BM Signal Generator / Audio Track

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fhhowdy

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BM Signal Generator / Audio Track

PostWed Apr 17, 2019 1:48 pm

Disclaimer:
I'm new to timecode syncing/workflows, so I apologize as I try to build up my knowledge around this.

Setup:
I'm in the process of migrating a multi-camera (consumer grade) setup to a prosumer grade setup. We currently have four cameras that record independently, as well as an independent master mix that is synced in post.

I'm in the process of bringing those camera feeds into hyperdeck minis, synced with a BM signal generator. So far, early tests have been good. But the audio feed is another question. The audio is currently multitracked over Dante to a dedicated computer to record for post mixing/editing, which leads to my question.

Question:
Is the signal from the BM signal generator a standard SMPTE timecode? I haven't connected it to see, but my current assumption is that it is, and if so, I'll connect that to an open channel in my audio mix so I can have a timecode channel, which will allow me to sync in post. I do this with my mobile setup and tentacle units now, but before I hookup the BM reference out to my board and fry something, I thought I'd verify that my assumptions around BM are accurate.

Thoughts? Suggestions?

Thanks in advance!
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fhhowdy

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Re: BM Signal Generator / Audio Track

PostWed May 15, 2019 4:54 pm

So nobody has an answer for this?
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rogersuski

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Re: BM Signal Generator / Audio Track

PostWed May 15, 2019 5:14 pm

Assuming you are referring to the BMD Sync Generator. No, it is not a SMPTE timecode generator.
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fhhowdy

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Re: BM Signal Generator / Audio Track

PostThu May 16, 2019 12:54 am

Thank you for the clarification. So, can I connect a standard SMPTE clock to the reference input, or do the BM devices only accept the BM Signal generator signals?
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Dave Del Vecchio

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Re: BM Signal Generator / Audio Track

PostThu May 16, 2019 3:59 am

No, the reference input is only for a genlock/synchronization signal from something like the Blackmagic Sync Generator. This reference signal is an analog video signal (for HD video tri-level sync is usually used), that is used to time the start of a frame across different devices. The reference signal is just a pulse, it carries no time information (no numbers) and every pulse is exactly the same. But this pulse is sufficient to make sure each device starts its frame at exactly the same instant.

So if you had multiple cameras that you need to capture frames at exactly the same instant, you might use a sync generator (sometimes also called genlock) and feed the signal to each camera. This is essential when shooting 3D video with two cameras so that the left and right camera capture at exactly the same instant. It's also very important for 360 VR applications involving multiple cameras for similar reasons.

Without a reference sync signal sent to each camera, the individual frames from each camera may be off from each other by a few milliseconds in time. For a multi-camera shoot where you just cut between different camera angles, this usually doesn't matter much, because the cut itself hides any sub-frame timing differences. You'd really only notice this when viewing frames from different cameras simultaneously (which is why a sync generator is important for 3D and VR applications).

Timecode is really just a counter, it tells you what time a particular frame was captured. A timecode counter can start at an arbitrary time (like 00:00:00 in record-run applications), or it can be set to the current time (time-of-day) as a clock. Timecode is also different type of signal than reference/sync signals. Timecode comes in different formats, but as a dedicated input connector, usually LTC timecode is expected which is an analog audio signal. In the case of LTC timecode, the analog audio signal is actually used to encode what is essentially digital information, namely the numbers representing the current time/frame count.

Timecode can be used to match up frames captured from roughly the same time from different cameras, and can be used to match a video frame with the audio samples captured at the same time. So if a timecode signal is fed to multiple cameras (or video recorders) and an external audio recorder, each device can mark the content being recorded with the corresponding time stamps to assist with this.

Keep in mind that with timecode alone (without a sync reference), the frames marked with the same timecode from different sources will all be from slightly different moments in time if you were to compare them side by side. But since the differences will be less than 1 frame (less than 33ms if shooting 30 fps), for many applications this is good enough.

We should probably add an item to the FAQ for the difference between timecode and sync reference signals, as this is a frequent source of confusion.
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jawsborne

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Re: BM Signal Generator / Audio Track

PostThu May 16, 2019 4:05 am

Sync (AKA reference or genlock) and timecode are two different things.

Reference is used when cameras are fed into a switcher and you need the timing between the generation of frames to be in sync when switching cameras. You would send the reference signal from the BMD Sync Generator to each camera's reference input as well as the switcher's reference input to accomplish this, but sounds like that is not what you are looking to do. And FYI The hyperdecks have reference input in case you are using them as playback sources on a switcher input.

I don't believe the Hyperdeck minis can support timecode input.

The Hyperdeck Studio Pro 2 has timecode input via XLR.
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fhhowdy

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Re: BM Signal Generator / Audio Track

PostSun May 19, 2019 4:08 am

Wow. Thank you both for the clarification. I'm new to this, and have been trying to get a setup that fixes post-production sync that have plagued us for the past year. We have 4 hyperdeck minis, so this information is great. Now I just need to figure out the best way to sync our audio back, since I have no direct way to reference it (it's post-produced, so using an existing audio feed isn't a direct option) to the video recordings.

Thanks again!
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roger.suski

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Re: BM Signal Generator / Audio Track

PostSun May 19, 2019 4:17 am

If you have the budget, you could get one of the switchers that has 3 aux out - that would at least give you 3 timecode locked recordings from your cameras -- you could fake it if you only hot punch program and put your 4th camera in preview at all times and use that as an additional "aux"

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