Re: kAudioUnitProperty_PresentationLatency usage
Re: kAudioUnitProperty_PresentationLatency usage
- Subject: Re: kAudioUnitProperty_PresentationLatency usage
- From: William Stewart <email@hidden>
- Date: Mon, 17 Oct 2005 14:57:04 -0700
The idea of this property is to describe the time it takes/has taken,
for audio that an AU is dealing with to be finally presented.
For example, if you were providing a metering of your output, then
you would need to adjust the display of the metering based on the
presentation latency if you want this to accurately reflect what the
user is actually hearing - in other words, it takes "presentation
latency" time for your output samples to be presented. For input of
course, its the reverse - how long it took from the "nominal time" of
the sample hitting the input circuitry until your AU saw it. If you
don't take this into account for the metering, then your meters will
be metering audio the user has yet to hear.
So, these figures are really there to provide you a context - its not
your latency that is the issue, but rather the latencies before
(input) or after (output) from your AU that this property reports.
There is no way for you to know this, so its information optionally
that is provided to you by the host - its a write only property too
of course.
Mostly, I think, this doesn't really matter, but as you get into
rendering threads that are rendering say a 1/2 second ahead, or you
get into AUs that do their processing with significant latencies (say
hardware accelerated), then this can start to have a noticeable affect.
Bill
On 17/10/2005, at 1:31 PM, Marc Poirier wrote:
I'm a little unsure of how to use the
kAudioUnitProperty_PresentationLatency property. In my scenario, I
have an AU that provides a visual meters for the input level and
the output level. What I've been doing is to adjust the display of
the input level by the value for PresentationLatency on the input
scope, and adjust the display of the output level by the value for
PresentationLatency on the output scope. But then I started
doubting that this makes sense. My AU itself has no latency in its
DSP, so why would I ever want a different PresentationLatency
latency value for input vs. output? In cases where my AU might be
getting different values, then what that would do would be to make
the level meters out of sync with each other. So far as my AU is
concerned, the input is occurring at the same time that its
corresponding output is being generated. So then I wonder, do I
just want to offset both displays by PresentationLatency on the
output scope? Or do I want to offset both displays by the sum of
the PresentationLatency on both input and output scopes? Or
something else? I am very unsure...
thanks,
Marc
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