Sync-gen with BMMCC

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MichaelF

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Sync-gen with BMMCC

PostFri May 25, 2018 4:11 pm

Hi all,

I have connected a Black Magic Sync Generator to the Ref Input of three Black Magic Micro Cinema Cameras.
The Sync Generator has its LED on, and I have set this to the 1080p30 setting (ON/OFF/OFF/ON) using the switches. However, on visual inspection it is clear that the frames are in fact not genlocked, as they are out of sync by half a frame or so, even when manually aligning as close as possible in after effects.

As a test, I used the cameras menus to set the camera to record at 60fps, and recorded a sequence. I expected that the sync-gen would override this to force them to save at 30 frames per second. However, this is not the case and the saved videos are in fact recorded at 60fps.

Is there something in the BMMCC settings that will force it to listen to the sync-gen? Or is there something else I am doing incorrectly?

Thanks
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Denny Smith

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Re: Sync-gen with BMMCC

PostFri May 25, 2018 6:59 pm

Michael, do you know how gen lock works? Genlockmuses an analog video signal, either black burst or Tri-Level to get glitch free switching between various video,inputs, including live switch cameras to “Sync” their video signal. The Ref. input is not a digital Time Code Sync, and genlocked Cameras can still drift on their internally generated T/C. Genlocking several cameras together does not align their internal recording to frame accuracy, it will align their video output to a common video recorder, to insure a clean switch between them.

You need to set frame rates on the generator to match the csmera settings, it will not over ride the camera setup.
Cheers
Denny Smith
SHA Productions
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MichaelF

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Re: Sync-gen with BMMCC

PostFri May 25, 2018 8:34 pm

Hi Denny,

Thanks for your reply.

I *was* under the impression that genlock would act as a global shutter signal - each time the tri-level signal is sent, every camera would simultaneously "click" their shutter. (I am aware that it doesn't fix timecoding, and I am happy to deal with this separately).

However, my experiments have shown that this isn't the case, as the different cameras are not capturing frames at the same time as each other. To be specific about the experiment I have performed: I filmed a stopwatch with all three genlocked cameras. I found a frame captured by camera 1 which showed the time on the videotaped stopwatch as 12.819s. However no frame from camera 2 showed a similar time - in fact the closest stopwatch time from a frame in camera 2 was 12.835s. So the cameras aren't taking photos at the same time.

If I have misunderstood how genlock should work then please do point me towards a suitable resource, or I'd be interested in a fix which achieves my goal of having the cameras click at the same time.

Thanks

Michael
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Denny Smith

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Re: Sync-gen with BMMCC

PostFri May 25, 2018 10:21 pm

Yes, Genlock does not control a camera’s shutter, it only writes a sync signal track at the start of each clip.
Aside from T/C in, which the Micro cameras do not support, not sure how you could do this.
Cheers
Denny Smith
SHA Productions
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George Leon

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Re: Sync-gen with BMMCC

PostFri Mar 15, 2019 6:27 pm

Denny Smith wrote:Yes, Genlock does not control a camera’s shutter, it only writes a sync signal track at the start of each clip.


That's incorrect information. Genlock does sync the sensors together. The Aja10 can do this (I use it myself to sync two bodies for 3D capture).

Note that the framerate setting on the Aja10 has to match the recording framerate. More recent BMMCC firmware now has Genlock indication on the cam's overlays ('REF' icon IIRC).
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Denny Smith

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Re: Sync-gen with BMMCC

PostSat Mar 16, 2019 5:43 pm

George, I didn’t say it did not sync the sensor readout (which is what a control track/GenLock does). What I said it that Genlock does Not control the shutter, each camera has to be set to the same resolution and shutter setting. Broadcast cameras in the SD days, were fixed resolution/shutter, so their was no need for genlock to control the shutter, only sync multiple cameras together to prevent switching glitches/errors.
Cheers
Denny Smith
SHA Productions
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Greg_E

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Re: Sync-gen with BMMCC

PostSat Mar 16, 2019 7:41 pm

But then there is still a timing issue to be resolved. Just because they are locked does not mean that the signals coming out of them have the same point in time where they sync. Transit time still happens in digital, so cable length still matters, as does any internal processing in each camera, running through a DA, etc. None of that may be the case, but just blindly locking two cameras together does not guarantee that the frames happen at the same time.
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Denny Smith

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Re: Sync-gen with BMMCC

PostSun Mar 17, 2019 4:51 am

Correct, it does not.
Cheers
Denny Smith
SHA Productions
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Howard Roll

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 8:31 am

MichaelF wrote:I found a frame captured by camera 1 which showed the time on the videotaped stopwatch as 12.819s. However no frame from camera 2 showed a similar time - in fact the closest stopwatch time from a frame in camera 2 was 12.835s. So the cameras aren't taking photos at the same time.


