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Audio production chain for 120fps HFR
Old 19th October 2013
Gear Head

Audio production chain for 120fps HFR

I'm involved in a test run for a later HFR production. They want to shoot at 120fps and then stress the usual production workflow to see where the pitfalls are.

The destinations are:
- DCP 2K/120fps
- DCP 2K/24fps
- Blu-ray 24p
- DVD NTSC (30fps)
- DVD PAL (25fps)

For the production sound I thought of setting my SoundDevices788 to 30fps, as this should work at the same speed than 120fps. But what about spotting with time code or reconforming?

I don't know yet if they edit at 120fps. But I'm sure the video for sound dept. will have some lower frame rate. My guess is that it will be at 30fps and so the session file will probably run at 30fps and have to be conformed to 24 and 25 frame rates.
The supervising editor already told me that they don't want to use optical conversation, which would be much easier.
Since I never worked with 30fps progressive video sourced from anything other than 24fps, what is the pull factor in percent between 30 to 24 and 30 to 25?

Has anybody already done audio post for HFR production and has any helpful suggestions and hints? Would be much appreciated.
Old 20th October 2013
Hi Josh, I know some of what you about to get into.

Likely the workflow will follow the 24fps speed. The higher frame rate of 120fps is just a factor of 5:1. So a second will likely remain a second over 120 frames and also 24 frames. The usual pull and speed changes will likely be required for your 25 / 29.97 versions and be derived from the 24fps version.

Sounds like a cool project PM me if you'd like more gossip off line.

All the best

Old 20th October 2013
Lives for gear
iluvcapra's Avatar
Originally Posted by Josh L. View Post
But what about spotting with time code or reconforming?
I've never done a project like this, but the available WAV file metadata represents this data in a fps-independent way
  • BEXT/Broadcast WAV metadata will encode timecode in a framerate-independent way; a BEXT metadata chunk that starts at noon in 24fps will be identical to one that starts at noon at 120fps.
  • iXML metadata advertises a recorded frame rate as a ratio of integers, but the spotting time of the file is represented in the same way as in BEXT, with a sample count since midnight.

The short story is, when you tell a Sound Devices to record at "24fps" or "30fps", it has no effect on the BEXT metadata, and only changes a couple bytes of the iXML, and neither Titan nor Pro Tools seem to read those for the purpose of syncing. The fps setting on the recorder changes the SMPTE signal that comes and goes on the TC Lemo, the TC value that the LCD displays, and that's about it.

Don't do this, but in theory, if you jammed your slate at 30fps, then switched the 788 to 24fps, recorded some takes, switched it to 25fps, recorded some, and then switched it to 30fps to re-jam the slate, nobody in Pro Tools land would ever know the difference.
Old 22nd October 2013
Pulldown factors are:
29.97 -> 24 is 0.1%
29.97 -> 25 is 4.1%
29.97 -> 23.98 is 0%

4.1% is about 72 cents in pitch.
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