Re: Measuring Input/Output latencies
Re: Measuring Input/Output latencies
- Subject: Re: Measuring Input/Output latencies
- From: Brian Willoughby <email@hidden>
- Date: Wed, 24 Aug 2011 14:52:11 -0700
On Aug 24, 2011, at 03:57, Stéphane Letz wrote:
In the context of the JACK server, we are using a tool called
"jack_iodelay" (developed by Fons Adriaensen) which allows to
measure IO latency using a calibrated signal.
2) Measured latency differs from the driver reported latency, we
were assuming that:
Input latency = Input latency safety offset + Input latency + IO
buffer size
Output latency = IO buffer size + Output latency + Output
latency safety offset
So global IO latency would be : Input latency safety offset +
Input latency + 2 * IO buffer size + Output latency + Output
latency safety offset
But we see this reported "extra loopback latency".
Does this value correspond to some additional ADC/DAC conversion
latency?
There were many numbers in your email, but I saw no indication that
you were clearly adding all CoreAudio latency values. Keep in mind
that a CoreAudio driver has both a device latency and individual
stream latencies. Some device drivers do not bother to report
individual stream latencies, but the idea is that a device with
analog I/O through converters and digital I/O through various digital
audio connections would report the converter latency only on the
analog streams and a much lower latency for the digital streams.
Other devices have a topology which prevents latency from being
determined on individual channels this way, but at the very least
your code should look for both values and add them.
To reiterate, total audio channel latency = device latency + stream
latency. This is true for both input and output.
There is, of course, always the possibility that given hardware is
not accurately reported in the CoreAudio drivers. I suppose even
Apple might not have every last hardware variation covered. I
actually have no information on the coverage there, so I'm not
claiming that Apple is inaccurate or accurate in all cases.
Brian Willoughby
Sound Consulting
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