At 60p each frame is .016 seconds. 12.819+.016=12.835. How are they not in sync? Im guessing you can’t read the timer because of the motion blur, crank the shutter way up so you can read the numbers.

Good Luck
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 12:42 pm

Howard Roll wrote:
MichaelF wrote:At 60p each frame is .016 seconds. 12.819+.016=12.835. How are they not in sync? Im guessing you can’t read the timer because of the motion blur, crank the shutter way up so you can read the numbers.

Good Luck


Genlock is designed to provide sub-frame sync (usually at the scanline level or better). That means that when the cams' sensors are locked, a frame from each should show exactly the same image. If one shows a slightly different timer, no matter how small the difference, they are not genlocked.
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 1:15 pm

Denny Smith wrote:George, I didn’t say it did not sync the sensor readout (which is what a control track/GenLock does). What I said it that Genlock does Not control the shutter, each camera has to be set to the same resolution and shutter setting. Broadcast cameras in the SD days, were fixed resolution/shutter, so their was no need for genlock to control the shutter, only sync multiple cameras together to prevent switching glitches/errors.


Roger, I misread you.

Greg_E wrote:None of that may be the case, but just blindly locking two cameras together does not guarantee that the frames happen at the same time.


True, it only guarantees that they are captured at the same time and the images are of exactly the same moment..
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Greg_E

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 1:34 pm

But they may not be the exact same moment... If one camera has a 12 inch cable and the other camera has a 100 foot cable, they are not going to sync at the same time.

Typically, if the cable length is all within a reasonable difference, you can consider the signals to reach at the same time. Still a fundamental of building multi-camera systems is to make all the cables the same length whenever possible. But that still doesn't mean that 5 cameras (same make and model) will have the same timing. You really need a real waveform monitor with external sync to see. Best case that I would consider is one frame off until I can measure and prove them to be closer.
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 2:05 pm

Greg_E wrote:But they may not be the exact same moment... If one camera has a 12 inch cable and the other camera has a 100 foot cable, they are not going to sync at the same time.


Sure, Genlock signal cable length matters.

But that still doesn't mean that 5 cameras (same make and model) will have the same timing. You really need a real waveform monitor with external sync to see. Best case that I would consider is one frame off until I can measure and prove them to be closer.


What could throw the timing off if the cable length is the same?
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Howard Roll

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 3:29 pm

Greg_E wrote:Best case that I would consider is one frame off until I can measure and prove them to be closer.


If they were one frame off they’d be in sync. The most they can be out is half a frame.
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 3:55 pm

Howard Roll wrote:
Greg_E wrote:Best case that I would consider is one frame off until I can measure and prove them to be closer.


If they were one frame off they’d be in sync. The most they can be out is half a frame.


That's true at the capture end, and if you can slip the whole-frame offset that's fine (that's my scenario as I shoot 3D and can slip in post). But if you need them to be in-sync in real-time, eg. for broadcast, then it's an issue.
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Gary Adams

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 4:41 pm

Hello Everyone. There is some incorrect information in this thread so I would like to help a little. When wanting to lock multiple Micro Cinema Cameras together, you need to match all the settings in all cameras. Like frame rate, shutter, and resolution. The reference input must have the same frame rate as the cameras or the reference will be ignored. You can easily verify if the cameras are locked by looking at the HDMI monitor. There will be a "Ref" indication. If it is not there, the camera is free running. When locked, all the images should be the same given the same shot. Also, cable length has no bearing on lock. At 140 ns per foot, it would take a long cable to make any sort of difference. I hope this helps.

Regards, Gary
Gary Adams
Blackmagic Design
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Denny Smith

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 4:46 pm

For Broadcast if they are within one frame, they are sync’d. Genlock for Broadcast provides a glitch free switching, syncing the svide signal from each camera, it is not an absolute time code sync frame lock. Not does it match the time code on the two Genlock cameras. For that you need TC sync, not Genlock (which is an analog pulse system and not a master clock sync system). Unless TC locked (which the Micro camera dies not provide), individual cameras can have internal clock drift, even if started at the same point in time.
Cheers
Last edited by Denny Smith on Mon Mar 18, 2019 8:35 pm, edited 1 time in total.
Denny Smith
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 4:51 pm

Denny Smith wrote:Unless TC locked (which the Micro camera dies not provide), individual cameras can have internal clock drift, even if started at the same point in time.


