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BMD Pocket Cinema Camera Stereoscopic Collaboration
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  • So the procedure is to just restart until sync is close?

  • 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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  • 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.

  • 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?

  • 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?

  • 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.

  • 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.)

  • 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.

  • 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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  • 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).

  • 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.

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

  • 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 ...

  • 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.

  • 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.

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

  • 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.

  • re. splitter, yes that is the plan, though the hyper format works great with some things, and stacking (overlapping) the bodies gets you a bit narrower (see my pic earlier in the thread).

    I haven't tested LANC yet. The physical buttons only move you out of play back into record mode, they don't immediately start recording. And that switch out of play (or possibly going into play) is resetting the capture clock.

    Although I expect that triggering this with LANC won't help sync, as I believe LANC itself is slaved to the camera's frame rate, and so any sync error is built into the protocol and can't be corrected (ie. if you start record on both via LANC, you still get the delay between the bodies before the command kicks in on each one). But we could at least see if both capture clocks are reset to the same error as before. That should tell us that with a physical button mod, we should be able to sync them. However that depends on how fast the physical buttons respond (eg. if they are polled, how quickly, even better if they're using interrupts).

    I can't see any way of doing this right now without a button mod, unless LANC does something unexpected.

  • Not seen this type before - nice, quite expensive though and not much more flexible than the cheap one I linked. Lower profile, but you also need to get a power connector in there, that will be larger anyway. Also the contacts are exposed and would need to be sealed. I'd also worry how many plug cycles it can take before it snaps.

    EDIT: also looking at this pic, it might interfere with the power connector?

    Could be very useful for some things though, especially inside a housing.

  • For HDMI you should get these low profile ribbon cables.

    Also you should consider because of their body size building a beamsplitter for them. They'll be a lot more useful in that configuration. I have a beamsplitter myself but no pockets to try them with.

    Does the record command from a lanc switch them from playback to record instantly, or simply switch from playback to live?

  • Quick cable tips (yes I made those mistakes : ):

    Get right-angle cables to get the bodies as closely together as possible. But don't get these:

    • mini HDMI:

    -- cable pointing down: it blocks the power socket! (sure does, sigh)
    -- cable pointing up: could work, but will block mic & headphone sockets!

    This should work without blocking anything (pointing towards you):

    EDIT: but that won't work if you do body stacking (overlapping the bodies). You'd need a cable pointing away from you instead (haven't seen one yet).

    • 1.7mm power:

    I thought I was being smart getting this DIY cable (usually listed for Asus EEEs) as they have narrower sockets than the rest: Except the connector barrels seem to be proprietary as they're only 7.5mm in length, shorter than the standard 9-10mm the Pocket needs. Works, but they fall out if you breathe on them (really).

    We want the (usually yellow) type which is 2.5mm (outer diameter), 0.7mm (internal diameter) 9-10mm barrel length. Sometimes listed as "2.5x0.7mm"

    (there are thicker ones like 4.0mm or 5.5mm outer, these won't fit!)

  • re. my tests above, can someone try to verify?

  • Some drift stats from my 1 serial no apart bodies (measured with a CRT and Camera Sync Tester):

    (shout if you see any mistakes)

    Tested at 29.97 fps: single-frame time = 1000ms / 29.97fps = 33.367 ms worst-case sync (1/2 frame) = 16.683 ms

    CRT @ 640x480 60Hz: 1 refresh cycle = 1000ms / 60Hz = 16.667 ms

    Drift from synced: 5mins: ~80 pixels / 480 pixels height = 0.1667 * refresh cycle (16.667ms) = 2.777 ms

    = 16.62 % bad (0% = perfect sync, 100% = worst)

    So around 0.555 ms / min

    Not sure how that works out in practice. Will measure my GH2's soon as I know what their real-world drift is like.

  • Quick update: got the bodies, but I have to send them back as they have screen issues.

    Had a short time to play with them today though. The following things reset the sensor sync:

    • power on (boot)
    • changing RAW to any Prores mode or vice versa (but not changing from one Prores mode to another)
    • changing from Play to Record mode (or maybe it's going into Play mode that does it)

    The following DON'T:

    • connecting/disconnecting native lens
    • record start/stop
    • changing frame rate
    • changing shutter speed
    • auto-iris or auto-zoom
    • toggling peaking
    • toggling 1:1 focus zoom

    Have I missed anything?

    Simply delivering power to the bodies simultaneously (without powerup) does not sync them (the clock is reset during boot).

    So perhaps a hardware button mod is required to power both bodies up simultaneously OR to record simultaneously. For the 2nd, the idea is that you manually go into play mode, and then simul-record to get to record mode & reset the capture clocks. Assuming it works that way, and that the button scanning is fast enough to not cause a significant random delay. Bonus is that you can also start recording simultaneously that way (no LANC required).

    Although a button hack is relatively easy, a non-invasive method would be better. Next I'll write a LANC controller, both to measure sync without a CRT and to see what can be done there.