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Extremely simple high speed data "modem"

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Hehe, I've finally tried what I was thinking about since one year ago. How silly we can be not to take 1 hour for something interesting and useful....

So my idea was simply to throw RS232 data into a video TX, and trying to get it back on the ground. There are 2 ways, the audio channel and the video one. I was doubtful about the ability of the audio one to work because of the low bandwidth. But the (theoretically) 5MHz of the video one should be ok.

The set-up was like this:

A microcontroller was sending a 0-255 loop out of its RS232 port (logic level). That signal was directly fed into the input of the TX. I've connected the scope on the RX output to see what happened.

I've tested a 50mW standard TX, and a tiny one like we have in our 10mW kits (in the same case than the camera).

The audio channel definitively has low BW. The signal gets distorted quite quickly, but I think it should work maybe up to 2400-4800 bps.

Not precisely tested yet.

On the video one the signal looks VERY good, with really vertical edges. What is surprising is that the effect of the decoupling cap can still be seen on the L and H states even at high baud rates. Never mind as this is digital :P

As the output signal of the 50mW's receiver looked not too bad, I've tried to feed it directly into the PC's RS232, and it worked!! I've been able to transmit at 38400 bps with no errors!

On the 10mW one there was a bias that prevented the PC to understand the data. It worked for 1 sec when powering up the RX with the TX on, but the levels definitively are not adjusted.

Conclusion: I'll go on trying on this way, and make a small circuitry to reform a more RS232-compliant signal out of the RX. The good solution would be to use a zener and a normal diode to clamp the signal between 0-5V, and then to feed it to a MAX232. I think that to make it simpler and cheaper 2 transistors would be enough, by transforming the signal into +/- 6V with a push-pull style amp, powered by the 12V of the TX and using a voltage divider to get a centered ground.

I haven't been able to go faster because the microcontroller already was at full speed :P

I'll try to get a second PC to try higher rates, but seeing the result I think it would be possible to go as fast as the PC will hold, which often is 115200bps.

That would be VERY interesting, as now video sets are extremely cheap compared to radio modems... and if a 115200bps rate can be sustained with good range a simple setup would beat the $1000 beasts... (I had both ends in the same room, so the range was not taken into account there).

As usually RXs are big, we'll have to use the small felsweb sets to make our bidirectional links :P

Well, that would use 3 channels for uplink, data and video downlink, but at least if it works.... I really hope that I can get something decent out of the audio channel, as this would save a TX/RX pair. At least when no high rates are needed.

Hehe, his means eliminating the tinytrak, and simply connecting the GPS output to the TX, RX output to the RS232 of the PC, and here you go with VisualGPS without having to bother with the soundcard drivers... neat! :P

Well, I'll post more info when I've continued the tests..



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That sounds very promising. There have been several success stories about using the audio channel for data with very simple interfaces.

On this forum there is a old post about this. Also, I believe Cyberflyer reported how his worked. I played aound with the data on the audio channel too and confirmed it can work with not too much trouble. I have also run across other discussions that mentioned useful RS-232 data rates, but I do not recall any that went beyond about 24K baud.

The video channel would probably support megabit data rates (and higher) of unmodulated serial data (wirelss video is just a form of high speed analog serial data). As such, it should easily allow you to transmit unmodulated 115K baud with simple external circuitry.

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That's what I thought. The problem then becomes the need of a MCU which supports that rate as I've never seen anything usual above 115k. That's already quite a lot though.

On the audio channel I bet it will depend on the TX. On both 10mW and 50mW at 9600baud the signal was already very dirty, it was even difficult to see the edges.

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I have to admit that I never tried direct serial connection through audio channel.

I use 9600 bd modem and it works fine with Felsweb's audio.

Video channel may have enough bandwidth for direct serial high rate data connection, but why not use Wi-Fi connection right away?

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Yes, a decent modulation must be much better, but of course less simple on both sides. I'll try with my felsweb also when I have time.


1) Power is limited to 100mW on all devices (at least in Europe).

2) When you've managed to communicate with a PCMCIA, USB or whatever type Wi-Fi adapter with a microcontroller please let me know :P

3) Implementing the full protocol to be able to transmit data directly to the PC must certainly be a real pain!

Not to say this is not possible because it surely is, from my point of view it will take too much development time compared to simpler ways which are known working. At least for the moment.

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  • 7 months later...

I would add a series resistor (like 10k) inbetween, which will limit the current in the input. Another problem which could happen is that it would saturate the modulator's input and cause it to drift too much from its frequency. But I'm nearly sure it will work.

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Argh, you got me in again.

OK, forget about the resistor :lol:

I've just given it a try: hook up the TX line of a PC's COM port into the video in of a BW 100mW TX, and plugged the video out of the receiver (the dual output one) into another PC's RX line (was I supposed to do this? :huh::P ).

Directly, zero external component.

115200bps transmission works perfectly, even with a WLAN card's antenna 5cm away from the receiver's antenna :)

Stupid I don't have faster COM ports...

I've had a look at the signals. The PC's +/-10V get clamped to +/- 0.8V by the TX, and the receiver puts out 0 / +3.4V.

That's odd that the PC accepts the 0V, as the standard defines at least +/- 3V.

BUT, it works. To do things properly, I would advise using a MAX232 between the receiver's output and the PC's COM port. So the levels would be correct. It worked on the 2 PCs I have here, but other ones could have UARTs that don't accept it.

I'll have to do some range testing now, as I have to prepare something to test the Aerocomm modules I'll take the opportunity to compare :)

Edited by Kilrah
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  • 2 weeks later...


first of all, i've to say that this is a great forum.

I'am still reading since weeks now but this is my first post.

Regarding high speed wireless data transmition.

I think there is a way to do if by WiFi.

Look here:


As i've heard they are not extremly expensive so this might be a option for semipro




PS: Sorry

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I wonder if these Wi-Fi devices are still reliable over 1 mile of range... :unsure:

And it would also be needed to check if all the protocol stuff doesn't insert too much delay for real-time control. But that could be an idea, especially for $120. I think the main concern would be range, with 25mW only.

Also consider you have to power it and interface to it with 3.3V.

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