Q about PAL2NTSC audio using time stretch...

I commented in my PAL2NTSC guide about some cases where it is desirable when converting from PAL to NTSC to slow the video to 23.976 fps, but time expanding an audio track with pitch correction (instead of a straight slow-down).

Sony SoundForge has a nice Time Expansion tool.  I calculated (wrongly it seems) that the expansion percentage should be 104.27% (which 25/23.976).  Testing it the audio was out of sync towards the end.

Should that be an even 104% or more like 25/24 (104.17%) instead?  Has anyone any experience with this?

Hmm.  Lots of views, but no comments.

Re-checked at 104.27%, the audio needs to run longer, not shorter.  That means MORE than 104.27%.  I can't think of any number that would correctly correspond to that change.

23.976 = 24000/1001

Actual ratio is therefore 1.0427083.

However, it is preferable to specifiy a final length to the nearest millisecond (can SoundForge do this?) rather than a ratio - even a small miscalculation or rounding error could lead to a noticeable missync after 90 mins.

Thanks.  That is indeed the problem.  At 104.27% SoundForge is more than 2 seconds off of the correct length.

Using a time calculator it seems I am only getting 104.245%.  SoundForge isn't too good at math it would seem.

It does have the ability to let you enter a final length.  Good because there's more flexibility, bad because there's now an extra step of using a time calculator.

Edit:  Hmm.  Even entering a time directly it misses the target.  Wt?

Any suggestions for another software that does the same thing?

Doctor M said:

Any suggestions for another software that does the same thing?

I found SoundStretch to be useful for simple rate changes; it can also do tempo changes with pitch retention.

I see it uses SoundTouch, which I already use with BeSweet/BeLight.  I figure what I need is the '-rate' setting since it seems to preserve the pitch, I just can't find the right number.

It SHOULD in theory be "-tempo -4.2709376042709376042709376042709" but that ends up long.  I stupidly tried 1-(23.976/25) = -4.096

The final length looks right and the pitch seems unchanged with pretty good quality (mind you it is a commentary track).

I'll see if it successfully syncs later.

Success. 

-tempo -4.096

Syncs fine.  The audio quality is surprisingly good for a freeware alternative.

Um, Besweet can use Aften for AC3 encoding.  AC3Enc is also available.

SoundTouch is strictly used for changing pitch, tempo and rate.  It can be used with any output format AFAIK.

When recoding AC3 to AC3 I always decode and make any changes as uncompressed PCM.

When I'm ready to encode back to AC3 I use SoftEncode.  It has been out of print for awhile, but it's still commercial grade DD encoding compared to someone's sourceforge project.