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Bosse

Modulation of GPS serial interface

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Hi Everybody.

I wonder, is it possible to transfer GPS data by simply connecting the proper two leads (TxD and SigGND or something) from a typical GPS-reciver, to the input of the audio-channel of a typical video-transmitter ?

Or does it require modem circuits and stuff ?

Best Regards,

Bosse

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Bosse,

It has been a long time since I worked with packet radio, so someone else may have a better answer for you, but I will give it a try. You would definately need some type of modem. The normal NMEA data from a GPS is at 4800 baud which is probably too fast for the audio channel of of these simple video transmitters. If you can set a lower baud rate on your GPS it might be possible to make it work. You wouldn't use an ordinary computer modem since they are full duplex. You would need something like this Ham packet modem on each end. http://perso.wanadoo.fr/f5rrs/eq_baycom.html

I don't know how good the audio channel is on your video transmitter and receiver, so you may have to run even slower than 1200 baud. Again, it has been a very long time since I messed with this stuff, so hopefully someone else will have a more definative answer.

Jeff

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You need a TNC (terminal node controller). They modulate 300bps to 9600bps data on the audio channel of your video Tx. Several RC-CAM members have done this and there are a few threads on their efforts at the "old" MSN board. Here is one of them: Locar's APRS project.

With luck, one of them will chime in and share more about what they did.

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Thanks for the link Mr.RC-Cam. I just wish there were some way to search the old site. Hopefully this site here will grow quickly to have a good knowledge base to search. Thanks again for setting up this great site.

Things have sure come a long way since I was into packet radio about 20 years ago. And I thought I didn't need audio on my video transmitter. To sumarize some stuff from the other board.

This page has a lot of info on APRS. http://web.ew.usna.edu/~bruninga/aprs.html

Here is a cheap APRS encoder that goes inbetween the 4800 baud output of your GPS and the audio input of a transmitter. http://www.byonics.com/tinytrak/index.html

And here is another one that is smaller, and also has analog and digital inputs, so you can include battery voltages, altimeter readings etc. in your telemetry transmissions. http://web.usna.navy.mil/~bruninga/mim22.html

To receive, you can get programs that use the sound card on your PC. You just connect the audio out of your receiver into the mike or aux inputs on your sound card. Or you can get a TNC which connects between the audio out of your receiver, and the serial port on a PC or PDA.

Hopefully someone who has actually used this stuff will chime in.

Jeff

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Hi all

I have used Tiny track II , UIVIEW and a tiny FM 144mhz tx. I think this

is not good enough. It is so many really nice telemetry programs, with 3d view

and hi-speed tracking. All of this programs needs pumping in NMEA sentence

as fast as GPS can deliver to function properly.

Is it possible to make a simple interface just deliver audio of the serial

from the GPS and also decode it on a laptop on the ground. On the TX/RX

side it should be possible to use a audio channel (videotx) if it is clean enough.

Maybe some of you with good knowlegde can figure something out.

Per

http://home.online.no/~la9ex/

Edited by PMT-Flyer

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Is it possible to make a simple interface just deliver audio of the serial

from the GPS and also decode it on a laptop on the ground. On the TX/RX

I am currently trying to build an improved version of MIM-2 telemetry module. I like the idea of MIM-2 but it's too slow. It seems to be possible to hit 9600 bps over audio band of the video connection using FSK modem. If anybody has any ideas how to get 10 analog readings plus GPS info 10-20 times per second using audio subcarrier please let me know.

Regards,

cyber-flyer

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If anybody has any ideas how to get 10 analog readings plus GPS info 10-20 times per second using audio subcarrier please let me know

Hi.

I *think* there is a theroretical limit (according to Nyquist's Theoreme) of less than 10 kbit/sec if the bandwidth is 20 kHz, wich I would concider being a good audio-channel, at least in respect to bandwidth. I could have the numbers wrong but the theory should apply if used right.

http://www.wikipedia.org/wiki/Nyquist_theorem.

I guess a separate radio-link for telemetry-data is the right way to go, if demands are as high, as quoted above.

I was hoping there would be no need for FSK (wave form wrinkling), if link speed is kept low, 1200 baud perhaps. ;)

I want to start off by going quick and dirty. I would be happy with readings every 3-5 seconds or so.

Best regards,

Bosse

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On a second thought, I think the theoretical maximum link-speed would be 40 kbits/sec on 20 kHz bandwidth.

That could explain why two modems connected to standard telephone lines can not communicate faster than 33,6 kbit/sec.

Best Regards,

Bosse

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Modern modem designs do not use typical frequency (FSK) modulation. If they did, telephone lines, which have under 3Khz bandwith, would limit us to the lowly 1200 baud domain (where we were stuck for years). Instead, today's modems use phase/amplitude/frequency detection methods (or subsets of those parameters).

