HV20 and genlock/syncing for multi camera shoot

[Stuart Brontman]

I'm a fairly new owner of an HV20 (along with an XH A1) and love the camera. I've got a possible need to try and use two HV20s for stereoscopic video capture. Is it possible to get perfect sync of the two streams? I know there's no built-in ability, but if you captured (tethered) to dual PCs with dual Intensity cards (when supported) using the Cineform codec, would the pure digital output allow for perfect sync in post. I figured I could use a sound or visual marker to accomplish the sync in the timeline. I know in standard prosumer or consumer DV cams there is always a little drift, but I would assume pure digital eliminates that.

Any thoughts?

Thanks.

Stuart

[David Newman]

Drift between the camera will not be a huge concern, however lack on frame sync at startup will be. Having the cameras out of phase causes 3D artifacts for motion. You need to see if there is a way to have the frames sync -- you might need to crack open the cameras for that.

[Stuart Brontman]

If I understand you, you're saying despite the digital signal, the start-up difference of the two cameras will create a slight difference on frame-to-frame sync, even though each stream stays steady.

What about external TBC and genlock control? Wouldn't that synchronize the two cameras? There must be a way to do this without "cracking open" the cameras.

[David Newman]

The camera needs to gather image data at the same time, not with an offset up to 1/48 of a second, so post syncronization will not help. There may be ways other than opening the cameras, I just don't know them.

[Dave Blackhurst]

Since you're dealing with fractions of a second, sync may be difficult - only thing I can think of is to use the remote to trigger both cams simultaneously - presuming they both start at the same time, that might work?

[Andrew Plumb]

Looking at the HDMI spec (v1.3a), there is a set bidirectional control signals that may allow for something like an Intensity card to talk with a camera and affect timing.

...To quote Section 8.1, "The CEC channel is optionally used for higher-level user functions such as automatic setup tasks or tasks typically associated with infrared remote control usage."

If that's the case and actually functional within the HV20's HDMI, two Intensity cards in one computer may be able to enforce synchronized capture between multiple cameras.

The folks with Blackmagic Design would know best which devils lie in the details.

[Andrew Plumb]

Of course, right on the Intensity spec page it sez:

Device Control: None. Use FireWire if capturing from HDMI cameras.

...so maybe you'll have to wait for a future rev. for HDMI-based control if it's a hardware limitation that can't be got around in firmware.

[Stuart Brontman]

One thing still confuses me about sync, genlock, etc... I'm trying to capture two streams. One stream is for the left eye, the other for the right eye. Even if the cameras start at separate times, why can't the footage be put into sync in post in my NLE? I'm not switching between left and right cameras, just recording both streams. These streams are rendered separately and then projected for use with shutter glasses. Wouldn't an audio reference point on two direct-to-disk recordings provide a sync point? Needless to say, I'm trying to avoid purchasing two XH G1 cameras, considering I just bought an XH A1 and HV 20!

[Noah Yuan-Vogel]

I wonder how much of a difference such a lag would be. So with syncing in post, if you started two cameras at the same time, youd get an average lag of 1/96th sec, right? People have been using those mirror shutter attachments for recording stereoscopic images in 60i for quite a while which I imagine result in a consistent eye-to-eye lag of 1/60th, which on average is MORE lag than youll get by turning on two 24fps cameras (xl-a1 and hv20) and syncing them as well as possible in post, right? so maybe its no big deal. I dont know a ton about this subject so my real suggestion would be to test it and see what comes out.

[Andrew Plumb]

Think of it this way. If you have two cameras recording at the same frame-rate, your two video streams can be corrected to within plus/minus half a frame period.

Without any kind of synchronization at 24fps, there's 1/24 secs between frames. One camera could lead or lag the other by up to 1/48 secs. That's the best you'd be able to align the frames. Depending on the application, for slow movement that's probably ok, but fast motion could get tricky and sick-inducing.

Bump up the frame-rate and you'll have less relative frame error to worry about. That may mean capturing interlaced though. Synchronizing two 60i streams gives you twice the number of interlace-related artifacts to worry about.

Knowing how sensitive HMDs (VR and wearable computing applications) can be to misaligned optics and the like, trying to capture stereoscopic with different makes of cameras is just asking for trouble. You're welcome to give it a try, but if you're already considering one HV20 in the mix I'd suggest at least borrowing and trying it with a second HV20.

...I'll be interested to see how it works out. With so much automation in the HV20 (OIS and IAF), I'm curious to know how "wobbly eyed" it will feel with the two cameras operating independently.

[Stuart Brontman]

I forgot to mention I'd either pick up another HV20 or XH A1 for the shoot. The "advantage" of the HV20 is the ability to keep the lenses mounted more closely to approximate human eye separation. I would turn off ALL auto functions and run the camera in totally manual operation. I would be recording at 60i with either camera.

I guess renting two XH G1 cameras is an option as well... If I can't get reasonable sync I'll have to look towards that IF the separation of the lenses is reasonable. I'll have to do some measurements...

[Stuart Brontman]

Hmmm... I just found this in my surfing:

http://www.blackmagic-design.com/pro...ensity/on-air/

Maybe this is the answer I'm looking for with the HV20. Once the Intensity cards play nicely with the HV20, it looks like complete sync is very feasible. I always intended to shoot this project tethered to a PC and capturing into Prospect HD.

The question now is will the "On-air" feature allow simultaneous recording of both cameras, or only one at a time? Looks like I'll need to surf some more for that one. Anyone know this feature well enough to say if dual recording is available?

[Noah Yuan-Vogel]

Yeah I was going to suggest that but I'm not sure it handles recording two simultaneous streams but rather syncing two streams and switching between them while recording one stream. I could be wrong though.

[Stuart Brontman]

Looks like I'll be calling Blackmagic on Monday to confirm its capabilities. I'll report back what I find.

[Marcelo Arend]

Look this:

http://www.dvinfo.net/conf/showthread.php?t=94478