25 to 23.976 Conversion Critic Needed

[mb571]

I will give a detailed explanation of the process that I used to convert a Kipper (25fps) DivX to NTSC SVCD, in hopes that someone can comment on the reason for progressively worsening audio sync.

Opened the original in VD: had 6 bad frames so I dubbed it clearing up the problem.
Info: 25 fps, 170501 frames, 1:53:40

I extracted the audio FP, and converted to 44kHz.

Video: changed the frame rate to 23.976, no audio, saved as avi.
New Info: 23.976 fps, 170501 frames, 1:58:31

Opened extracted wav in Cool Edit. Length: 1:53:39.379
Stretched it to 1:58:31.3197
(170501 frames divided by 23.976 = 7111.3197 = 1:58:31.3197)
Saved as wav.

Encoded the two new files to mpeg2 with TMPGEnc.

Any criticisms would be appreciated.

Mark

[Vejita-sama]

I normally use the Assumefps(23.976 false) command in avisynth and frameserve to CCE to conver PAL to NTSC (then run 2:3 pulldown).

https://www.videohelp.com/forum/viewtopic.php?p=487748&highlight=assumefps+avisynth+mpeg2dec#487748

Or do a forum search for 'assumefps' and see what you get. There should be no sync problems with the audio as the total runtime remains the same.

[antoniosgubba]

I do the same thing but in reverse, going from 23.976 to 25...I use Goldwave for the audio stretching...I found Cool Edit a little inaccurate in the resultant audio time it produced.

[antoniosgubba]

Hmmmm...hang on...I just checked your figures.

You stretched the audio from 1:53:39.379 to 1:58:31.3197

If converting from 25fps to 23.976fps then you need to multiply the time by 1.042709376 (25/23.976)

1:53:39.379 is 6819.379 seconds

6819.379 multiplied by 1.042709376 is 7110.630

7110.630 seconds is 1:58:30.630 which is nearly a second different to what you converted it to!!...this sort of discrepency will certainly cause very noticable audio/video sync!

Try again...but stretch the audio to 1:58:30.630

Gubba

[antoniosgubba]

By the way...sometimes the original video is NOT exactly the same length as the original audio...so never use video frame numbers to calculate the final audio time..use the above method..ie multiplying the original audio by 1.042709376. This preserves the correct RATIO of video to audio when going from 25 to 23.976.

[Xesdeeni]

Try the new audio time scaling utility that preserves audio pitch. More info at http://forum.doom9.org/showthread.php?s=&threadid=49800.

Xesdeeni

[mb571]

Very good antoniosgubba ! I have success after you corrected my logic behind the stretch to -______ calculation.
Thanks,

Mark

[antoniosgubba]

Cool!...glad it worked.