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Audio track as modem data NMEA


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#1 ThomasScherrer

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Posted 13 February 2007 - 01:50 AM

Adding a modem to the onboard GPS and OSD system,
and then transfering the unaltered GPS NMEA stream at 4800 baud
using the audio in the 2.4GHz video transmittersystem,
the idea is to mount a tomtom pda next to the tv monitor,
so it is harder to get lost when flying at a new location.

I find it hard to get the CML ic type FX469,
any ideas about how to get a handfull of them in SMD ?
or any other cool modem ic you can recommend please ?
Thomas Scherrer OZ2CPU www.webx.dk

#2 Kilrah

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Posted 13 February 2007 - 01:53 AM

Quite a few people have done that in the past years, mainly using this:
http://byonics.com/tinytrak/

and grabbing the audo on the ground with a PC line in.

#3 ThomasScherrer

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Posted 13 February 2007 - 02:32 AM

thanks for the link, a cool product by the way,
but this is not what I want,

I want a 4800 baud downlink modem,
that sends down the nmea UNMODIFIED !
so I can input it in realtime into a tomtom pda on the ground.
Thomas Scherrer OZ2CPU www.webx.dk

#4 Kilrah

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Posted 13 February 2007 - 02:49 AM

Instead of Tomtom, look for "GPS Tuner".
That thing lets you put any image as a map and works very well. Tomtom is very good for street navigation, but it won't accept to put you anywhere else than on a street even if you're in the middle of a field, it will choose the nearest road...

#5 Mr.RC-Cam

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Posted 13 February 2007 - 07:51 AM

There is enough bandwidth (>20KHz) in the typical audio channel to skip the 4800 baud modem. Just level condition the GPS's serial data and feed it into the Tx's audio input. On the Rx end, just clean up the edges and translate to the signal levels you need. Very simple hardware, very simple solution.
- Thomas

#6 Kilrah

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Posted 13 February 2007 - 02:10 PM

I had done that with the video channel, that worked well even at 115kbaud. I was having much more trouble with the modules I was using with the audio one though, even at low speeds...

#7 ThomasScherrer

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Posted 14 February 2007 - 05:01 AM

Yes I also think its a cool idea,
but serial data needs DC !
and all radio links are not DC coubled
I have measured the Audio signal chain to 100Hz to 10khz at -3dB
maybe I am lucky since the serial from the GPS is sending almost constantly.

I have another solution, add one more component in each end,
a biphase encoder/decoder, now I have DC handeling.

Edited by ThomasScherrer, 14 February 2007 - 05:03 AM.

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#8 Kilrah

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Posted 14 February 2007 - 06:01 AM

but serial data needs DC !

We don't care much. As it's more the transitions that matter rather than the levels it doesn't hurt if they aren't really flat. At least it worked perfectly for me, the signal being fast enough not to see the AC coupling too much.

That's why Mr.RC-Cam mentioned cleaning up the edges and restoring the levels. When I did my test it was even more "rough", that is TTL serial out directly into video TX input, and RX output directly into the PC's RS232 RX. And that worked ;)

Edited by Kilrah, 14 February 2007 - 06:02 AM.


#9 Lars DK

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Posted 14 February 2007 - 02:46 PM

:D :D Thomas jeg skulle aldrig have givet dig linket til dette fora, nu hesker du jo herinde :D :D

Edited by Lars DK, 14 February 2007 - 02:46 PM.


#10 ThomasScherrer

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Posted 16 February 2007 - 03:23 PM

YES; thanks Lars..

Kilrah: I tried to modulate the RS232 from the GPS directly into the audio,
and on the RX side clean it up using schmittrigger inverter and a MAS232,
it looks like crud on the scope and 9 out of 10 chars go fine, that is the best I can do this way, so it is useless..

We have however created a special firmware in a tiny2313 microcontroller,
this will machester encode end decode, and generate a constand biphased tone, when no input chars are present in the pauses between nmea pakages.
now it is rock stable !
Thomas Scherrer OZ2CPU www.webx.dk

#11 Mark Harris

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Posted 16 February 2007 - 03:30 PM

Glad to hear you got it stable, sounds very interesting what your doing!

#12 Mr.RC-Cam

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Posted 16 February 2007 - 04:00 PM

It's great to hear you found a solution. Did you implement it as an on/off keyed (OOK) sort of Manchester modulator?

BTW, the various little 8-pin MCU's are life savers! They seem to solve all of life's little problems.
- Thomas

#13 mieczotronix

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Posted 19 February 2007 - 06:39 AM

I did something you're talking about using MX614 chips
They are hardware 1200 bps modems needing very few external components (actually 4 caps and a crystal). You only need to connect it to uC USART and it works perfectly.

