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Wireless Helmet Cam - How To (Read 1227 times)
GrizzlyGuy
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Wireless Helmet Cam - How To
05/06/07 at 16:25:22
 
I finally finished building my first wireless helmet cam system. First off, a big thanks to Pa Pa Jack who found the video radios and got this whole movement started!   Smiley

Here is the finished transmitter assembly sitting next to my helmet:

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And here it is next to my Rack Cam setup:

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Here is a look inside with the cover removed:

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And here it is with the circuit board pulled out into maintenance position:

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I decided to put pin & header connectors on the lines from the transmitter module, and on the cable that goes to my helmet cam. This makes for easy maintenance and swapping-out of the helmet cam cable for different helmet cam models.

I actually have two helmet cams from the same manufacturer (helmetcamera.com), but they have two different connectors since one was generation II while the other is generation III. So I actually have two different cable assemblies made up already.

The transmitter works fine, here it is undergoing some bench testing. Notice that the picture on the LCD of my HC-96 is coming from the helmet cam connected to the transmitter (cam pointing at the Sony tape). The receiver is connected to the HC-96:

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The key ingredients are the 2.4 GHz transmitter module and receiver from Future Hobbies:

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The two 3.5mm phono jacks you see in the bottom of the transmitter are for audio in and video in/out. The video in/out can be used with a field monitor to check the camera shot right at the transmitter. It can also be used to feed in video from a different camera, such as the one that Pa Pa Jack is evaluating (or from a video cam like the HC-96 if you like).

If you are an electro-geek, you'll know the other parts you need. If you aren't, I'll hopefully follow up later with a parts list and construction details.
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Pa Pa Jack
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Re: Wireless Helmet Cam - How To
Reply #1 - 05/06/07 at 17:43:31
 
Thanks for the credit.

Very nice professional setup. I am glad you offered to do a tutorial, I was afraid someone would ask me how i did it and after all the tests and smoke, I'm not sure I remember how. Grin

But again, I have to ask, WHERE"S THE VIDEO? Rolling on floor laughing
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GrizzlyGuy
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Re: Wireless Helmet Cam - How To
Reply #2 - 05/11/07 at 09:40:52
 
I tested the wireless system on a ride Tuesday while filming for my Multimedia file viewing and clickable links are available for registered members only!!  You need to Login or Register!!
. I rode hard and the good news is that the transmitter survived all the bumps, jolts and vibrations while mounted on my rack.  Smiley
Here is a picture of it on my rack. The extra piece of metal that you see is a piece of sheet steel that I bent and bolted on for extra heat sinking (the transmitter and regulator get hot):

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The bad news is that I don't think this is going to work for us.  Sad The reason is something called multipath interference.

What happens with radio waves is that the main signal will travel directly from the transmitter to the receiver over the shortest path available. But other signals will leave the transmitter, bounce off of something, and make their way to the receiver as well. Since the signals took different paths that have different distances, and all signals travel at the same speed (speed of light), it takes the reflected signal a bit longer to make it to the receiver than the main signal. This causes the two signals to be out of phase at the receiver, and they can partially cancel each other. When that happens you will see dropouts or noise.

If the transmitter and receiver were both stationary, this is really no problem. You can adjust the positions of the antennas to get a solid signal. But since we are riding around, the transmitter and receiver are both moving in relation to the environment. So the signals reflected from trees, rocks, etc. are constantly changing. No matter how you have your antennas oriented, there will still be multipath interference.

Here are some 3 screen shots from Final Cut Pro showing my 3 cams. Each screen shot is one frame later in time than the prior one. The wireless system was connected to my rack cam, which is the video image in the top-right. You can see horizontal lines in each of these frames. Those are due to the effects of multipath interference. The wired helmet cam and the handheld cam sitting on the ground (which I run over in this segment) show no such lines:

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Here is another one from the same area which shows an even worse line:

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What I found was that if I was riding through an open area with no nearby trees or boulders, the interference was at a minimum. Much of the video in these areas is mostly line-free. But if I was in a forested area, the lines got really bad. This is another confirmation that the cause of the problem really is multipath interference.

So what can be done to fix this problem? Nothing, really. We are at the mercy of physics.  Sad

It is possible to reduce the effects (but not eliminate them) by a variety of techniques that really aren't practical for ATVs. One way would be to use a diversity receiver with multiple antennas. This type of receiver constantly scans between its input antennas and chooses the one with the best signal to feed on through to the video camera. So while this would be an improvement, there could be multipath effects on ALL the antennas at a given time. So the effect would be to choose the lesser of two evils.

Another way to reduce the effects would be to mount directional antennas on the quad for both the transmitter and the receiver. Since most of the signal would then be directed directly at the receiver, there would be less signal available to bounce off of trees. And what did bounce off would be seen as weaker by the receiver since it is coming from the wrong direction.   Although this would improve things, riding around with two directional antennas rigidly mounted to the quad wouldn't be the most convenient thing to do, and the antennas might even rattle and vibrate into junk.

Yet another way would be to use digital signal processing to compensate for these effects. The expense of this kind of stuff is far beyond what any of us would be willing to pay, and building your own DSP system is no small task. As in unless you are an Electrical Engineer with years of experience in that particular sub-field, don't go there.  Wink

Yet another way would be to use a different frequency range. My system is operating at 2.4 GHz. If I instead chose a system operating down at 432 MHz, it could be a different story. Since the wavelength down there is longer, the reflections could be less (takes a relatively larger thing to reflect a radio signal effectively). So if you really want to stick with wireless, that is the route I'd experiment with. Even better if you could go all the way down to 50 MHz or so, but video transmissions are illegal down that low in frequency. Unless you happen to be a commercial broadcaster licensed to operate on TV channel 2, that is. Wink

Note that using higher power wouldn't help. All the signals would simply get stronger by the same amount, so you'd still have the same effect as these now-stronger signals interfere with each other in the same relative amount.

Now for some good news. If you were interested in this primarily for a wireless rack cam like me, there is a simple, inexpensive and effective solution: buy yourself a 10-foot long A/V cable and call it a day. I brought one with me just in case, and used it for the second half of my ride. The rack cam video was then fine.

I am going to make one improvement to my cable to reduce wiring even more. I'm going to chop off the left/right audio RCA's, connect those two audio lines in parallel, and use them to carry 12V from my cam pack out to my rack cam. That eliminates needing to run a separate 12V line to it, or have a separate battery for it.

So there you go. It was a neat geek experiment and any time you can learn something from an experiment, we geeks think it is a good thing (whether or not you come out with what you wanted). I may just put my transmitter out in the Grizzly Den so that I can keep an eye on what the Grizzlies are up to in there during the long winter.  Grin
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