Re-Timing AC3 audio

[Zenobia]

Hi all,

My first post.

I have some .ac3 and .dts encoded files which were supplied to me at 23.98 fps - which I need to re-time to PAL 25 fps. Is it possible to do this with Audacity?

[kozikowski]

Almost certainly not in Audacity 1.2.

Audacity 1.3, I believe will directly open AC3 files .. but... In the same sense that an Audacity Project isn't sound, AC3 is a sound manager. If you decode it, you may (almost certainly) find yourself staring at six sound channels plus the loudness contour profiles. "Make the gunshots here twice as loud as everything else."

If you manipulated the streams -- even if you got all of them in, I would expect a scrambled show -- or one that didn't sound like the original.

Just for continuity, it's not 23.98. That number is rounded based on the difference between The Real World and US NTSC. It's a fudge number. The real one is...

23.976023976

And that's the kind of accuracy you need to generate if you expect a half-hour show to track over 30 minutes.

Audacity won't do that, plus, speed changes never sound very good in Audacity anyway.

Most of the video editors I'm familiar with handle conversions like this either internally or by adding "PAL" or "NTSC" packages (depending on where you start from.) The older Final Cuts have Nattress with which I've done some terrific speed conversions. They're not absolutely perfect, but most people just can't tell. Highly recommended.

http://www.nattress.com/

Koz

[Zenobia]

Thanks - I contacted Nattress but he says his filters won't allow the re-timing of 6 channel audio, so I'm back to Audacity which I'm told by others can re-time the audio, but I'm unsure as to how exactly.

[kozikowski]

Since we last "spoke," one other other elves posted that this may not be such an awful idea after all -- at least in the much more modern Audacity versions...


stevethefiddle wrote:

Just a point about changing speed in Audacity. While the "Change Tempo" and "Change Pitch" effects produce increasing amounts of distortion as the amount of stretch is increased, the "Change Speed" effect in Audacity 1.3 is very good for small speed changes (not so in Audacity 1.2.x).

I can't remember how Audacity 1.2 did speed changes but I remember it wasn't very good, but this seems to have been greatly improved in Audacity 1.3. This may be platform/build dependent as there are 2 alternative libraries that may be used for resampling (libresample and libsamplerate).

A simple test for a 2% shift in tempo:
Select a track and apply Change Speed = -2.0 (minus 2.0)
Change it back with Change Speed = 2.04075 (value must be typed)
On my test set-up (using libsamplerate, High quality conversion set to "Best Sinc Interpolator") is good within 0.0004% on length and no noticeable difference in spectrum. Audacity 1.2.6 on Windows shows a marked change in the spectrum which is particularly noticeable when testing with a sine wave.
=======================================================

So now I'm sitting in in the corner with a cup of tea calculating the speed change you would need to do the conversion....

Koz

[kozikowski]

Whilst I juggle infinite decimal places, how long is the show?

Koz

[Zenobia]

Thanks for your help!

PAL (25 fps) = 102m 53s (the NTSC needs to match this)

NTSC (23.976 fps) = 107m 25s

I don't know if this is of any use?

link: PAL-NTSC-FILM-Converter

Suggestion for Audacity coders - next version have pre-sets for doing PAL / NTSC retiming.

[kozikowski]

I think with you on the forum, the people interested in cross-standard video sync converters come to....... two.

Without scaring you, there are quite a number of very serious Audacity problems pending and the Audacity Development Team just isn't that big. If all of them went down to the pub for a pint at the same time, they'd have to call in reinforcements to make the minimum for a good table.

That said, the developers were not unmindful of PAL or NTSC. There are SMPTE Time Code conversions available and they even pay attention to Drop Frame and Non-Drop Time Code [shudder.]

Even if I do find all the right numbers (video rate is different from number of frames), it may just flat not work. I pulled up a 60 minute stereo show from the archives and told Audacity to make a show 95% of the length. It gave me a 30 minute show. I'm goin' with 'no' on that one.

It's not that hard. If you're on Audacity 1.3.11 (NOT 1.3.10), you select all the tracks you wish to affect with the effect. Effect > Change Speed. That panel will give you a percent change value and fair warning, it will round off to the nearest five numbers. 95.904095903 magically becomes 95.904.

While I'm struggling with this, you need to make sure you can open your show into all the tracks reliably. Do you know that you can make it back into AC3? Ever try?

Oh, and since I know you're doing this whole thing on your laptop at the airport, you'll need, conservatively, about 6.2GB of hard drive space. AC3 is highly compressed using some very, very nice tools, but still. Once you remove the compression, the Real Show arrives.

Koz

[steve]

PAL (25 fps) = 102m 53s (the NTSC needs to match this)

NTSC (23.976 fps) = 107m 25s

To stretch 107:25 to 102:53, change the speed by 4.406

[kozikowski]

However, I am pushed to ask (now that I think about it without the holiday pressure), how did you get the video? The object of successful television standards conversion is to start and end with a, say, 60 minute show. 60.000 minutes, both sides. The framerate is irrelevant. The only time you get duration conversions is when you play film "at the wrong speed" in order to jump between countries -- or as it is pointed out, play a 24.000 fps movie film into a Movie DVD production which runs at 23.976.

There is another conversion process in effect here, as well. "24 Frame" video cameras do not deliver the show at 23.976. They deliver it at 29.97 with 3:2 pulldown added to make up the difference. That's so they can be viewed on an NTSC monitor during production, but the first thing we do is convert back to 23.976 for editing because trying to edit a speed conversion can create all sorts of framing problems in the finished show -- particularly if you try to Brute Force convert to PAL.

Is that what happened? Somebody tried to "drag their finger" on the video (to create a metaphor) to slow it down or push on the accelerator to speed it up? Proper standards conversion is hard and yes, that does give you a different show duration. PAL has really sharp pictures and terrible motion. NTSC has really good football motion, but fuzzy pictures. If you don't do the conversion right, you get furry pictures with ratty motion. They add up the wrong way.

We had an ADAC standards converter and an Oki-Data converter at work. It took three people to move either one of them.

Koz

[Zenobia]

I have been supplied with audio files (ac3 and dts) that were encoded in the US and subsequently used on a US 23.98fps (progressive) DVD.

I am working on a UK PAL 25 fps release, which is derived from a Digital Beta master tape (but sadly the master tape does not include the newly created 5.1 Dolby Digital EX and 6.1 DTS mixes.

ps: I'm working on a Mac Pro with plenty of disk space.