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DAT/Film Synch Sound

Published : 21st October 2004

Basic question. For synch sound, I've been recording sound on DAT and shooting film at 30 fps, since my understanding was that DAT only does 30 fps. Is there a way to record DAT at 24 fps?

If film is shot at 24 fps and sound is recorded on DAT, can synch sound only be achieved by either speeding up the film or slowing down the sound in post?

Charlie Clemmons
Wiltonwood Productions
Wilton, CT


Film is shot at an 'actual' frame rate of 24 or whatever...But, you're applying 'actual' frame rates to an audio recording's time code which is not the same thing. The time code you record with a (i.e. time code) DAT could be anything without changing the 'speed' of audio.

It's just a numbering system. TC is used for syncing audio to picture and essentially it is only important for them to keep a steady relationship to each other. That's why TC 30fps is chosen for film projects running 24 frames...and 29.97 when running HD/23.976.

One second of audio is still a second of audio whatever time code is attached to it. Speed of (digitally recorded) audio is changed by changing it's sample rate.

Hope it makes some sense - it's a bit early in the morning and we wrapped late....

Karl Lohninger
Sound mixer etc.
Los Angeles


Charles M. Clemmons wrote :

>Basic question. For synch sound, I've been recording sound on DAT >and shooting film at 30 fps, since my understanding was that DAT only >does 30 fps. Is there a way to record DAT at 24 fps?

Sound does not run at a framerate like the film does. It kinda just runs, however the timecode often associated with the sound does have a framerate. You should set this for the same as the camera framerate, which should be up to you (and the decided final media). Your DAT recorders TC-part or external TC-generator should have a framerate setting.

Best Regards

Anders Thorsby
Clapper/loader & video assistant
Copenhagen, Denmark


DAT recorders do not record at "30 fps" or any other film frame rate, they record at their sampling rate of 44.1 or 48 KHz. (If you shot film at a crystal locked speed of 17 frames per second and projected film at that speed, you could in theory maintain sync).

They'll normally play back at the sampling rate they recorded at. The issue with using DAT in film sync arises in NTSC video transfers of film: film shot at 24 is "pulled down" to an effective 23.976 on transfer.

So in order to maintain sync *in a video transfer situation* you must "pull down" the playback speed of the DAT recording by an equivalent amount i.e. by a factor of .001 and, if you are going back to the integer frame rate of projected film at 24 fps you will need to pull the speed of your audio back up at some point. If you are going to stay on NTSC frame rate video, your work is done.

One trick to effect the pull down is to use a timecode DAT recorder and record TC at an even 30. Then, in playback you lock a DAT machine that can accept an external clock source to 29.97 NDF - this can "pull down" its playback speed by
.001.

Actually with a Sony 5030 (right # ?) you don't need an external clock.

It's been a while since I've dealt with this stuff, maybe Karl will
knock back an espresso or two and help out !

You can also pull down in appropriate versions of Pro Tools, etc

Sam Wells


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