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Need Modem advice/help...

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I found this on R/C Groups...

"If you will be using a wireless video Tx on the R/C model then there may be no reason to employ any modulation schemes at all. The audio channel on these often have very high bandwidth and good DC fidelity. You can merely connect your TTL level RS-232 output to the audio input (attenuate as needed). On the Rx end, just level shift and buffer the signal, then feed into a microcontroller or host PC's serial port. Baud rates of 2400 to 19.2K are possible on some systems using this simple Muntz'd technique.

This trickery was reported by a fellow on the RC-CAM forum. RC-CAM"

I want to get serial data to Google Earth to take advantage of the new realtime GPS feature. APRS and TinyTrack is working for me but I want a realtime RS-232 serial stream to feed to other, non-APRS programs.

Can someone please point me in the direction to see what other's have done. I only need unidirectional telemetry data from my UAV (video w/ OSD and moving map) so I don't want to add extra transmitters (just use the two channels).

Thanks. Jeff

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I prototyped this awhile ago using the audio channel. It was a quickie proof of concept project. Summarized as follows:

I attenuated the microcontroller's TTL serial data and fed it into the audio input of an AirWave Tx. I set up the input level so that on the Rx end the signal had good rail to rail swings.

To condition the Rx end, I buffered the audio with a op-amp and fed that into a AC to DC rectified filter that gave me a DC level that was proportional to the Rx'd serial signal levels. That was divided by two and used to set the threshold on a comparator, which became my data slicer. The output of the comparator was fed to a MAX232. I tested it at 4800 baud by sending large text files. They were error free. In case you ask, I never went to a high baud rate since I only needed 4800 baud.

However, another fellow tried something like this and had some errors. So, he has devised a simple modulation scheme to resolve the issue. See http://www.rc-cam.com/forum/index.php?show...t=0entry11934

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Thanks for the quick reply. I follow what you’re suggesting and I will attempt to breadboard it up even though we’re at the outskirts of my electronics knowledge (I work in high voltage and make most of my electrical connections with a 15/16” box wrench).

I’ve also posted a request for details on the other guy’s approach. Thanks.

Using the audio channel seems like such a no-brainer; I don’t know why people choose APRS over a serial connection to a GPS. Is adding a Maxstream SO simple that it makes wasting an audio channel OK?

I’d rather pay $50 for a simple serial modem set than a TinyTrack3 (Great product, I have more than one. They are not realtime however).



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I will attempt to breadboard it...

Good luck with it. If your audio is AC coupled inside the tin sardine cans, and you cannot circumvent that, then it may be necessary to ensure that there are no pauses in serial data. If the data is from a GPS then I suspect you will be able to get things to work (they have almost no time for long pauses in between sentences).

I’ve also posted a request for details on the other guy’s approach.

His NRZ modulation scheme should be very robust. Hopefully he can help you out.

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