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BMD Pocket Cinema Camera Stereoscopic Collaboration
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  • Just tested my GH2 rig, serial numbers about 2 months apart (ie. pot luck): same test setup = 140pixels/5 min = 4.86 ms drift.

    So 0.97 ms/min.

    With this setup I've been able to shoot small to medium motion for about 5 mins before missync artifacts became noticeable. Most of my shots are locked down, but things like people walking across the frame, and birds doing the same (even in the distance) start to fall apart around 4-5 mins.

    The Pockets (1 serial no. apart) are 1.75x better synced.

    Of course 2 samples doesn't tell you anything definite, but for workaround sync close serials are worth trying for.

  • Are you still considering wiring the clocks together? Seems if you are going for a button mod might as well go all the way.

  • Yes, but as I said before, I will wait a bit until I can trust I won't need the warranty (again). Anyone want to go first? : ).

    And I can't be sure it will succeed (it is a super-tiny board & I don't have much electronics experience at that level), so I want to get workaround sync working first. That should also benefit more people as I expect a clock hack will be tricky & risky.

  • Basically once I open'er up, I'll try the button mod and start poking around with my scope for clocks (I have some better ideas now where to look). But it seems to be a multilayer board with many SMD components, and that is tricky stuff without the right gear (that I don't have). It just depends which part(s) needs desoldering. And I'd like the option to get it back in if it doesn't work out, as I can't afford to trash a body.

    I could use some expert advice for the clock hacking, so if you know any electronics whizzkids, ask them to drop by.

    In the meantime I'll get AVR-based LANC sync measurement and record start/stop working to see if there's a non-invasive solution. Simultaneous record triggering will be useful anyway.

  • I finally got my Pockets back from my vendor (roundtrip via BMD). Long wait, but they got replaced with brand new consecutive serials.

    I just tried my new scope on the LANC data lines, interesting. The video shows the data bursts each camera sends (one per video frame). Perfect sync is when the yellow & blue bursts line up.

    In the video I'm going from Record mode into Play mode, and then back into Record on the blue camera, twice. Each way, you can see how the signal vanishes, then reappears but falls behind briefly (it's like the clock slows down while the chip is busy doing setup), until it suddenly fixes in a new position:

    (the click going into Record is the native m43 lens aperture changing).

    Same thing happens changing from RAW <> Prores.

    Question, is the delay before the signal stabilises always the same duration (and the same on both bodies) or variable? If consistent then a Record button mod should work ...

  • Can you try it without the m43 lens mounting? The delay might be the camera communicating with the lens.

  • It's the same (there could be an imperceptible difference, I'd have to time it accurately to be sure).

    The clock also doesn't change if you (dis)connect the lens, so lens comms seem to have no effect on sync.

    EDIT: the same lag + jump even happens at boot, so it seems to be a shared setup procedure.

  • Disregard all my previous CRT timings, I either did them wrong or it's my CRT - it's an ancient VGA monitor that only supports one resolution & refresh rate (badly), god knows if it even ran at the settings I thought.

    (assuming LANC drift is directly related to sync) my new Pockets have 0.1ms / min drift. Worst-case sync (half a frame) @ 24p is ~21ms, it would take 3.5 hours to hit that (not that you want to be anywhere near it) - nice.

    I'll check over time to see if it changes, temperature might have an effect. I'm also interested if both cams running off the same PSU/battery vs their own batteries/PSUs has any effect (slight voltage difference).

  • I've been using this cheap dual-cam bar for my stereo rig for ages:, and now with the Pockets.

    As the pockets have identical mounting holes on top, if you don't need access to the top buttons (eg. using LANC or a button mod for record), you could actually mount a 2nd bar upside down - that should give total rigidity (useful in vibration scenarios) - sorta like a stereoscopic cage. And as the bar's tightening screws also have screw holes, you can still mount stuff to them as before.

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  • Timings are quite variable. I ran another test from cold over 15mins (same PSU for both as before), and got down to 0.68ms / min!

    I then tried BM batteries over 5mins, back to my previous ~1ms/min. So it's fine to power from two different sources - but sharing power is probably safer for dual LANC control (depending on the LANC circuit) or if the clock gets cloned to the other body.

    Record start/stop has no effect on the clock.

    Interestingly, the new SD formatting function once reset the clock when I came out of it on completion, but now I can't reproduce it. I tried formatting empty cards and with recordings, neither cause it now - must be a combination of things.

  • On question to people using a BMPCC and a LANC control: How fast is max speed manual fokus?

    (Not using "focus to infinity", but incremental "focus one step further" signals.)

