25p to 24p Blu Ray - Audio issues?

[neilwilkes]

I need to know if there is a way of taking an audio stream mixed at 48kHz to sync up to a film that has to be converted from 25p to 24p - with no pitch shifting involved whatsoever.
How easy is this to do? I feel certain it must be possible, assuming a hardware conversion of the original footage. After all, a second of running time is a second of running time, right? So it must all be down to the way the footage is converted.....
Reason I am asking is because the artists manager has been informed by "experts" that footage shot at 25p cannot be taken to 24p (resolution is 1920 x 1080/25p) as it presents "insurmountable problems"

Pitch shifting is not an option

[eoats]

Lots to sort out.

24P is maybe really 23.97fps?

No, a second is not always a second if it's made of 24 pieces that are each of them 1/23.976 long.

[kk@jamsync.com]

Originally Posted by neilwilkes

I need to know if there is a way of taking an audio stream mixed at 48kHz to sync up to a film that has to be converted from 25p to 24p - with no pitch shifting involved whatsoever.
How easy is this to do? I feel certain it must be possible, assuming a hardware conversion of the original footage. After all, a second of running time is a second of running time, right? So it must all be down to the way the footage is converted.....
Reason I am asking is because the artists manager has been informed by "experts" that footage shot at 25p cannot be taken to 24p (resolution is 1920 x 1080/25p) as it presents "insurmountable problems"

Pitch shifting is not an option

I don't think you're going to be happy. The traditional way is to use something like Cinema Tools and change the header to 23.98 (or 23.976 if your app has that selection).
Then you pitch shift. Sorry. Otherwise you'd be dropping frames and the video would look choppy. There may be hardware video converters out there, but I'd be leery of how they'd look.

Check out this tutorial: Frame Rates: Conforming from 25P to 24P on Vimeo.

Here's another way using Compressor that retains the exact length (it's at the bottom of the page)...but I'm betting it ain't pretty.

ProVideo Coalition.com: Lights Camera Action by PVC Contributors

[kk@jamsync.com]

Originally Posted by eoats

24P is maybe really 23.97fps?

Round up....23.98 fps.

[ErikG]

HI Neil
I do not know enough about Blu Ray in particular, but as far as I know there's still no good method today to create a good looking 24-25 or 25-24 picture conversion apart from speeding the footage up/down. Of course you could do what we did in the old days of video to the picture on videotape, just add one extra stop frame a second, but that comes with its own perils with less than perfect sync (+- almost a frame) and ugly camera pans.

But it does sound weird that the most modern home videoformat today shouldnt be able to handle the most common video speeds as well as film speeds.

[Steven1145]

Hi Neil,
here in France, just as you have in England, all the PAL stuff (25fps) is slowed down for film (24fps), just as the reverse is true.
There are no algos to convert the picture directly while keeping the nominal film or video speed. AVID gets round this by adding two "ghost" fields or one "ghost" frame per second outputting 24fps footage to 25fps media (be it tape or files).
Our Vcube does the same. In each case you can clearly see the ghost frame on pans or camera movements, and it never looks good.

We're quite used to varispeeding and pitching (or not) the audio that goes with the picture.

In your case the picture is goind from 25 to 24. The only option to keep the running time the same (staying at nominal speed) would be to remove a frame per second. It's probably even worse than adding a ghost frame. The "experts" are right in that no hardware exists to do this, a part from slowing stuff down.
And yes, BluRay is usually at 24fps, as it allows for quite easy playback on 60Hz compatible displays (whereas 25fps does not), which - if I understood correctly - matches the same 3-2-3 conversion that you get in telecine (or is that 2-3-2...). If someone wants to chime in to correct me...

[neilwilkes]

Thanks Steven
So - even if everything is converted as well as possible, and audio is converted without adjusting pitch, the actual speed of the final audio will be different to the original?
Sorry to be so picky here, but I need to be absolutely clear in my mind before I have to go back to the management & tell them that this just is not going to fly.

Oh, how I wish someone had mentioned this before filming. I have always known 25p cannot be used in BD discs, but was unsure that speed issues would still remain.
Not good news...........

[minister]

I don't know all the ins and out of the latest converting tools for picture. I do know that there are plug-ins for AVID and FCP, and also After Effects does a good job of converting footage without a speed change. But on the projects where they wanted to not risk any motion problems, the 24-->25 change was done as frame=a frame. So I had to adjust speeds.

When knowing this, my plan is adjust stems and remix. I realize you are saying that a pitch change is not an option, but:

The best, most accurate speed change I know is the SRC trick.

For changing pitch:

Dialogue : Digidesign's X-Form (Izotope) or Waves Soundshifter.

Music: Pitch N' Time Pro.

FX and BG's? Who cares. I leave as is.

Those are my thoughts if you get stuck and have to change speeds.

[Geert van den Berg]

We have done this a couple of times, yes you can hear it and it depends on the type of material if the results can be deemed exceptable. We once did a movie which already had a very slow pace and that became even worse but the pitchshift worked okay. (we use Waves Soundshifter for timecorrection/pitchshifting)

For video the results will look fine, if you make sure the playback speed slows down, in other words 1 frame stays 1 frame.

(As a sidenote I recently converted some 24p material to 25p for dubbing, keeping the filmspeed the same, Apple Compressor did a great job with the frame blending, only noticable on very fast movements and even then that was acceptable.)

[Steven1145]

Concerning pitch shifting for 25 to 24 conversion: as I said we do this often. The usual modus operandi is to first varispeed in Nuendo (length of clip changes, but so does pitch) and then pitch back to the original pitch with Waves Soundshifter. Since V6 it is actually quite good (before that it was crap, and sync went immediately down the drain).

The other option is to use SoundShifter in ProTools, where you can directly do a time compression/expansion of the clip. Quality remains good.

You take less chances in damaging the audio if you process stems separately, and then recombine them.
Golden ears will undoubtedly hear the processing at times, but I find that it is quite acceptable, the more so since there aren't many other solutions... You could do just a plain vanilla varispeed to match the pic, you wont have any pitch processing but the pitch will be altered by 1/4 tone.