Re: Stream/Device Latency
Re: Stream/Device Latency
- Subject: Re: Stream/Device Latency
- From: Jeff Moore <email@hidden>
- Date: Thu, 30 May 2002 11:43:31 -0700
On Thursday, May 30, 2002, at 09:07 AM, Andrew Kimpton wrote:
I'm attempting to use kAudioDevicePropertyLatency ('ltnc') to
establish the latencies in my input and output streams. Running with a
TiBook (667), Mac OS X 10.1.4 and using the built in audio hardware
getting the value of this property always seems to return 0 (and no
errors). I notice in TN 2029 mention of a
kAudioDevicePropertyStreamLatency but this doesn't seem to be in the
Framework headers I have installed (I have the latest dev tools)
should I actually be getting the Stream Latency property (if so what's
the 'selector' code) or are they the same thing but the built-in
hardware driver for the TiBook just returns zero for the latency ?
The drivers for the built-in don't currently report the correct values
for the latency property.
One thing about calculating the latency of things you should remember
is that the driver's safety offset affects the latency as well. This is
the number of frames the HAL must leave in front of the engine's output
head to assure things get transferred correctly. If you were trying to
calculate the earliest a frame of audio will hit the wire, you would
sum the start time with the safety offset and the latency.
The HAL doesn't really give you a whole lot of guarantees about what
the first output time stamp will be, so you don't want to rely on the
start up latency of AudioDeviceStart() by starting the IOProc before
you want to start outputting data. In that case the output time stamp
will already reflect the safety offset but not the latency.
--
Jeff Moore
Core Audio
Apple
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