Re: IOAudioDevice Startup Latency
Re: IOAudioDevice Startup Latency
- Subject: Re: IOAudioDevice Startup Latency
- From: Jeff Moore <email@hidden>
- Date: Thu, 12 Dec 2002 12:17:07 -0800
I think you are saying that you are observing an offset between the
video and the audio when digitizing with Quicktime. Correct?
Does your vdig provide a clock component? If not, it should.
Does your IOAudio driver have the correct values for the latency and
safety offset properties? It's easy to get these wrong. The latency is
the number of samples frames at the current sample rate it takes from
when the DMA gets the data to when it shows up on the line. The safety
offset describes not just the physical requirements of the DMA
hardware, but also needs to reflect the jitter in the time stamps. You
need to spend some time measuring this and increase your safety offset
accordingly.
There are some other wrinkles in doing some of this stuff, particularly
on the QT side. There's probably a lot of stuff that we just can't
cover in email, so I would encourage you to get in touch with Developer
Relations.
On Thursday, December 12, 2002, at 06:23 AM, Dirk Musfeldt wrote:
No, I don't think that is true.
I don't know how to measure exactly what is happening, but I have
audio DMA
as well as video DMA.
The audio DMA goes into my IOAudioEngine and is output by CoreAudio
and the
video DMA goes into a QuickTime vdig.
I have observed a latency between the audio output from CoreAudio and
the
video output as well as between the audio output from CoreAudio and the
DAC's line out signal.
The video output and the DAC's signal are in sync so that I can't
imagine
the latency being in the DMA stream.
With kind regards
Dirk Musfeldt
Meilenstein Mac OS Software
Neue Strasse 5
D-31582 Nienburg
Germany
Tel: +49 (0) 5021 91 24 44
Fax: +49 (0) 5021 91 24 45
<http://www.meilenstein.de/>
Von: Jeff Moore <email@hidden>
Datum: Wed, 11 Dec 2002 11:24:00 -0800
An: CoreAudio API <email@hidden>
Betreff: Re: IOAudioDevice Startup Latency
So if I'm understanding what you are saying correctly, you have
observed a latency between when the data is bussed to your card (by
DMA
or some other way?) and when your card's DAC's put the signal on the
line. If that's right, that sounds like a hardware issue with how your
card interfaces with the PCI bus. This is probably not the best forum
for your question in that case, but perhaps I'm misunderstanding?
On Wednesday, December 11, 2002, at 07:11 AM, Dirk Musfeldt wrote:
Hi all,
I have a PCI device that has audio and video output over PCI as well
as a
line out port.
I observe an offset between the audio over PCI and the audio over the
line
out port. I have difficulties to measure the exact amount of the
offset but
I think it is about 500 msec.
Of course there has to be an offset but I think it should be less
than
500
msec.
How could I minimize the offset?
With kind regards
Dirk Musfeldt
Meilenstein Mac OS Software
Neue Strasse 5
D-31582 Nienburg
Germany
Tel: +49 (0) 5021 91 24 44
Fax: +49 (0) 5021 91 24 45
<http://www.meilenstein.de/>
--
Jeff Moore
Core Audio
Apple
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--
Jeff Moore
Core Audio
Apple
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