeline. This is just a visual test so it is hard to determine
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exactly how far off things are, but the audio clearly occurs before
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the indicator reaches the audio in the timeline.
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Typically I see the initial inOutput.mSampleTime is ~8k.
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now.mSampleTime in the first callback is ~4k. Substracting out the 3k
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samples leaves only 1k sample positions to perform the callback and
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start playing the buffer. This seems kinda short but not unrealistic?
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Jimmy D.
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-----Original Message-----
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From: Jeff Moore [mailto:email@hidden]
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Sent: Friday, August 16, 2002 12:02 AM
>
To: CoreAudio (E-mail)
>
Subject: Re: inOutputTime accuracy?
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>
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That certainly sounds odd.
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I have done tests where I have physically looped the output back to the
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input. I can then put a pulse out of the output at a known time and see
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where it comes up again in the input. This lets you see how the time
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stamps line up with reality. When I do this, the error is no where near
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3K samples, and it's definitely never early. I was using the built-in
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hardware the last time I ran this test.
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Can you tell me a bit more about what hardware you're using and what
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system version? Also, can you describe exactly how you are determining
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that things are 3K samples early?
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>
--
Jeff Moore
Core Audio
Apple
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