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Post by markcars » Mon Aug 27, 2018 12:45 am

I am trying to record from 3 cameras (canon xl-h1)s to 3 digital recorders on a multi camera shoot (3 in this case).
I am not sure if the genlock/ref signal is to be fed to the cameras or to the recorders as the recording is not being done on the cameras themselves.

If I send the external genlock(or reference) signal to the cameras, does the recorder also need a reference signal? I am a bit confused on this. Thanks.

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Re: Genlock

Post by w9wi » Mon Aug 27, 2018 6:13 am

If you're recording each camera to its own recorder, you don't need genlock. Genlock is necessary only if you're connecting the cameras to a video switcher and recording all three on the *same* recorder. If you did need genlock it would need to be applied to all three cameras. It would not strictly need to be applied to the recorder but it usually is.

(The idea is..

Cameras, even digital ones, "scan" the picture. They start in the upper left and move to the right and bottom.
If you want to switch between cameras, you need the scan to be in the same place on both cameras when you switch. Otherwise, the scan will need to "jump" from somewhere in the middle of the picture to somewhere else in the middle of the picture, causing an ugly glitch.
If you want to do effects (dissolve, wipe, title, whatever) it gets even uglier -- you can't predict where the title will appear on the screen; the entire screen jumps at the end of the dissolve; etc..

Genlock forces every locked device to start the upper left of the scan at precisely the same time.

You don't need genlock when recording separately. The digital recordings will all play out of the same player in the edit suite (or out of multiple players which *are* genlocked) and that will guarantee the scans start at the same time.

Hopefully that makes sense...)
Doug Smith W9WI
Pleasant View, TN EM66

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Re: Genlock

Post by dcwar » Tue Aug 28, 2018 12:53 am

Wow, I haven’t seen the XL-H1 in a good ten+ years.

For multicam field shoots, you generally want to make sure each camera is being fed identical genlock and timecode.** If you’re using field HD-SDI recorders (e.g. AJA, Sound Devices, Convergent Design), they simply convert the incoming video into a file. So, you definitely want to make sure the cameras themselves are the devices receiving genlock. That way, when the editor ‘groups’ the footage together in their NLE, everything matches up, and will stay locked together over the duration of the group / multiclip.

(Without genlock, each camera will be slightly off from each other in time, in different ways each time you roll. This will require your editor to manually sync each take… so head and tail slates will be helpful to them. But even then, without genlock each of the cameras will drift out of sync from each other over the duration of the recording [and each one differently from all the rest]. The longer your cameras are rolling, the greater the problem becomes. For very long takes, all cameras can go in and out of sync with each other in ways that are simply incredible... and will make your editor will want to inflict harm on the production crew - because it can get pretty hairy.)

** = With that camera, if you’re feeding genlock, you should also be feeding timecode. (Although, it’s been so long I don’t honestly remember if that camera actually conveys the incoming timecode data over the HD-SDI output. I suspect it doesn’t - so you may have to route timecode directly to your recorder.)

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Re: Genlock

Post by Steve-Pietras » Thu Sep 13, 2018 11:10 am

If you are indeed recording on three separate media devices (no switcher) a quick and dirty way to sync the cameras in post is to put them all into record and slate them together with a phone app like "clapperboard" that shows seconds. It will get you close enough in post that you can sync each video track. A loud and purposeful hand clap works too but a little more tricky. I have used both methods shooting on broadcast camera and gopro.


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Re: Genlock

Post by PID_Stop » Thu Sep 13, 2018 12:04 pm

We generally think of genlock as a means of keeping sources in phase, since that's necessary for clean transitions. It's easy to forget that in order to enforce a phase relationship, you're more fundamentally forcing each source's internal oscillator to run at exactly the same frequency.

That becomes unusually relevant in a case like this, where the local oscillator determines the precise frame rate and sample rate; if each recorder is left to free-run, the slight differences in frequency will eventually create time discrepancies that will tend to get worse over the duration of the recording. If you're just recording a short clip, you likely will never see a difference; for something like a several-hour production, it could yield a perceptible error toward the end.


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