Mp3's importing LONGER than source

[Dabbler]

I was asked to export a piece of music I had cut as an mp3. Composer writes me back to say the cue is drifting out of synch. Session is 48k/24B.

I did a bounce to disk, MP3 Interleaved 16B 48k, slowest encoding speed, 192kbit/s.

Reimported the new MP3 file into the same session and lined up against the original material the MP3 conversion is longer by 15 timecode frames on a 2min22sec clip. It's subtle but this music is cut very tight to picture and so you notice it after a bit. Not a pullup/down issue.

Seems to be happening on both Pro-Tools version 9(.0.6) and version 10.

I did a quick search on the DUC and couldn't find anything relevant so I'm posting.

Thoughts?

[Dabbler]

I was asked to export a piece of music I had cut as an mp3. Composer writes me back to say the cue is drifting out of synch. Session is 48k/24B.

I did a bounce to disk, MP3 Interleaved 16B 48k, slowest encoding speed, 192kbit/s.

Reimported the new MP3 file into the same session and lined up against the original material the MP3 conversion is longer by 15 timecode frames on a 2min22sec clip. Not a pullup/down issue.

Seems to be happening on both Pro-Tools version 9(.0.6) and version 10.

I did a quick search on the DUC and couldn't find anything relevant so I'm posting.

Thoughts?

[minister]

Don't use MP3's if you care about sync. When you convert to MP3, it alters the timing so it can do the masking and compression. Stick with PCM files.

[Kasper]

The obvious answer is .... don't send out MP3s as your final mix. I would not trust it.

Any reason why you have to send Mp3s?

Try opening the mp3 in QT player, export as a wav, import that into PT.
Any difference?

kasper

[Dabbler]

A) Obviously it's not a final mix, just a demo for the director and composer. (feature Post-Production)

B) I sent MP3 because they asked for MP3, I hate the format personally. Clearly I don't use it much otherwise I'd have run into this before.

C) I'll try the QT thing, but it still doesn't solve the problem of shipping MP3s in proper synch when requested.

Is this a known bug? Or is it just life with MP3 conversions?

[fretfingers]

This shouldn't be happening. 320k MP3 is broadcast standard here in Toronto for radio commercials. Though we're only talking 30 seconds. I'll try a longer export at work tomorrow and see if I get similar results.

[minister]

Originally Posted by Dabbler

A) Obviously it's not a final mix, just a demo for the director and composer. (feature Post-Production)

B) I sent MP3 because they asked for MP3, I hate the format personally. Clearly I don't use it much otherwise I'd have run into this before.

C) I'll try the QT thing, but it still doesn't solve the problem of shipping MP3s in proper synch when requested.

Is this a known bug? Or is it just life with MP3 conversions?

This is how MP3's are made.

Originally Posted by fretfingers

This shouldn't be happening. 320k MP3 is broadcast standard here in Toronto for radio commercials. Though we're only talking 30 seconds.

Why should it not be happening? 320kbps for a 30 second radio is fine, because it is radio, it will still be "about" 30 seconds, but not EXACTLY. Test it for yourself. Over a longer period of time, and when frame accuracy counts, all bets are off for retaining EXACT timing. IOW, it SHOULD be happening. MP3's are made by "windowing" audio into granules that are 384 samples long, if I recall. Timing precision with a window like that is impossible vis á vis PCM.

If you are curious to know more about MP3's:

http://wiki.hydrogenaudio.org/index.php?title=MP3

As I said, as Kasper said, do not send MP3's if you care about timing. Just tell the director that the timing issue is due to MP3 encoding and decoding and you need to send a wave or aiff. I mean, it's a 2 1/2 minute clip, the file can't be that big. Sendspace, FTP, Yousendit......

[Kasper]

Originally Posted by minister

I mean, it's a 2 1/2 minute clip, the file can't be that big.

↑↑↑ we have a winner ↑↑↑

[Rich Breen]

Originally Posted by Dabbler

A) Obviously it's not a final mix, just a demo for the director and composer. (feature Post-Production)

B) I sent MP3 because they asked for MP3, I hate the format personally. Clearly I don't use it much otherwise I'd have run into this before.

C) I'll try the QT thing, but it still doesn't solve the problem of shipping MP3s in proper synch when requested.

Is this a known bug? Or is it just life with MP3 conversions?

Well everyone's certainly jumping on the timing ambiguities inherent in mp3, and it's true that the start time is completely ambiguous (IME start can fall anywhere within a 30msec window), but once true start time is lined up, sync *should* hold. Seems to me, there's something else going on here. Did you line up the imported file's actual music start against the music start of the original file? Does it slowly drift out while listening against each other? I'd be inclined to look more carefully into this; mp3 and other perceptual coders should not change the actual length of material (and certainly not by 1/2 second on 2 1/2 minutes of material) - if they did, AC3 would never work as a sound-for-video encoder.

best,
rich

[lexaudio]

Originally Posted by Rich Breen

Well everyone's certainly jumping on the timing ambiguities inherent in mp3, and it's true that the start time is completely ambiguous (IME start can fall anywhere within a 30msec window), but once true start time is lined up, sync *should* hold. Seems to me, there's something else going on here. Did you line up the imported file's actual music start against the music start of the original file? Does it slowly drift out while listening against each other? I'd be inclined to look more carefully into this; mp3 and other perceptual coders should not change the actual length of material (and certainly not by 1/2 second on 2 1/2 minutes of material) - if they did, AC3 would never work as a sound-for-video encoder.

best,
rich

+1

Something weird here. I bet its a sample rate thing.

I've never had an mp3 drift unless that sample rate was wrong.

Sounds like the director is working at 44.1 and you are working at 48k.
I've run across that before, and the edit never knows how to check or change it.