You may see/hear what I've been working on here:


http://www.youtube.com/watch?v=kty8jOsFFmI

oops, sorry, I haven't noticed you've mentioned those chips.
I use the same controller (Atmega8) to read data from GPS (4800 bps), process them and output them to MX614 (at 1200 bps), to monitor voltages, ppm signal (currently working on it) and some other stuff maybe.

BTW
It seems that I've been doing almost the same stuff you did. I.e. I also dug into the AVROSD avrfreaks project and have succesfully changed letter size (double height) and ported it to Atmega8. But I'havent reached the NTSC->PAL issue yet, so your code will save me some hours.

Edited by mieczotronix, 19 February 2007 - 10:59 AM.


#14 jparisse

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Posted 22 February 2007 - 05:05 PM

We have however created a special firmware in a tiny2313 microcontroller,
this will machester encode end decode, and generate a constand biphased tone, when no input chars are present in the pauses between nmea pakages.
now it is rock stable !


Are there any details you can share? I'd like to also see your technique and attempt to duplicate it. Thanks.

I'd like to feed the serial data to Goofle Earth's realtime GPS feature.

Jeff ;)

Edited by jparisse, 22 February 2007 - 05:06 PM.


#15 mieczotronix

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Posted 22 February 2007 - 10:23 PM

Only now it occurred to me that you did this without FSK (just a manchester-encoded waveform, right?)
Yeh tell sth more, that would be pretty interresting
What kind of conditioning circuit you need after the receiver to convert audio signals to TTL?
What transmission speeds you are able to maintain with your solution?

#16 ThomasScherrer

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Posted 23 February 2007 - 01:50 AM

the manchester code has a zero dc value, so it is good for radio links,
the sw is made my dzl.dk the hardware needs only a resistor and capacitor on each end !
the serial must be 4800 baud
the manchester is 5000khz this is made a bit faster than the rs232 on purpose
to be able to be ready for the next start and to be able to create leadin and detect leadin,
the audio channel in my 2.4GHz link has 10khz bw at -3db !
so 5khz is no problem,
but rs232 is impossible to transfer on radio links without using a modem of some sort.
it is not tested enough to share, if we share it before it is perfect, we will have to waste too much time on support questions, on a products that is given away for free, not uptimal for my time , you seee,.. i hope..
Thomas Scherrer OZ2CPU www.webx.dk

#17 ThomasScherrer

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Posted 23 February 2007 - 05:28 AM

here you go, now it is all for free !
http://dzl.dk/projec...odem/modem.html
Thomas Scherrer OZ2CPU www.webx.dk

#18 jparisse

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Posted 23 February 2007 - 09:35 PM

Thank you Thomas...

So.... You're into Tesla coils? I'm Jeff Parisse, owner of kVA Effects. Are you a member of the pupman list? We make giant Tesla coils for advertising and entertainment.

Check out www.reinmedia.com/kva it is our new site about to go on line. We can normally be found at www.teslacoil.com

Back to the modem: The killer application that will cause a demand for this circuit is the realtime GPS feature addition to Google Earth. This lets Google Earth to be used as a moving map program with TONS of advantages over other map programs. I believe you should commercialize this for ALL the A/P, RPV and UAV guys that have video transmitters. Let me know what you think of such an idea.

Jeff W. Parisse
Director
kVA Effects
www.teslacoil.com

#19 ThomasScherrer

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Posted 23 February 2007 - 11:29 PM

Tesla:
yes, me and dzl do sometimes meet up and do crasy things, last year we did a puplic show, with a tesla coil it made 1.5 meters sparks and nice blue flames,
100 of people was screaming and clapping, that was funny.

Modem:
now the model schematic and hex files are out on the web free to dl
and free to use for all. if someone want to sell it as ready made, they are welcome, if so, please send 2% of the money to dzl for the sw work :-)
we dont have time to sell more things, we have jobs and kids and 1000 of funny projects, all we care about is to have max time for fun.

People that construct cool RC planes, with cool video gear,
will be able to make an AVR programmer,
and do this simple hw all by them self, it is unsupportet, and we will not answer questions and we can not help if they can not find out how it should work.
if no skils, ask a dude near your self to program and test it :-)

By the way I have no adea about all this google earth realtime function ?
it is an option you need to purchase ? how much ?
can I just input the NMEA into the serial port of an online PC ?
and have it track on the screen on the globe in any zoom ?
is it possible to do something like this if the PC is not online ?
Thomas Scherrer OZ2CPU www.webx.dk

#20 Kilrah

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Posted 24 February 2007 - 06:59 AM

You indeed need to buy the Google Earth Plus version to connect the GPS.