[mythtv-users] How Does Time Stretch Work?

Marc Randolph mrand at pobox.com
Fri Oct 17 22:11:04 UTC 2008

On Fri, Oct 17, 2008 at 4:51 PM, Ivan Kowalenko
<kichigaimentat at gmail.com> wrote:
> Evening. I've got a moderately technical question, it's not exactly a
> tech support question, but I'm wondering what kind of strategy does
> MythTV use to do time stretching? I mean, if I bump playback speed to
> 1.1x, does it just drop frames, or attempt to play the video back at a
> higher frame rate (like if I went to 2.0x, would it try to play back
> at ~60 FPS?). Or does it do something completely different?
> I'll be honesty, I don't know jack about coding, but I find these
> kinds of details interesting.

Howdy Ivan,

For the video, I assume they drop frames.  The human eye is very
forgiving.  But the audio is a mystery to me, and I did some serious
work on digital audio compression back in college. This reference
page: http://www.cuymedia.net/mythtv-trunk/ points out that
libmythsoundtouch is a pitch preserving audio resampling library used
for the time-stretch feature.
This library, and a couple of sections on how it works, looks to be
described here:

http://www.surina.net/soundtouch/README.html About algorithms

Interesting stuff.  If anyone knows more, let's hear it!


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