increasing or decreasing the playback speed of the videos

[parag]

please add the most important feature to change the playback speed of the output videos by increasing or decreasing their playback speed

[AnonCrow]

Already there (though perhaps could be named in a more user-friendly way).
In the picture tab, open the frame rate window.
When decreasing playback speed, set 'do not skip any frame' to true.
When increasing playback speed, set 'force duplicate frames to be encoded' to true. Won't naturally create/interpolate any new information like fancy 120/240Hz televisions.

Any audio time stretching will have to be done elsewhere (like Audacity).

Of course if the playback is done on computer , VLC can interactively change the playback speed /w correct audio pitch. Heck , even plain old MS Windows media player can do it. Many DVD players and some PMPs also have limited functionality; from about 0.5x to 2x speeds.

[AnonCrow]

Elaborating on the issue:
viewtopic.php?&t=7273

http://forum.doom9.org/showthread.php?t=120348

http://www.divx-digest.com/software/avifrate.html

http://forum.videohelp.com/topic78178.html

[rasheedsuun]

I kind if understand what you're getting at.