Video and comms question

You could insert data in the first few video lines of the video signal. See LM1881 sync stripper data sheet, for the basic circuit. Use a switched PNP current source to drive the coax with data at a suitable time. You could put say one or two bytes in a video line, quite easily, but with a bit more care you could get say 10 bytes per line.

Because video is normally a 75R source and 75R termination you need to drive approx 37R to say 0.5V. If you require two way data you could use the odd/even flag from the

Obviously you will need a micro at either end, which are locked to the video, and a simple line counter to select the correct line, will simplify the software.

Elantec/intersil make better video sync strippers that the LM1881, that may offer a more flexible approach

martin

Thanks Martin.

As you may have gathered, I am not skilled in the art (or mystery) of electronics (I am a mechanical guy) and will need to gather more info and assistance on taking this further. One issue is going to be cost but I guess there are some pretty cheap micros or PICs about that could be considered.

The other issue is going to be how effective the data speed will be given that I am trying to control something dynamic (it's not a vehicle or airplane but the action is similar). I can't see that having a bidirectional link is that important although could be useful in the future.

I had been hoping that I could just load the 27MHz (If that's what it is) RC control straight onto the video signal. How naive is that?

Thanks again. Any further assistance always welcomed.

Flint

you could feed the 27MHz into the video feed, but i can't really suggest an optimal method. Notch filters may be rquired to prevent it upsetting the video.

But back to basics:

How fast is the data going, I dont know much about RC protocols, and i'm too lazy to google

martin

Didnt you say "video feedback" ?? so RC to the equipment, video back from the equipment ?? you cant piggyback the RC data on the video that way if the signal sources are at opposite ends of the cable :)

Get a camera with modulated channel 2/3 output, and make sure your RC is running on a VERY different freqency.... you said 27mhz RC ?? thats far enough away from TV freqencies at 54 to 66 mhz for channels 2/3

setup low-pass/high-pass filters at each end to isolate the RC/video signals .. you will probably need to attenuate the RC signal a LOT since its designed for over the air rather than wireline. Set the frequency split at about 40-45 mhz -- dont have to be horribly accurate, nor do you need a filter with sharp roll-off. The technique is called duplexing and ham radio repeaters do the same thing so they can transmit and recieve at the same time over a single antenna.. there are lots of online resources including applets that will design the filters for you.

you could even feed DC power over the same coax along with the video/rc signals if you like :) an inductor at each end to block the RF from the DC powered circuits, and a capacitor to couple the RF to/from the coax

I just can't restrain myself: TV rcvr--Servos

All you need is somebody to design the stuff for you, or tell you what you need to buy. :-)

And just FYI, RC protocol is almost audio, so no worries. :-) (I once had to generate servo control signals for a robot, and so I went down to Futaba and asked the tech. It's something like a 1-20 ms pulse at a 100 ms period, or something way slow like that. Please don't quote my numbers!)

Good Luck! Rich

Some days you just can't see the wood for the trees

martin

I'd be thinking maybe the twisted pair is about 1000X simpler and cheaper?. (wouldn't want to knock one up without a customer underpinning the week's work :)

Why not, exactly?

Thanks, Rich

Easy to do, if you understand what to do. There is endless combinations on what you can do. Bandsplit is just the start, directional couplers and tx/rx bridging add to the things that can be done. You can sucessfully send power one way, voice, data, and video both ways all at the same time.

Just the same, i would have just used used an additional RF link for the video, just like the RC flyers do.

because if I understand correctly -- the video is sourced from the operational equipment end of the proposed cable -- and the RC is sourced from the control at the opposite end -- you dont have both signal sources at the same end to combine them

also -- RC is a continuous signal -- would be a huge problem to pick up the signal from the RC transmitter and convert it to something that can be combined with the video signal -- would be better off sticking the RC on an audio subcarrier IF the RC signal and video were sourced at the same end of the cable -- but that doesnt appear to be the spec -- RC to equipment, video feedback from equipment

what have I been saying from my first post ?? every one else has been talking about embedding the RC signal in the video

They do that with stinger missiles, using a very thin coax.

definately.

both could work.

ordinary video occupies a basically the frequencies from 0 to 5Mhz anything above that is available for signalling.

Bye. Jasen