This is where you lose me. Genlock keeps the sensors in sync continuously, so the capture frames stay in perfect sync and the cameras never drift against each other. They don't just start in sync. I assume you're talking about something else?
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Gary Adams

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 5:09 pm

Given multiple cameras, the recordings will not all start the same for sure. The time code on the individual clips will not match as well. If they are genlocked for recording, once you line up the same frame on each camera (like in Resolve), they will be identical for the duration unless there is a dropped frame of course. That would be the purpose of the reference in this case.

Regards, Gary
Gary Adams
Blackmagic Design
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Denny Smith

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 8:33 pm

George, youmare mxing two different issues. Genlock syncs the camera’s so they will output to a switcher, a sync’d signal, it does not jam sync the camera’s timeclock (only Time Code Sync (TC) does that, and as Gary says, it will create a matched set of recordings, length wise, but will not match the time code on each camera. You need to do a goggle search on “GenLock vs Time Code Sync” for a full explanation.
Cheers
Denny Smith
SHA Productions
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 8:37 pm

Denny Smith wrote:George, youmare mxing two different issues. Genlock syncs the camera’s so they will output to a switcher, a sync’d signal, it does not jam sync the camera’s timeclock (only Time Code Sync (TC) does that, and as Gary says, it will create a matched set of recordings, length wise, but will not match the time code on each camera. You need to do a goggle search on “GenLock vs Time Code Sync” for a full explanation.


Denny, I see what you're getting at. However the OP asked about genlock sync, not timecode sync:

MichaelF wrote:I have connected a Black Magic Sync Generator to the Ref Input of three Black Magic Micro Cinema Cameras.
The Sync Generator has its LED on, and I have set this to the 1080p30 setting (ON/OFF/OFF/ON) using the switches. However, on visual inspection it is clear that the frames are in fact not genlocked, as they are out of sync by half a frame or so, even when manually aligning as close as possible in after effects.


So you have in fact confused the thread (and me : ) by talking about timecode sync.
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George Leon

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Re: Sync-gen with BMMCC

PostMon Mar 18, 2019 8:44 pm

.. however, now we're talking about timecode:

you say that in the BMMCC's for example, even when they're genlocked, the internally created timecode is generated off a separate free-running lock, and so can still drift between bodies.

Are you sure that's true? I don't know either way, however as a programmer, it seems to make sense that the internal timecode would be generated directly off the frame capture clock, ie. directly ticked by each captured frame. A free running clock that is not slaved to the genlocked clock in some way seems less likely.

As I said, don't know for sure - do you?

Or BM, can you clarify?
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Gary Adams

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Re: Sync-gen with BMMCC

PostTue Mar 19, 2019 12:04 am

Hello George. That's a good question. So the internal clocks of the cameras will drift with respect to each other because they have no connection to each other. However, time code is "grabbed" at the start of each recording and based on frame rate will not drift within the recorded clip. So again with multiple cameras, if genlocked, the starting time code will not be the same given the start of the clips may not be the same but there will be a fixed offset in the time code of each clip since the frame rates are locked together. There will be no drift within the clips.

Regards, Gary
Gary Adams
Blackmagic Design
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Denny Smith

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Re: Sync-gen with BMMCC

PostTue Mar 19, 2019 12:21 am

Thank you Gary, George seems to keep thinking GenLock is a timecode feed, like a LTC, or Word Clock would generate.

George, GenLock is not time code specific, it does not have any timecode information in it. What Genlock is, is an analog generator-lock signal usually consists of vertical and horizontal synchronizing pulses together with chrominance phase reference in the form of colorburst. No picture information is usually carried to avoid disturbing the timing signals, and the name reference, black and burst, color black, or black burst is usually given to such a signal. A composite colour video signal inherently carries the same reference signals and can be used as a generator-locking signal, albeit at the risk of being disturbed by out-of-specification picture signals. It does Not generate a master clock timecode signal.

In use, a Genlock Generator can be used to synchronize as few as two isolated sources (e.g., a television camera and a videotape machine feeding a vision mixer (production switcher)), or in a wider facility where all the video sources are locked to a single synchronizing pulse generator (e.g., a fast-paced sporting event featuring multiple cameras and recording devices). Generator locking can also be used to ensure that multiple CRT monitors that appear in a movie are flicker-free. Generator locking is also used to synchronize two cameras for Stereoscopic 3D video recordingcamera . The key here is synchronizing, using an analog video pulse signal, not a digital timecode master clock feed. The pulse generated however, is controlled by a frequency clock in the Genlock Generator.
Cheers
Denny Smith
SHA Productions

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