playback enhancements

I've been looking into issues related to implementing the
variable-speed 
playback enhancement that would be part of this project.
I've managed to 
make audacity "appear" to play back at variable
speeds (the playback 
cursor moves faster/slower as I adjust the slider) but the
actual sound 
stays at a constant speed. The minor change I made to do
this is to 
cause the slider-adjustment event to reset the high-low
value in the 
toolbar's private TimeTrack (which it now uses instead of
ignores).

I just noticed that if I start playing back at one speed,
then change to 
a different speed, playback continues until about 3-5
seconds after the 
change, when all sound output stops. It looks like the Mixer
loads about 
5 seconds of samples in advance, so is it possible that once
this runs 
out, whatever reloads this buffer is realizing I've changed
something 
and suddenly dealing with it (in a strange way)? Where
should I look for 
a method that refreshes the pre-loaded sound?

I've traced the act of actually playing a sound from the
button press 
down to the AudioIO and Mixer classes, but I can't seem to
find the code 
that takes it to the level below that. I see that AudioIO is
shared 
among three different threads. That suggests to me that
maybe this is as 
low-level as I would need to look. Maybe I just need to find
out more 
about how the PortAudio library interfaces with AudioIO?

One more small issue: Is there a way to print to standard
out in this 
project? I would like to be able to print debugging
messages.

Thanks,
Michael

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Hi Michael,

Have you looked into how playback is handled by the
Transcription toolbar?
For instance, start playing something and move the slider on
the
transcription to slow down or speed up playback.

You are correct in that the handling of this is in AudioIO
with the main
handling in audacityAudioCallback() (look for mSeek), but
there are several
other functions that play supporting roles as well.

For debugging output, use wxLogDebug() if you're on Windows
and wxPrintf()
if on Linux or Mac.

Leland

On 3/26/08 11:28 PM, "Michael Brooks"
<michael.brooks11gmail.com> wrote:

> I've been looking into issues related to implementing
the variable-speed
> playback enhancement that would be part of this
project. I've managed to
> make audacity "appear" to play back at
variable speeds (the playback
> cursor moves faster/slower as I adjust the slider) but
the actual sound
> stays at a constant speed. The minor change I made to
do this is to
> cause the slider-adjustment event to reset the high-low
value in the
> toolbar's private TimeTrack (which it now uses instead
of ignores).
> 
> I just noticed that if I start playing back at one
speed, then change to
> a different speed, playback continues until about 3-5
seconds after the
> change, when all sound output stops. It looks like the
Mixer loads about
> 5 seconds of samples in advance, so is it possible that
once this runs
> out, whatever reloads this buffer is realizing I've
changed something
> and suddenly dealing with it (in a strange way)? Where
should I look for
> a method that refreshes the pre-loaded sound?
> 
> I've traced the act of actually playing a sound from
the button press
> down to the AudioIO and Mixer classes, but I can't seem
to find the code
> that takes it to the level below that. I see that
AudioIO is shared
> among three different threads. That suggests to me that
maybe this is as
> low-level as I would need to look. Maybe I just need to
find out more
> about how the PortAudio library interfaces with
AudioIO?
> 
> One more small issue: Is there a way to print to
standard out in this
> project? I would like to be able to print debugging
messages.
> 
> Thanks,
> Michael
> 
>
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> Check out the new SourceForge.net Marketplace.
> It's the best place to buy or sell services for
> just about anything Open Source.
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239;13503038;w?http://sf.net/marketplace
> _______________________________________________
> Audacity-devel mailing list
> Audacity-devellists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/audacity
-devel
> 



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