Capturing and correcting PAL-speed audio of 24fps film without artefacts

[chen lung]

I have a PAL VHS of a 24fps film, so there's speed-up.

Is there such a thing as digitising/capturing such audio at a rate slightly higher than 48/96 kHz (e.g. 100 kHz) and simply reducing the numbers to naturally achieve the correct speed? The appeal was being able to avoid artefacts (as a result of re-mapping/sampling), though I don't know for sure (incase there's other factors I'm unaware) - I'm not an expert.

I don't know what the exact rate would be, but I'd likely do it all in Audacity, for example. The likely purpose would be syncing it to the 24fps (or 23.976) video of a Blu-ray.

[chen lung]

Just to say: I had a quick look beforehand, but couldn't seem to find topics on this.

Thanks!

[LMotlow]

VHS PAL is standard definition and is usually captured at 720x576 @25 fps. Standard definition BluRay is 720x576 @25fps, and is encoded as interlaced. It uses frame duping, pulldown, or speed-up to achieve 25fps. 24fps is not valid for standard definition BluRay.
https://www.videohelp.com/hd#tech

[chen lung]

VHS PAL is standard definition 720x576 @25 fps. Standard definition BluRay is 720x576 @25fps, and is encoded as interlaced. It uses frame duping, pulldown, or speed-up to achieve 25fps. 24fps is not valid for standard definition BluRay.
https://www.videohelp.com/hd#tech

The Blu-ray is 1080p/24 - I want to marry it with a soundtrack that's not officially available on digital home video.

[jagabo]

It's trivial. Take you PAL audio and slow it down. Some audio editors have PAL/NTSC filters just for that. But in Audacity you can just use Effect -> Change Speed. Reduce it by the amount necessary. For example, 23.976 / 25.0 = 0.95904. Or more with greater precision 24000 / 1001 / 25 = 0.959040959. This will increase the running time and restore the pitch.

But the difference is speed is rarely the only difference between two releases of a movie.

[chen lung]

It's trivial. Take you PAL audio and slow it down. Some audio editors have PAL/NTSC filters just for that. But in Audacity you can just use Effect -> Change Speed. Reduce it by the amount necessary. For example, 23.976 / 25.0 = 0.95904. Or more with greater precision 24000 / 1001 / 25 = 0.959040959. This will increase the running time and restore the pitch.

But the difference is speed is rarely the only difference between two releases of a movie.

I'm aware of that method, but it's not what I want, as I believe it will result in slight quality loss.

[LMotlow]

VHS PAL is standard definition 720x576 @25 fps. Standard definition BluRay is 720x576 @25fps, and is encoded as interlaced. It uses frame duping, pulldown, or speed-up to achieve 25fps. 24fps is not valid for standard definition BluRay.
https://www.videohelp.com/hd#tech

The Blu-ray is 1080p/24 - I want to marry it with a soundtrack that's not officially available on digital home video.

Good luck.

[manono]

I'm aware of that method, but it's not what I want, as I believe it will result in slight quality loss.

You believe? Have you tried? You cap at 720x576 25fps, extract the audio and slow it down. I use BeSweet but you can also use Audacity, eac3to or any number of audio programs. If I've done it once I've done it a thousand times.

And as jagabo mentioned, there's a very good chance the slowed and lengthened audio won't fit the video in your Blu-Ray. Never know till you try.

[JVRaines]

Is there such a thing as digitising/capturing such audio at a rate slightly higher than 48/96 kHz (e.g. 100 kHz) and simply reducing the numbers to naturally achieve the correct speed?

Yes. You can change the indicated sample rate of the file without changing the sample data. This will alter the duration and pitch of the audio stream. I don't know if Audacity will do this without resampling, but here's a way with FFmpeg:

ffmpeg -i myinput.wav -af asetrate=44100 myoutput.wav

[chen lung]

And as jagabo mentioned, there's a very good chance the slowed and lengthened audio won't fit the video in your Blu-Ray. Never know till you try.

I've not performed such tasks so I can't comment, though I'd be surprised if it didn't sync.

Is there such a thing as digitising/capturing such audio at a rate slightly higher than 48/96 kHz (e.g. 100 kHz) and simply reducing the numbers to naturally achieve the correct speed?

Yes. You can change the indicated sample rate of the file without changing the sample data. This will alter the duration and pitch of the audio stream. I don't know if Audacity will do this without resampling, but here's a way with FFmpeg:

ffmpeg -i myinput.wav -af asetrate=44100 myoutput.wav

I'm useless at command line, but yes, I believe Audacity does:

1. Set project rate to 100kHz (though I'd like to know the precise number).
2. Capture the audio.
3. Once captured, click the track name>Rate>change to 96kHz.

You can adjust the speed (in the Effects menu), but I believe this will, as you mention, resample.

[jagabo]

1. Set project rate to 100kHz (though I'd like to know the precise number).

It's trival algebra:

for 25 fps to 24 fps the rate is:

96000 * 25 / 24 = 10000

for 25 fps to 24000/1001 fps it's:

96000 * 25 / (24000/1001) = 100100