Saturday 17 February 2007

The 24-25 issue in PAL film editing

There is this '24-25 thing' that crops up every now and then, in film post in PAL countries, especially India.

I was at the Institute last weekend. The Institute in the Indian film industry refers to the Film and Television Institute of India. Its at Pune and many of India's finest Film and TV personnel and artistes passed out of there. Along with my partner Anand Subaya we did a short workshop on editing.

One of the things that came up with the students one evening was this '24-25 thing'. The students there have some rather interesting issues, some I'd never thought of.

Anyway, this '24-25 thing' comes about because...
1. Film is shot at 24 fps.
2. Sound is recorded separately on Nagra/DAT/disk
3. Film is edited on a computer.
4. But it needs to be transferred to a tape to be transferred a computer.
5. Tape runs at 25 fps in PAL countries.

So...
6. Film is transferred to tape at 25 fps.
7. Because an accurate relationship has to be set up between film and tape.
8. So that in the end a cut list can be made to cut the negative accurately.
9. This transfer from film to tape is at 25 fps
10. That speeds up the film. (by 4%)
11. But editing systems need to run (play back) at 24 fps to stay in sync with sound.

So where's the problem?

As long as you're editing the film, and making cut lists, there is no problem. It works and we've been doing it for over a decade now.

But you need to pass on the edited picture as video on a tape for sound track laying and mixing. Since in Avid or FCP (computer-based) editing, you don't cut the negative till after the sound is done.

This transfer from computer to tape needs to be at the right frame rate so that after mixing and recording to optical neg, the optical neg matches the cut picture neg.

So that's the problem. Most people, especially editors from the video world simply don't handle this correctly.

And to make it worse, no system except telecines, Avid Film composers (and new Media Composers), and FCP systems - only these systems are capable of playing a 24 fps video source to a 25 fps video monitor or tape recorder (Beta/Digi/DV whatever) while keeping the duration intact.

Smoke, Quantel eQ, etc. nothing at all can do this.

What's the problem again?

1. Film is shot at 24 fps. So 10 sec of time is recorded to 240 frames on film.
2. Film is transferred to tape at 25 fps. So all 240 frames of film are transferred to 240 frames on tape. But since tape runs at 25 fps, these 240 frames run in 240/25 that is 9.6 sec.
3. Avid and FCP systems capture this at 25 fps. But they convert captured footage to the frame rate of 24 fps. So the original 240 frames are transferred to Avid/FCP.
4. And these (Avid/FCP) machines play them back 24 per sec. So 240 frames now play out in 10 sec.
5. That's how they sync with sound that was recorded.
5. But if after capture and editing, you play them back on an Avid or FCP and record that to tape, you'll get a 10 sec shot on tape which will be 250 frames.
6. So 10 sec on a timeline is 10 sec on tape. 10 sec on video tape is 250 frames.

So where do these extra 250-240=10 frames come from?

Avid and FCP systems repeat one frame every second so they add 1 frame each second. 10 secs = 10 frames. So 240 frames become 250 frames.

As an aside, the problem that students at FTII have is more complex.

They shoot at 24 fps. Process and print. Then cut these rush prints on a flatbed or Steenbeck. Steenbeck runs picture and sound (on mag film or sepmag) together at 24 fps.

But - and this is interesting - budding film-makers there shoot this edited rush film off the Steenbeck screen to a Handycam. They then capture these Handycam DV tapes to a sound software to do sound sweetening, over-dubbing, and other sound post.

And - this is serious stuff - their sound software outputs a wav or aiff file that doesn't sync with the visual. On the face of it, this seems more likely to be an issue of the Steenbeck not playing back at a tight 24 fps rather than a '24-25 thing'.

Anyway, this FTII 24-25 Steenbeck telecine issue is just an aside.

Even seasoned film-makers don't have much of a clue about this 24-25 thing. At a trial screening for a forthcoming feature someone noticed a small part of a scene being 'out-sync' At the interval, he promptly asked me if it was the '24-25 thing'

One thing to get very clearly. The '24-25 thing' invariably creates a mis-match between video and audio which is perfectly 1 frame per sec. So after 5 secs you're 5 frames out-sync. After 20 secs you're 20 frames out-sync. And if a 2000 feet reel is mis-matched because someone messed up the '24-25 thing' you'll be out-sync by a whopping 1 minute! Meaning at the end of the reel, picture will end a whole minute after sound or vice-versa.

So what's the best solution to this '24-25 thing'?

Things to remember

1. Understand how each stage of the film was handled.
2. Convert from 25 to 24 and vice versa if necessary (only possible in FCP)
3. Never capture an Avid or FCP dump back for editing.
4. Prefer handing out Quicktime movies rather than tape dumps for sound post.
5. Sound has no fps. It has a duration in seconds only. (Read this twice)
6. EDLs have no fps. They generate timelines of the fps of the source fps. (Read this thrice)

And if all this fails and you're completely stuck with a picture and sound that doesn't match, then I'm available for consulting. I've figured that so much time and energy has been spent on this '24-25 thing' that there's some commercial value in helping out. So write me and we can talk.

Else figure out who's gonna pay for the stock and studio time if you mess up.

I'm still available.

3 comments:

  1. I once had a student who recorded his film off a Steenbeck with a camcorder and did the sound editing on a computer without telling me. When we got to the mix all of his sound was out of sync. Steenbecks are not crystal controlled. They do not play back at a strict 24 fps. You can watch and edit sync sound on a Steenbeck because the mag sound and picture are locked to the same motor and play back at the same, slightly variable speed. But separate one element from the other and you WILL have sync problems.

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  2. Another wonderful post for digital film students.. Lot of food for thought. I am thinking of buying a Canon 7D for learning 24P work flow with FCP. Lot of producers want the the film out on a DV. It is obvious that shooting in 24 P would be fatal.

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  3. Hello sir,

    My name is prathyush,i am an Editing student of Satyajit Ray Film Institute. Recently for one of my project, shot in 35mm 4 perf ,i captured in Avid dv xpress pro 3.1 as film project (24p 35mm 4 perf Sound transferred at 100% film rate) and edited it.For the sound post production the sound recordist ask me to send the video reference at 25fps Quick time .MOV (DV PAL CODEC) in a hard disk and i did that.After the sound post production i got a mono track and i matched it with my BIP to BIP in Time line .It was in SYNC.And i checked the Pro tools project it was 25 fps.My doubt is whether in final married print it will be out of sync because the final film is projected @ 24fps .I am expecting an answer from you as soon as possible,i will be really thankful to you if u reply me.

    yours faithfully prathyush

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