Thursday 3 October 2013

24-25 fps in the digital age


There's still a fair bit of confusion in the minds of professionals who work in cinema about this '24-25thing'. By cinema, I  mean movies that get made for showing in theatres. And this confusion is deep amongst sound recordists and sound engineers. And to a smaller extent, with VFX and DI 'artists'.

In the film days, video from telecine (conversion of film to video) came out with a 25fps timecode. The editing system tracked 25fps timecode. But it played back film originated material at 24fps.
Sound recorded by sound recordists could happen at 24fps timecode or 25fps timecode. But recordists often recorded with 25fps timecode. 

Sound has no fps denoting speed. Sound is a continuous phenomena with no 'frames'. 'Frames' happens in video and film only. But some sound recordists still believe that there is such a thing as timecode rate and frame rate and they have to be different. Duh?

In digital filming - shooting with Red/Alexa/F65 etc - we shoot at 24fps. And we edit digital 'rushes' at 24fps. So there's no 25fps timecode or frame rate employed anywhere in the chain. And this significant fact has escaped many a sound recordist and in India they continue recording sound at 25fps even for digital 'films'. Even some Oscar, and multiple Filmfare or National award winning sound recordists continue to record sound at 25fps. Again, duh?

They claim that it works just fine even if the film is being shot on digital and the editing is being done at 24fps. Maybe it does, but its largely unnecessary. Maybe their ProTools sessions still track 25fps timecode that's why. But it's simply illogical to run the sound session at 25fps when the picture edit sessions are running at a frame rate of 24fps and a timecode of 24fps.

fps in the film days

I've written a fair bit in this on this before and you can check it out here
(http://neilsadwelkar.blogspot.in/2007/02/this-24-25-issue-in-pal-film-editing.html)

But very briefly, its like this... 
When we shot film - 35mm or 16mm film - the film had to be converted to video for editing on a Avid or FCP or whatever. This is called telecine. In India we always did telecine from film to video at 25fps. Meaning film ran at 25fps and tape ran at 25fps.

I just met a very senior sound recordist last week who told me film was telecine'd at 24fps. Plain wrong belief. We never did telecine at 24 fps for a film who's negative had to be cut later. Or even a film where DI had to be done. Never ever.

Never, ever, meaning not even one of the 160+ films I've supervised the DI for, (some I was part of the editing) had telecine at 24 fps. Not even one. And yes, one hundred and sixty plus is the number of films who's DI I have personally supervised.

Why?
Because tape runs at 25fps in India so the only way to have a one to one correspondence between film frames, and video frames was to run the film at 25fps in telecine. And you need this one to one correspondence because the telecine records a log file carrying key codes which is the only way to identify any given frame in film.

The editing system - Avid or FCP receives this log and tags the video with it. So that when you are finished editing, a 'cut list' can be put out and a human can read this to cut the negative. Yes, before DI happened to humanity, negative used to be cut. This was till about 2006.

One small other fact. If the telecine runs film which had been shot at 24fps at 25fps instead of 24fps, does that not speed up the film? How does sound sync in that case? It sync because, yes, the film at 25fps is 4% faster, but when the editing system receives this 25fps from tape, it automatically slows it down by exactly 4% to make it run at 24fps and hence sync with sound. 

Avid does it automatically during capture in a film project (not otherwise). While FCP does it after capture. Both editing systems are capable of playing back a 24fps picture with 25fps timecode. Yes, that's exactly how it works. 

And yes, there are still some sound recordists and engineers who have, apparently not grasped this fact. And some of these have won Oscars and some have won the national Film award for their work. So they are very accomplished and produce some amazing work in sound. But this small technical fact still escapes them.

Why am I harping on the award thing? Because every time I alert a young sound recordist about this, he or she points me to that senior award winning guy or gal who still 'does it' at 25fps. 

I believe that this 24fps picture with 24fps timecode, married to sound with 25fps timecode is the single most common reason for 'loose sync' in digital film sound work that many complain about. If you don't believe me, try for once recording sound with 24fps timecode, do your ProTools post on a 24fps session and then check how tight the sync remains when editing in Avid or FCP.

Or listen to that 'award-winning recordist' who doesn't have his fundas straight.

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