[mythtv] Time Stretch with PVR-350 (Possible solution to adjust audio pitch)

[Louie Ilievski]

> Allan,
>
> AFAIK it is there if you're not using a PVR-350 to do the decoding,
> i.e. software MPEG2 decompression; the 350 doesn't adjust pitch (well
> I've heared it maybe can do it for x1.5 but then the driver would have
> to implement it).
> There's a checkbox in the setup where you can specify that you are
> using the 350 audio out (so the volume controls etc don't work). I'm
> suggesting adding an option where you tell MythTV that the 350-audio
> is redirected to line-in of your soundcard; line-in is real-time
> pitch-adjusted and played back through your soundcard to your stereo.
>
> Frank,
>
> you might be able to re-use the soundtouch libraries currently used by
> Myth to do the pitch-adjust if you are using software MPEG2
> decompression. I was trying with sox as I know how to work that one.
>
> N.

What exactly does that checkbox do, anyway?  I never really understood it.

Also, here's another idea for this timestretch idea:

Rather than doing all this fancy conversion from the line in and spitting it 
out of line-out (which, I would THINK might cause a little latency), how 
about when a value for timestretch is selected other than 1.0, mute the sound 
on the PVR-350, and directly play the sound straight out of the sound card, 
using the existing soundtouch libraries and such.  All the code for this is 
pretty much there because this is how it's done with any other system using 
nVidia cards or something else, where the audio comes out of the sound card.  
Alternatively, just allow for a user option to only use the sound card's 
audio out, and avoid all this switching and muting/unmuting of the 350.

Would this work?  I don't know a whole lot about the MPEG2 standard, but I 
believe the audio stream is separate from the video, right?  So it would be 
easy and not very cpu-intensive to just take the audio and do the work on it 
while letting the 350 do the video decoding.  I wonder if it would be a 
problem to keep the audio in sync this way?

~Lou
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