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

[John Harvey]

My understanding of the checkbox is that it allows you to control the volume
of the tv if you feed the output of the 350 to the line in of your audio
card. This requires the code to open the audio device though even though it
isn't actually used for audio playback. By ticking this box you say that you
aren't using the audio card for tv playback so it won't try to open it (and
possibly fail) but this means you can't control the volume.

As for keeping the audio and video in sync I suspect it will be a complete
nightmare. The delay in ivtv's mpeg playback is quite large and I suspect
not very well defined. 

You would probably need to grab the information from the vsync and number of
frames played etc. and use that to keep it all in sync. 

John

> -----Original Message-----
> From: mythtv-dev-bounces at mythtv.org [mailto:mythtv-dev-bounces at mythtv.org]
> On Behalf Of Louie Ilievski
> Sent: 30 April 2005 21:02
> To: mythtv-dev at mythtv.org
> Subject: Re: [mythtv] Time Stretch with PVR-350 (Possible solution to
> adjustaudio pitch)
> 
> > 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