Need help with single external battery for dual HV20s
 

Hello,

I'm posting here, rather than the HV20 or stereoscopic sections, because my question is a DIY one more than anything.

I'm trying to shoot stereoscopic video with a pair of HV20s and I have an idea for how I might sync the cameras' 60hz clocks, so as to avoid having frames mis-synced by, at most, 1/120th of a second when shooting 60i. I figure, if I can turn both cameras on at exactly the same time, then they'll probably be pretty much in sync, for at least a few minutes or so. So how to turn them on at exactly the same time? I've noticed that if I take the battery out of the camera, put the camera's power switch in the "on" position, then plug in its ac adapter, the camera immediately turns on. Although the cameras' batteries are 7.4V, the DC input is labeled 8.4V. I'm figuring that if I can find a y-plug with two of the male jacks that my cameras' DC inputs take, plug them into the battery pack-less cameras, turn the cameras switches to "on" then plug the other end of the y-plug into some kind of external 8.4V battery, then at that moment power will simultaneously be provided to both cameras, starting their clocks at the same time, thus in sync. There are plenty of 8.4V battery packs sold online, but what about the y plug I'd need?

Any of this sound plausible? Got any advice?

Thanks!

Paul

Call me old fashion, but why not just use a clap-board at the head of the shoot, as a reference in post?

I'm already planning to use a clapboard, but without doing anything to sync the cameras' 60hz clocks, I run the risk of having frames mis-synced by, at most 1/120th of a second, 1/240th of a second on average. There's nothing you can easily do in a timeline to sync 60i material that is 1/240th of a second out of sync. For fast-moving objects such mis-sync is perceptible. What's worse is if I shoot 24p with a 180 degree shutter, without syncing the cameras' clocks, my frames could be as much as 1/48th of a second out of sync, 1/96th of a second on average. This is very perceptible. If the HV20 had LANC I could use a Ste-Fra LANC or a LANC Shepherd to power on the cameras at exactly the same time, thus establishing a good initial clock sync between the cameras. Without LANC, I have to devise my own way of powering on the cameras at the same moment. A single battery, with an on/off switch, powering both cameras is, I believe, my best bet. Anyone know how to make such a thing?

Hi Paul...........
 

Should be a doddle if you're familiar with the business end of a soldering iron.

Should be able to source necessary components from Radio Shack (they still exist?) or similar. Gonna need 2 concentric 3.1 mm (by my measure) power plugs, a suitable switch if required, some .5 mm stranded twin core cable (you could use very thin co - ax if some is to hand) and a matching connector for the chosen power pack. Oh, and the ubiquitous black electrical tape. 10 minutes with some side cutters, a knife and a soldering iron and you're good to go.

The centre pin on the power plugs is positive BTW.

CS

Thanks for the advice, Chris!

I'm actually not familiar with soldering irons, but I think it's about time I became.

So I take it what I need to do is get an 8.4V DC battery with a detachable device-connecting wire, detach the wire from the battery, and cut off the device-connecting end of this wire. I'll then have two bare wire ends, one positive, one negative. I then solder two individual wires to each end of my original wire ends, giving me four wire ends, two positive, two negative. I then take my two camera-compatible concentric-barrel adapters and solder one of my positive wire ends to the inner barrel of each adapter, and solder one of my negative wire-ends to the outer barrel of each adapter. I now have a y plug which I then plug into the battery, resulting in power flowing to each adapter, thus each camera, simultaneously. Is this about right?

Oh, and is it important that my soldering be very precise? I mean, if, say, I solder one plug a little bit closer to the battery than the other, will the camera connected to the closer plug be getting more power than the other camera?

Thanks a ton for the help!

Paul

Paul, you've got my head spinning with all this out of sync gibberish.

The whole point of using a clap-board is so you can very easily sync your tracks in post.
It doesn't matter if you start one camera before the other, as long as all cameras shoot the clap-board at the same time.

Then just line up the claps on the timeline.

No precision required....
 

as long as the joints are tight, sound and are staggered - ie, don't cut the wires exactly the same length, as the joints will all be next to each other and the risks of a short circuit go up dramatically.

In other words always cut, say, the positive lead shorter than the negative and the joining bit of wire has a longer positive than the negative. Make sure their well wrapped in tape to prevent shorts even with the staggered arrangement.

This doesn't work for the plugs and switch as all the leads need to be the same length, of course.

The theory as you outlined sounds fine. You should be able to find some basic instructions on soldering on the web and it would be prudent to spend half an hour just practising your technique before attacking anything that can melt. You'll also find that a second or even third pair of hands is required at some point - I use a set of surgical clamps to grip anything really small whilst soldering, you'll probably find a wooden clothes peg will just about do the job at a push.

If your going to be buying a soldering iron for this, don't get anything too massive, the small basic units should be just fine. As I said, a sharp knife and a pair of side cutters are the primary other tools required.

Good luck

CS

Thanks, Chris!

I'll let you know how it goes!

Paul, cannot comment on validity of your idea of turning on both cameras from the editing point of view, but can offer a few bits from the technical angle.
1. Do not be surprised if upon powering both cameras on at the same time they are not in exact sync. The power circuit always has a filter to smooth voltage imperfections (little spikes, noise, etc.). Upon powering, the filter slows down the voltage ramp up by a few milliseconds. If the cameras have slightly different filtering properties due to components tolerance, one of them might ramp up before another.
2. There is no such thing as a 8.4V battery. A Li-Ion battery pack (it has 2 cells inside the pack) is about 8.4V fully charged, about 6.5V discharged. It is traditionally called 7.2V pack as it is the level where it spends most of it's discharge time. If you see the 8.4V marking on your camera, it is a matter of semantics, or maybe the AC adapter is set to 8.4V - it can be anywhere between 7.2 and 8.4V.
3. If you do decide to go DIY route, keep in mind that crossing polarity or making other mistakes due to you being at the beginning of the learning curve is very unforgiving - blowing up a camera is very easy, the batteries themself store huge amount of energy, try not to short them...
HTH