    If have heard, that using LANC focus changing is really slow - can anybody confirm this with the BMPCC? (The min. timing of the LANC protocoll might also limit its usefullness for syncing the cams.)

  • I haven't got LANC working yet, somebody here already has though. I'll try when I have it running.

    LANC signals seem to be synced to each video frame, so any delay between the two body sensors is also present on the LANC signal. That means you can't use it for syncing - any command you send simultaneously will be delayed on one body if there is a sync difference. But if there's another way to sync them, then you can at least start record simultaneously (for convenience and you can do it from a distance) + monitor sync from the signal delay.

    LANC commands apparently have to be sent 5 times before they take effect (probably a workaround for unreliable signals). So there's potentially 5 video frames of latency in each command (~200ms @ 24p) which could explain the inc/dec focusing not being responsive.

  • Anybody know how much current is safe to draw from the Pocket's LANC?

    Also the VCC pin is 5V, but the data pin is ~3V. Why the difference? And do I send back 5V or 3V?

  • Wait, it's open-collector so I'm not sending voltage, just draining it. Still, how much can I draw off the 5V? I want to run the micro (should be fine) + small OLED display (max. 40ma @ 5V, though less in practice). Safe?

  • Finished the LANC code, next comes building the circuit & test.

    Just found that the record <> play mode clock-reset only happens when the camera is in ProRes, NOT in RAW where it doesn't miss a beat (nor mess with the aperture). But a reboot still does it.

    Can someone reproduce? Just toggle in/out of play mode - in ProRes it should pause/blank & flash the screen & change the native lens' aperture (that's the clock reset). In RAW it just instantly goes in and out here, no pause or visual glitches.

  • You know that TV show where the obnoxious kid at the end shouts "I made this!"?

    This is the LANC DRIFT3R:

    Current features:

    • controls one or two cams
    • displays sync drift of two in real-time (+ record state & framerate)
    • single touch-sensitive button for record (or testing other cmds)
    • powered by LANC (no battery)
    • colour OLED screen (just because)
    • as tiny as I could make it.
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    LANC_DRIFT3R side.jpg
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    LANC_DRIFT3R hand.jpg
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  • So the procedure is to just restart until sync is close?

  • For some early demo footage yes. But my plan is to try all available LANC commands next (which are unlikely to reset the clocks, record doesn't as expected), look for hidden ones, and failing that do a button mod to see if that will reliably reset the clocks. If a button mod works then I can verify its reliability with the drifter in the field.

    There may be some randomness in the clock reset. If I use LANC record to get the cams out of play mode, I don't get a reliable sync difference (well, there sometimes is a pattern to it, but not always). That doesn't necessarily mean that a button mod won't work, there could be some variability in the LANC response that you may not get with a hardware button mod.

    BTW, nobody responded to this

    "Can someone reproduce? Just toggle in/out of play mode - in ProRes it should pause/blank & flash the screen & change the native lens' aperture (that's the clock reset). In RAW it just instantly goes in and out here, no pause or visual glitches."

  • @_gl That control unit looks very good. What are you using as processing unit, an Arduino Micro?

  • Thanks @Pysco. It's running on an 8 pin AVR (Attiny) chip, code is straight AVR-GCC.

  • I guess you are limited by how often the lanc is polled.

  • Yeah, LANC is in sync with the framerate, so any sync difference between the bodies is on the LANC signals as well.

    I figured out that you could actually use LANC to sync the bodies genlock style, but it would require firmware additions. I know how it could be done - you'd then just wire the LANC ports together & set one cam to be the master. Apart from frame sync, you could even sync all the parameters (exposure, focus, iris, framerate, WB etc) in real-time aswell (that just requires extra LANC commands for each param, which are super-easy to implement).

    If BMD would open up the firmware, it could be added fairly easily.

  • All of that sounds really complicated to me ah ah ah :D I'm lost !

  • Which bit is giving you trouble? And don't say 'all of it' : ).

  • just read this after experimenting with 2 pockets in a sbs rig myself. We used this device and it worked out well in 50% of the shots. however, shots with a lot of motion ( we shot some martial arts in front of greenscreen) are having non acceptable time drift and temporal artefacts. I find the idea of 2 BMPCC´s (or even 2 Blackmagics) very tempting for low budget 3d with still a good quality (ie RAW). Shame that BM says it sees no demand for 3d rigs. From what i see here, the option to genlock 2 cameras via usb wouldnt be that complicated for then to embed. At least, even gopro managed to do so. _gl please go on after this quest - if i can help, please tell me