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Sound Devices 744T timecode questions
Hi all,
I have a question about the relationship between the frame rate and the sample rate in the 744T (and in general, actually).
As far as I understand, if you record audio at straight 48K, and have 24fps timecode, you have audio that will sync perfectly with 24fps video or film. One second of time code will be real time second. On the other hand, the manual says if you are going to be synching with 23.98 or 29.97 video and require audio pulldown, what you can do is select 48048kF, which will be audio that is recorded at 48048Hz but stamped at 48kHz, and this tricks an editing system that doesn’t recognize a 48048 sample rate into playing the sound back with pulldown already “built in.”
My question is, what happens if you record audio at 48K, but choose the 23.98 or 29.97 frame rates? Do you also get a pulldown? Because you can choose a 23.98 frame rate to sync to HD video cameras. Can you choose this being at a sample rate of 48K? If so, isn’t that a contradiction? If 48K is really 48K per second, then doesn’t it necessarily match up with 30 or 24fps video only? Because with a frame rate of 23.98 a second of video timecode is .1% slower than a realtime second. Does the Sound Devices recorder add a pulldown to your audio if you choose a fractional frame rate?
In general, does anyone know what different combinations of frame rate and sample rate are possible and what each combo does exactly?
Thanks!!
-Nayeli
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8 Replies
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Hello Nayeli and welcome to the Cow Audio Forum.
Every time someone explains it to me, it sticks for about 23 minutes and then slides right off. If someone doesn’t pop in here with the answer, I suggest you try a usenet newsgroup called rec.arts.movies.production.sound. Here’s an easy way to get there.
https://groups.google.com/group/rec.arts.movies.production.sound/topics
Regards,
Ty Ford
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Hi Nayeli-
According to my 702T manual, 48.048kF will only work with 30ND. It’s used when film is being shot at 24fps and transferred to NTSC.
23.976 (or 23.98) is the real frame rate that an HD camera is recording when it says 24fps. As I understand it, if you are working with an HD camera shooting at 24fps, you want to be recording at 23.98…the REAL frame rate of the camera. 24fps is ONLY used when you are shooting film and finishing on film (for projection). In other words, not for HD transfer for television.
Of course, you should ALWAYS check with the post-house or at least the producer as to what you should set your frame rate and frequency to. NEVER second guess them after-the-fact, because you don’t really know how they plan on using your recordings. Plus, if they tell you wrong, it’s not your fault.
Hopefully some of this helps :o)
Dave J
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“23.976 (or 23.98) is the real frame rate that an HD camera is recording when it says 24fps. As I understand it, if you are working with an HD camera shooting at 24fps, you want to be recording at 23.98…the REAL frame rate of the camera. 24fps is ONLY used when you are shooting film and finishing on film (for projection). In other words, not for HD transfer for television.”
Thanks for responding!
I have actually worked on film productions where we do a reverse TK to 23.98 instead of 24 because the post house prefers it that way. In those cases they recorded sound at 24fps/48kHz and relied on Final Cut Pro to do an automatic pulldown so that the audio would match the video running at 23.98. One time it for some reason was only working for second unit sound so we had to do a sample rate conversion in ProTools to the recorded audio.
In any case, my real question I think is, if you record with a 23.976 frame rate, but your sample rate is set to 48kHz, do you get audio that matches video running at 23.98? And if so, how is it different or the same as recording audio with 24fps frame rate but 48048Hz sample rate? It seems to me that in order to have audio that matches video at 23.98 you necessarily have to alter the sample rate, since by definition 48kHz has a reference to real time: 48000 samples per second. Does this make sense?
-Nayeli
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“I suggest you try a usenet newsgroup called rec.arts.movies.production.sound. ”
Thanks Ty, I’ll check that out!
-Nayeli
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“In any case, my real question I think is, if you record with a 23.976 frame rate, but your sample rate is set to 48kHz, do you get audio that matches video running at 23.98?”
Yes, as far as I know. Also, for those who are wondering, 23.98 is 23.976 rounded up.
As to recording 24fps @ 48.048, I think I understand what you’re asking. Hopefully someone else will post here and enlighten us :o)
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Bottom Line. Communicate with the Post Department and make sure they know what you are sending them. The optimum if you are using actual film @24 fps would be 48.048 (the pull-down brings it to straight 48K so no conversion is necessary)
If you are going to HD Video then Record at 48K, no conversion needed for post there either (the pulldown is already included). The problem is that 48.048K has not been around very long and both Avid and Final Cut Pro developed a work-arounds that were used before the sample rate existed. (basically playing back 48K at 47.52, if you play back 48.048 at 47.52 you end up with close to 2% pull-down instead of 1%)
This is confusing and frustrating for all parties, so I go back to “Communicate with the post department, if they have their game on they will ask for 48.048 because it makes the entire post process work at straight 48K no conversions. If they work in an older work-flow environment they will want 48K and they will deal with it.OK, Brain tired,
Best
DrewChaos is the beginning of everything.
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What you have to remember with audio is that there is no timecode within the audio file. The only reference to timecode, in an audio file, is the starting point. There is metadata in this file that indicates what the starting timecode point is, and this is represented in samples from midnight depending on the sample rate. An example would be at 48Khz the starting timecode is 00:01:28:00 – this would be representing in the .wav metadata as 4224000
For frame rates 24, 25 and 30 that same sample number of 4224000 will always represent 00:1:28:00 because as you mentioned ” One second of time code will be real time second.”
For the framerates 23.976 however 422400 represents 00:01:27:21. Now stay with me this is the tricky part.
If you recorded on set with 24fps pictures and are now editing at 23.976 this represents a 0.1% slow down…. so you will have to slow down your audio too. This is done by a sample rate convert from 48Khz to 48048 …. BUT you stamp it at 48Khz to retain that timecode point of 00:1:28:00 or 422400 and the sound will be 0.1% slower.I hope this makes sense, I can be very confusiong at times, it has taken me a while to get my head around it all.
Hope its helps
Brendan Croxon
brendan@musicandeffects.com -
Hello Brendan and welcome to the Cow Audio Forum.
Thanks for shining a light on this tricky topic. Please come back again.
Regards,
Ty Ford
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