Off-camera Movie sound: How does it work?
June 22, 2010 9:05 PM   Subscribe

Off-camera Movie sound: How does it work?

So this seems like a really basic concept, and it's kind of embarrassing that I don't know this, but I can't find a definitive answer by googling.

When recording sound off-camera, are there different "speeds" of sound recording to match different frame rates? That is, does the audio recorder need to be set up differently to sync with 24fps, as opposed to 30fps?

I have heard people say this is the case, but I'm not sure if I understood properly. Part of me thinks it makes sense, but the other part of me thinks it's all the same speed, 24p just has a frame occur less often.

For the sake of an example, let's say I want to shoot 24p video on a Canon 7D and record audio on an iPhone with an external mic and pro-quality recording software, as mentioned in that other thread yesterday. I would use a slate and then sync it up in Final Cut later. I want to know if I need to set some "24p" setting on the recording software, or if I can just press "record" and it will sync up correctly later.

I want to know the answer for practical use, but I'm also interested in understanding the concepts behind it, so if you're a sound guy, fire away!
posted by drjimmy11 to Media & Arts (10 answers total) 2 users marked this as a favorite
Response by poster: (Btw this is more for small-scale personal projects, not for a professional movie. When I shoot "real" stuff I hire a sound guy, but I've never actually sat down with him and had him show me his equipment. Which is why you don't know this.)
posted by drjimmy11 at 9:07 PM on June 22, 2010

Best answer: Nah, no need for different sound speeds. 24p records 24 frames per second; in your recording software, you'll see a 1-second block of video. Your 1-second block of sound will fit in there. Same thing would happen if you recorded a 30p would take up that same 1 second block.

The 24p vs. 30p thing just affects how long each frame is visible and has nothing to do with sound.
posted by DMan at 9:18 PM on June 22, 2010

Response by poster: Ok then, thanks! I'm not sure what the people I heard talking about "speeds" were referring to! It might have had to do with the 23.97 vs pure 24 issue when shooting film?
posted by drjimmy11 at 9:20 PM on June 22, 2010

You might have to stretch your audio tracks out if you change your frames per second (like how 24fps films are sped up to 25fps for PAL video) but this is something you'll almost never do and if you did, it'd be done in post-production well after shooting.
posted by zsazsa at 9:54 PM on June 22, 2010

With old movie tech you'd need to match the speeds of the sound and image as measured by the ratio of time to distance on the film, because both sound and image are printed on the same film that gets pulled through the projector. You need 1/24s of sound to take up the same physical length of film as 1/24s of image.

So I imagine.
posted by grobstein at 9:56 PM on June 22, 2010

Please don't record to your iPhone, that's not going to give you any better result than recording to the DSLR. At least spring for a basic recorder like a Tascam DR-100 or a Zoom H4n - devices actually designed to record sound and interface with professional mics.

In sound for picture, you generally record 24bit/48kHz. Sync shouldn't be an issue unless you're doing really long takes, the clocks of the camera and the recorder will eventually drift apart. Just remember to slate every take and you should be fine.

Oh, and don't forget the headphones!
posted by jjb at 10:08 PM on June 22, 2010 [1 favorite]

Response by poster: Yeah I was actually looking at other small recorders rather than iPhone at this point.

But anything will give better results than on-camera on a 7D- trust me. There are some add-ons now, but as the camera comes out of the box it lacks any sound controls whatsoever besides "on and off." Not even a headphone jack.
posted by drjimmy11 at 10:16 PM on June 22, 2010

There is a new Zoom recorder coming out to be entry level for DSLR off camera audio, smaller than the H4N with dual internal mics for $99. The H4N is still better, but double the price.

Always record at 48 Khz if possible, and you can absolutely have drift in audio video sync, though only a real concern over long single takes. As an example, if you did an interview over an hour, your external audio could lose sync from video by the end.

Before the firmware update, the 5D recorded at pure 30 fps, not 29.97, which resulted in some sync problems in editing programs like Final Cut Pro, but on an adjustment level of .01%.

For better or worse, the 5D and the like are technically limited to 12 minute clips, so sync drift is very unlikely to be an issue. Buy an external recorder, buy the PluralEyes plugin, and life will be good.
posted by shinynewnick at 11:07 PM on June 22, 2010

As far as I know.. actually YES you would have to setup the Audio Recorder differently when you are Shooting Video at a different Frame Rate. Over longer a long single shot there s some chance that the Audio / video Streams can drift out of Sync ie minor discrepancies cummulate and the video and Audio Fall out of Sync.

I think this is less of a problem with Digital Video and Digital Audio but back when you were using Film and Tape for Audio then it would have been a serious problem - hence you had SMPTE Clock which was used to keep your Film and your Audio in Sync. And I think you woudl 'stripe' your audio tape with a Framerate dependent Clock signal.

its still kind of used in digital today for Offline Editing cause you need to keep a very good record of exactly what frame / audio spot your eddits are occuring at.
posted by mary8nne at 9:02 AM on June 23, 2010

I'd strongly recommend against using the onboard mics in any of the various portable recorders.

The pickup patterns are too wide, you have exactly zero control/metering if you stick the thing on a boom, and there's no reason to record dialog in stereo.
posted by jjb at 4:33 PM on June 23, 2010

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