Audio and frame rates?

[Joe Riggs]

I'm trying to get a better understanding of how audio and frames rates work together, take an H4N for example, I could record video at 23.976 or 29.9, and the audio file from the H4N would match to either one of those. I don't have to set a frame rate for that (I'm not concerned with jam syncing time code or anything).


However, what happens if your film is shot and edited at 23.976 but your sound designer does all his work in 29.97? Will there be a four frame sync difference when you get the audio file back? I'm not familiar with most DAWs, do you need to set a frame rate there before you begin work?

 

[Michael Tiemann]

I'm trying to get a better understanding of how audio and frames rates work together, take an H4N for example, I could record video at 23.976 or 29.9, and the audio file from the H4N would match to either one of those. I don't have to set a frame rate for that (I'm not concerned with jam syncing time code or anything).


However, what happens if your film is shot and edited at 23.976 but your sound designer does all his work in 29.97? Will there be a four frame sync difference when you get the audio file back? I'm not familiar with most DAWs, do you need to set a frame rate there before you begin work?

There are frame rates (for video) and sample rates (for audio). There are also frame rates for capture and frame rates for distribution. Perhaps you captured something at 23.976 that needs to be broadcast at 29.97. That sucks, and the sound designer may well choose to work against the 29.97 frame rate so as to create the best result for distribution rather than 23.976 (which would be the best frame rate for you). In this case, they may do sound cues that fall precisely on 29.97 boundaries but which fall somewhere in the middle of 23.976 frame boundaries. Next time you'll capture video at the correct frame rate for your intended distribution.

Video sample rates are typically 48k, though more and more intermediate audio is mixed at 96K and then mastered down to 48k at the end. This has nothing to do with lining up audio and video cues, and only to do with the fidelity of the audio signal chain.

Back to your original question: if you know your distribution format is going to be 29.97, then for God's sake shoot at that frame rate and set all devices that care about frame rate and/or time code to a consistent value. If you have no idea what the distribution format is going to be, but you prefer shooting 23.976 because you like how it looks, then more power to you. Set all your devices so that their frame rates/time codes agree. If you then need to conform to a different rate, that can all happen at once. If you shoot and record at diverse rates, you are inviting confusion and scorn.

 

[Nick Morrison]

Set camera and sound recorder to same frame rate (23.98 or 29.97, etc), otherwise they will drift away from each other in your timeline (if this happens you often can timewarp the audio back in sync, but this can be a huge pain on a large project).

 

[Marc Wielage]

I believe the best answer is:

if you have an integer frame-rate (24fps, 30fps, 25.00, etc.) shoot with 24.00 or 30.00 or 25.00 timecode.

if you have non-integer frame-rate (23.976, 29.97), shoot with 23.976 or 29.97 timecode.

It's not impossible to sync up 30-frame audio timecode with 24.00fps video -- the trick is, if you have a timecode slate, the numbers will only match once a second, at :00 frames. You can split that up into half seconds and quarter seconds and so on, then do the math. In a perfect world, all timecode rates should match frame rates, project settings, audio settings, and timecode slate settings.

The Zoom H4N is a cheap, crappy, $279 audio recorder without built-in timecode, without any kind of external reference capability, and will drift over time. One of the reasons that more-costly recorders are necessary is you need one with an extremely accurate word-clock crystal referenced to an internal timecode generator. Without that, there's a good possibility sound sync will drift. Fostex, Tascam, Sound Devices, and many other companies make very good timecode-capable recorders that will stay locked to camera for hours and hours at a time.

 

[Joe Riggs]

if you know your distribution format is going to be 29.97, then for God's sake shoot at that frame rate

The distribution frame rate is 23.976, the so called "sound designer", an amateur, can only work in 29.97 (yes, I know, hire a professional, I would if it was up to me and the person hired wasn't related to the boss), so will there be drift in a 15 minute project when I get a mixdown WAV file back? Or any other ideas on how to keep sync?

 

[Michael Tiemann]

The distribution frame rate is 23.976, the so called "sound designer", an amateur, can only work in 29.97 (yes, I know, hire a professional, I would if it was up to me and the person hired wasn't related to the boss), so will there be drift in a 15 minute project when I get a mixdown WAV file back? Or any other ideas on how to keep sync?

I think that in this case, throw out the concept of sync. Put a 10 second count-down at the start and at the end of your video and ask for a 48K WAV file that locks to the counts and all your footage in between. You can then add that audio track to your 23.976 timeline and all should be good. If your sound designer is so amateur that they cannot think of a good, crisp impulse for the counters (and thus easy for you to align in post), you are truly screwed. Otherwise, no big deal. You don't need timecode to lock audio to video--just a reasonably stable sample-rate clock for your audio and a reasonably stable frame rate for your video.

 

[Nick Fury]

Hi Marc, Michael and OP,

Not to derail the thread, but...Did you all see ths from Zaxcom?


Our original concept for ERX was that it would be used in close range of a ZaxNet transmitter, always jamming to the timecode source to maintain perfect sync or to hold sync for short periods of time when it was out of range. As the system gained in popularity users started requesting that the ERX would hold its sync for a longer periods of time so it could be used as a standalone sync box – so we did!
ERX Version 2.0 has a new time code calibration feature for all ERX receivers, so each individual unit will calibrate itself to the ZaxNet source. A calibrated ERX virtually eliminates any timecode drift that had affected an unreferenced ERX in the past. Once calibrated, the ERX will have less than one frame of drift over a ten hour period.
An ERX will auto calibrate itself to any ZaxNet timecode signal in 11 minutes from boot up. Once it’s calibrated the auto calibration can be shut off and the ERX will either keep the offset value in its ROM or it can be set up to re-calibrate itself every time it boots up.
Software Version 2.0 can be loaded to any ERXTCD and features the new auto calibration mode. In addition there’s a new timecode entry menu for manually setting the internal timecode generator when not receiving an external signal and a new shortcut to enter the extended menu from the home screen by pressing the timecode button three times.

Just sayin'' ...Maybe an option for the OP down the line.

Thank you very much

Fury

 

[Marc Wielage]

Not to derail the thread, but...Did you all see this from Zaxcom?

Unfortunately, comparing a Zoom to a Zaxcom device is kind of like talking about a Yugo and a Ferrari. Not a lot of similarity between the two except they both sorta do the same things, only the former does it very badly and the other does it extremely well, for a lot more money.

You don't need timecode to lock audio to video--just a reasonably stable sample-rate clock for your audio and a reasonably stable frame rate for your video.

So tell me: what will prevent the picture and sound from drifting out of sync over a period of time?

I have worked on projects (even film projects) where we've done a clap at the head and a clap at the tail to aid in varispeeding, but the reality is that it's a ton of trouble compared to just buying or renting the right equipment at the outset and doing it right. There are $995 timecode recorders out there, you know.