What about izotope RX?

Why not use eac3to instead?
It has an option to pitch-correct audio from 25fps to 23.976, and it will also do a 2nd pass to fix any clipping it may find.
I slowed down many PAL audio tracks using eac3to, and I have NEVER encountered any issue.
Give it a try.

Does it correct from 25fps to 24fps?

(2026-01-07, 10:45 AM)Stamper Wrote: Does it correct from 25fps to 24fps?

Yes, and viceversa (from 24fps to 25fps).

Here's how to use it:

https://en.wikibooks.org/wiki/Eac3to/How_to_Use

(2026-01-07, 08:47 AM)emy54 Wrote: Why not use eac3to instead?
It has an option to pitch-correct audio from 25fps to 23.976, and it will also do a 2nd pass to fix any clipping it may find.
I slowed down many PAL audio tracks using eac3to, and I have NEVER encountered any issue.
Give it a try.

This is the method I use.
eac3to is a great and powerful piece of software.

(2026-01-07, 04:50 PM)CSchmidlapp Wrote:

(2026-01-07, 08:47 AM)emy54 Wrote: Why not use eac3to instead?
It has an option to pitch-correct audio from 25fps to 23.976, and it will also do a 2nd pass to fix any clipping it may find.
I slowed down many PAL audio tracks using eac3to, and I have NEVER encountered any issue.
Give it a try.

This is the method I use.
eac3to is a great and powerful piece of software.

I have used it for YEARS, to do this.
You definitely save A LOT of time, than by doing it manually.
And it also comes with a great sampling rate tool as well.
For any kind of other audio editing, of course, I use Nuendo.

(2026-01-05, 10:59 AM)Stamper Wrote: What about izotope RX?

That would be my recommendation, if you only want to change pitch, or only want to change speed. Its Radius algorithm is quite good.

It doesn't seem to offer an option to change both speed+pitch together, and it's a bit cumbersome to have to think about semitones, so I wouldn't recommend it if you have a track with both speed and pitch incorrect.

As for why, here's the details:

Changing pitch and speed is normally a single, coupled operation. To achieve a change of only one but not another, a pitch correction algorithm has to be used, which introduces significant quality variables.

If you have 25 FPS with wrong pitch, that's easy to fix, you can use any software, I prefer sox, lets me use an exact factor:
where RATE is TARGET_FRAME_RATE divided by SOURCE_FRAME_RATE, e.g. (24000/1001) / 25 = 0.95904095904096 for 25->23.976.

If you have 25 FPS with the right pitch, that's significantly harder, and results with heavily depend on the tool. Izotope RX's Radius algorithm performs very well, and is my go to option. Though, in those cases, I'd rather keep the wrong speed to avoid potential artifacts, from a double pitch correction (first time from original speed to 25, second time from 25 back to original).

eac3to can only perform a speed change, like the first sox command listed. Its syntax is convenient, and the operation is simple, but its output is non-deterministic (probably some dithering quietly going on), and in general it's a rather outdated tool, even with DG's support.

(2026-01-10, 08:25 AM)velocity Wrote: If you have 25 FPS with the right pitch, that's significantly harder, and results with heavily depend on the tool. Izotope RX's Radius algorithm performs very well, and is my go to option. Though, in those cases, I'd rather keep the wrong speed to avoid potential artifacts, from a double pitch correction (first time from original speed to 25, second time from 25 back to original).

Interesting.

A lot of the projects I'm working on at the moment are 25fps to 23.976fps, which includes the audio.

I'm currently using two methods
I normally just use Useacto...
, thus no pitch correction.

But I've been experimenting with AVIsynth...
Which I believe does pitch correction. (I change 'rate' with 'tempo' for none)

My understanding with PAL DVD's is they were just sped up with no pitch correction, so slowing them back down requires no pitch alterations?

Any help and advice on the best way I can do this would be great.
The timing of your posts in this thread is uncanny

(2026-01-10, 10:03 AM)CSchmidlapp Wrote: Any help and advice on the best way I can do this would be great.

If you can get your hand on Izotope RX Pro, that's I think your best bet. Tool is "Time & Pitch", set stretch ratio to 104.27083 and algorithm to "Radius (Best Quality)".

It's also possible to use sox for this, might be worth comparing results to RX:


No clue what algorithm that AviSynth function would be using, I try my best to avoid that software 

(2026-01-10, 10:03 AM)CSchmidlapp Wrote: My understanding with PAL DVD's is they were just sped up with no pitch correction, so slowing them back down requires no pitch alterations?

Most of them, yeah. Some newer ones do have pitch correction. 25 FPS Blu-rays and streaming are more commonly pitch corrected.

(2026-01-10, 05:03 PM)velocity Wrote: Most of them, yeah. Some newer ones do have pitch correction. 25 FPS Blu-rays and streaming are more commonly pitch corrected.

For TV shows/broadcasting content, even if they are in 25fps, they already have the pitch corrected.
ONLY movies are sped-up, and therefore have the pitch altered.