With a 20Khz audio channel, and QPSK modulation or the like, the baud rates would be enormous. So, it would be possible to squeeze a barnyard of data into very fast rates.

My point is that if you need lots of data sent over the camera's audio channel then it is possible. If all you need is 9600 baud, then standard FSK techniques will work. Beyond that, the higher tech modulation schemes will take care of the dirty work. To keep it simple, 56K modem modules from Cermatek or Xecom could be forced to work in our wireless video application.

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It seems to be possible to hit 9600 bps over audio band of the video connection using FSK modem.

Hamtronics sells a 9600 baud TNC that is a kit. The board is 2" x 4" but it looks like it could be cut down. http://www.hamtronics.com/data_9600.htm

Another interesting choice is the kit from TAPR. The nice thing about it is that it is a full duplex modem. So, your Rx decoding is easily taken care of. Sadly, this board is big. http://www.tapr.org/tapr/html/F9600.html

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I use the tiny track from byonics, and use my audio channel on my a/v tx. On the ground, it plugs into the mic jack of my laptop and I run AGWPE software that links the mic to a com port. In UIview or WINarps, just select com 1. Works pretty good if the audio signal is clear. Mine has a 2hz refresh rate on the gps data and moving map display. (gps location, bearing, speed, altitude) It can be turned up to 1hz, but I was getting errors.

Recently, I was so busy watching the video and telemetry, and aiming the antenna, I forgot that I had to fly :blink:

Nice crash.......

Another nice prog is baloon track, but it doesn't support AGWPE.

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My thought is that, if you can read some of the informatrion of an analog signal (wich I know is possible), by means of a digital com-port, with only passive external components and proper cabling, then the reverse operation should work as well.

Let it be with low link-speed but it should work.

Best regards,

Bosse

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I loooked around a bit to get a better hold on some of the modulation techniques.

FSK should be interpreted as Frequency Shift Modulation, wich is not same as FM (Frequency Modulation).

PSK is a quite recent modulation technique that involves manipulation of the wave forms to encode information. (Before, I thought that was called FSK, one never stops to learn)

QPSK is a development of PSK and is what is used on analog 33,6 kbit/sec datalinks over phone lines.

Like you say, bandwidth of most telephone lines is slightly less than 3 kHz. I thought it was higher in fact.

Note that 56K modems relies on digital phone lines and special analog/digital modem hardware at the other end. Two of those modems, even if they are 56k modems, will be limited to 33,6 kbit/sec, when used toghether.

If someone manage to get two 56K modems communicate at higher speed than 33,6 kbit/sec, then I am very interested to hear about how it was done.

I counted the number of characters from my old Garmin GPS 38.

It was roughly 600 bytes per second/dataset. That equals approximatly 6 kbit/sec, (baudrate was in fact only 4800 bits/sec, I guess I did not get samples every one second after all) wich would be 60 kbit/sec if updates were to be transferred ten times per second.

Best Regards,

Bosse

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The CMX469 would work fine. Pair that up with a MCU and you would be able to handle 4800 baud on your Tx and Rx ends.

But for the Tx side, you could eliminate the modem chip and do most of the work in the MCU. A 4-bit resistor based D/A ladder and some code would handle the tone generation. I think that is what Cyber-Flyer is looking for.

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Thank you everyone for feedback.

Especielly Per, who seems to have found excactly what I need. :D

If one wants to append data to the GPS dataset (to send it over same audio channel), I guess it would be possible to intercept it and buffer it with a microcontroller, append digitized signal levels from various sensors and then re-transmit it from the microcontroller.

Best Regards,

Bosse

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Per,

Have you had any success in contacting your CML distributor (wich should be "EU components").

I talked to their Girl in Stockholm, guarding their phone. She couldn't even confirm they are in fact a distributor of CML components here... At first she said (after some talk with some person sitting next to her, they would indeed sell to private persons, depending upon what products in quesion.. ?? But when I mentioned the particular chip I wanted, She said they don't have it and they don't sell to private persons anyway, Djävla skragga (darn scrag...)

They do not reply on e-mail either...

Maybe they can only be arsed to reply on requests from FMV or SAAB ? If so, they are wrong.

It would be quite annoying for me to be forced to buy the sodding chip from UK when it is supposed to be available right here...

ELFA is expensive and they do not sell "odd" chips but at least, they got some manners...

I guess I will have to ask CML in UK to send me a chip and I will have to muck with odd payment procedures...

Best Regards,

Bosse

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A few years ago I tried to get some CML components for a commerical telecom design. Obtaining samples from them turned out to be a huge hurdle and I eventually used competitors' ICs.

CML has a very interesting and unique line of IC's. Too bad they are not easier to order from (distributor choices are very limited).

BTW, I split the A/D discussion into its own thread in order to keep this one "on-topic."

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