Speed Correction (Time Stretch) In Audition.... Few Q's

[Liquid Drum Theater]

Hi,

I'm working on re-authoring a dvd with new audio and menus/chapters. I've done this before a few times with great results and no problems.

I've now come across a big problem. The audio I have seems to be too slow. When I line it up in Ulead Video Studio its in sync at first but as the song progresses it gets further out of sync, which I'm guessing is a tempo problem.

Now, everytime I 'shrink' the audio in Audition so that the song is faster, it introduces lots of wierd sounds and pops (sounds like its warped or something).

Any way to adjust the tempo without introducing all this noise??

The files I have are WAV and the video is mpg (NTSC).


Thanks for your time,
Simon.

[guns1inger]

Was it always NTSC ?

This sort of drift usually occurs when the framerate of the source has been altered, perhaps from PAL to NTSC 23.976 or NTSC 29.97 (depending on which way the audio drifts). If you need to change the framerate there may be better ways to do it.

I have not used Audition, but Sound Forge has a similar tool. The first thing to understand is that the change will introduce artifacts into the audio, and the second thing to understand is that the effect will be cumulative. This means that while you may need to take several goes to experiment and find out exactly how much change it required, once you find this figure you should go back to your original audio and apply a single change. If you make the changes incrementally then you will compound any artifacts and make the audio sound terrible.

The other thing you need to take into account is that a shift of more than around 5% will make these artifacts worse. This is usually OK for PAL/NTSC shifts as the difference is only around 4% if done correctly.

The first thing I would be looking at is why you have this audio shift to begin with. If you are just re-authoring an existing disc then this should not be happening unless there are other factors at play. Solving these would be preferable to changing the audio.

[Liquid Drum Theater]

I recieved the disc as NTSC. It was filmed in the US, so I guess it hasn't been encoded to PAL, but I have no idea if it has or not. When I copied the disc to HHD, it said 29FPS.

All I can see is that the audio goes out of sync. Now, it doesn't go out by a certain amount and stays like that for the whole show. Just by judging 1 song, it goes out of sync and continues to go further out until its about 1-2 secs behind...

I can only guess that the audio needs speed correcting, but if there is a simpler way that would be much easier.

When I tried the temp change, I had changed it from 7m55sec to 7m53sec just to see if it worked. But it just sounded all wierd, with pops, clicks, you name it.

Have I explained it ok?

Thanks,
Simon

EDIT: The audio drifts backwards (It falls behind the video as if its too slow).

[guns1inger]

Is the audio out of sync on the original disc, or only on your HDD ?
How did you copy it to your HDD ?
Are you replacing the audio with a new audio track ?
If so, where did this new track come from ?

[Liquid Drum Theater]

Is the audio out of sync on the original disc, or only on your HDD ?
How did you copy it to your HDD ?
Are you replacing the audio with a new audio track ?
If so, where did this new track come from ?

The audio on the disc is in sync (it is just the original audio from the camera).
I copied it using 'DVDShrink' without any compression. I've always used it to copy dvds without problems for well over 2 years now.
Yes, I have a new audio track to replace the existing audio. I ripped the dvd to HDD and then demuxed it so I have the raw video mpg.

This new audio sounds to me like its cassette sourced (although I was told it was DAT). The hissing sounds analog to me.

Also, I've tried mixing the new audio over the old audio to see if they line up, and it also drifts backwards with that method too.

[guns1inger]

So the 'new' audio could have come from any source, and if it was originally on audio cassette, even if it has been transferred to DAT along the way, it could well have stretched with time.

Audacity also has a time stretch filter. The biggest problem with the audacity filter however is that it does not pitch shift at the same time. This means you have to adjust for the length first, and once happy that it syncs up, you have to pitch shift the audio so it doesn't sound like chipmunks/Darth Vader.

What is wrong with the original audio that it needs to be changed ?

[Liquid Drum Theater]

So the 'new' audio could have come from any source, and if it was originally on audio cassette, even if it has been transferred to DAT along the way, it could well have stretched with time.

Audacity also has a time stretch filter. The biggest problem with the audacity filter however is that it does not pitch shift at the same time. This means you have to adjust for the length first, and once happy that it syncs up, you have to pitch shift the audio so it doesn't sound like chipmunks/Darth Vader.

What is wrong with the original audio that it needs to be changed ?

Thanks, I have Audacity so Ill give it a try and report back.

The original audio is very bad (as with most cam audio when it comes to music). This new audio is a big upgrade and will make the enjoyment of the dvd much better for me.

[Liquid Drum Theater]

Hmmm....

I did a quick speed change by 1% (sped up) and it sounded ok, but theres still wierd noises going on. Any settings I need to tweak to make it best possible??


Thanks.

[guns1inger]

Can you post a couple of second of both before and after ?

[Liquid Drum Theater]

Can you post a couple of second of both before and after ?

Sure, give me 10-15 min.

[Liquid Drum Theater]

Hmmm, I think its my headphones.

I've just changed to another set and it sounds 'normal' now. Thats strange, might have been a dodgy connector or something. Can clicks/pops be from the dodgy wire or something?

BTW, I just tried to sync up the 1% sped up and now it gradually drifts forward. I'm guessing I need to lower the 1% until it fits.


Thanks. I'll report back with any probs/progress.

[Liquid Drum Theater]

Think I've done it.

Its taken me all day and the first track ended up being sped up by 0.7% and by the last track I had to speed it up by 1%.

Just got to author and hope it plays back ok.