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Old 02-23-2020, 05:53 PM   #1
DiSoFilm
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Default MultiCam AudioSync vs. speed of sound

Does the following make sense, or am I mistaken in my chain of thoughts ?

Let's say that speed of sound = 330 m/s, and there are 50 frames in each second. This means that each frame equals 330/50= 6.6 meter, or sound travels with 6.6 meter/frame.

When I manually sync two audio sources, one 100% on left, and the other 100% on right, I believe I can sync them within one frame.

So, even with the most perfect audio sync, your video may be out of sync when differences in distance from cameras to audio source exceeds 6 meters, which is quite common when I do multicam concert recordings in large churches.

For me it is important that the video is sync'd to the frame, so when I have a split screen and there is overlap, e.g. conductor on two screens, the movements of the conductor match each other. One frame mismatch does make a difference, is my experience.

When I (only) use the Audio Sync of Edius, the video will not be sync'd due to the varying distances of each camera (some are 2 meters away, others 30 meters) to the audio source and the speed of sound being 6.6 m/f
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Old 02-23-2020, 06:47 PM   #2
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Yes you are correct. If you shoot video ( as I do from the back of the hall about 100 feet away from the stage ) the image will get to the camera a lot quicker than the sound. So to make lipsync from that distance in a closeup the sound has to be moved forward in the frame on the timeline. If you have cameras at different distances then the sound recorded at different positions could be out by more than a frame. I normally record sound both at the cameras and take a feed from the theatre mic ( which is high above the stage about 10 feet back from front of stage). Recording both to my GH5 there is usually a difference of about 3/4 frame. I normally shoot with three cameras from roughly the same point at the back of the theatre with different framing. I manually sync video with sound as their guide checking with multicam view though to ensure they are really in sync. I take sound from the GH5 ( which has shotgun channel one and theatre feed channel two) and export a wav file and take into Vegas so that I can accurately align channels and use the house feed for the real sound with a little mix of the GH5 mic for audience sound. Take file back into an A track in EDIUS as sound source for multicam.

Automatic audio syncing with cameras at stage level or 150 feet away could be out by at least a frame or more. If the subject matter is not critical it may not matter. However for dance or dialogue closeup it will not work. At least not good enough for me.
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Old 02-24-2020, 01:09 AM   #3
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Yes, it is simple physics. The speed of light is much faster than the speed of sound, and the speed of sound actually changes depending on the density of the air it is traveling through, which can be changed by temperature and altitude.

When I do multicam shoots, I try to keep all cameras approximately the same distance away from the stage, podium, etc., so an Edius waveform sync or PluralEyes sync will work if the room is smaller, and I also try to take a feed from the house sound system as my master audio source, either to a camera or a seperate recorder. The sound feed from the house sound mixer theoretically should be in sync with the video if feeding a camera, since electricity and light are the same 299 792 458 m/s. If the feed has to go to a separate recorder, it is still good since it is a clean signal of the show, (I will often have to add some reverb to get it to sound real, since the feed does not reflect the acoustic characteristics of the room, but this is a good thing, particularly if the room is extremely reverberant), and I will then manually nudge sync as required like Ron mentioned. Manual adjustment for the cameras that are not getting the house feed is also require if the room is bigger due to the massive difference in speed of light vs speed of sound but a smaller room works pretty good with just a straight waveform sync. I usually prefer a seperate recorder in a larger room or a very reverberant one so that the camera mics sound the same and an auto sync will work on the footage, as outlined below.

If the all the cameras are about the same distance from the action using their own mics, (not a house feed), and they all get synced together based on their waveform the, video from camera to camera should be in sync, even if the audio isn't in sync with the picture, a simple nudge of the house sound feed is usually all that is required to bring things back in sync if they are out, assuming you are not using camera sound. If using camera sound, then that would get nudged also, but since all audio should be in sync with each other, it should be the same nudge/offset adjustment for all audio. If cameras are all at different distances and the room is larger, then every camera needs to adjusted a different amount.

Another trick I have found that works pretty good if it is a banquet/podium kind of shoot, and is kind of similar to Ron's house mic approach, is to place my own mic in the form of either a wireless LAV or a Zoom H1 on the podium (I actually have a short articulating arm, clamp and shock mount for a Zoom H1 strictly for this purpose). Since it is so close to the people speaking, it is in sync with the video being recorded by the camera, and then camera mics can actually be used to slightly mix into the sound as a natural room reverb and audience sound if they are not too echoy, as Ron suggested, or if they are echoy, nudge them back in sync first.
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Last edited by BernH; 02-24-2020 at 07:46 PM.
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Old 02-24-2020, 03:46 PM   #4
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When I shoot multicam, I usually have a direct feed from the mixing board going into one of the cameras. This what I consider my main cam for sync as it will have the shortest to no delay between the sound and video. Every other camera gets synced to this camera visually (dragging video and audio together). Then, I sync any audio on the the other cameras after the visual video sync using the Audio Offset feature which goes down to the sample level. Or I export the audio out after the visual sync to a DAW and align there.

If it is a multi-channel music recording I try to use a recorder that has timecode jam sync. This keeps the audio recorder in sync with one of your cameras via an SDI yimecode feed. Very important for long form concerts or the audio recorder can drift away over time.

Usually, the main thing is dragging any live mics forward in time to match the direct feed. I find the spoken word to be easier to sync up than music. All of this is done with my ears and eyes! Dance is the easiest for visual syncing as you can find a jump and sync to when the foot just lands on the floor etc... The cameras will often have SDI cables with timecode sync and matching brands and models of cameras. To avoid any drifting I will often put a room ambient and the direct feed on the same camera so the audio/video sync will be the best it can be.

All of this stuff is to get it right during the event so post is easy and correct.
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