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ThomasScherrer

Audio track as modem data NMEA

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In all the recent A/V systems I've dug into, the audio sub carrier adjustments have gone away. I think in part because these designs now have ceramic filters that are adequately accurate right out of the box. But, maybe your Rx's have an adjustment for tweaking the sub carrier, so look for some coils that may do that. There's usually a variable cap for the local osc PLL -- don't touch that if the video looks good!

There are no standards when it comes to sub carrier use, so incorrect mixing of video equipment can cause problems. So if the equipment is incompatible then goofy things can go wrong, including problems with audio.

That has me a bit worried. It suggests that the two Rx's are not the same design (I understand that these are "identical" models, right?). Pop the sardine can lids off and show us some clear close up photos of the two unit's guts. Maybe we can see that they are not the same, even if they are advertised as such.

I agree that is not acceptable. These A/V designs are mostly hand-me-downs that have roots in consumer A/V sender applications. Most are designed to provide high bandwidth audio. The basic designs are good for about 10Khz, but some go up to 30KHz and higher. BTW, the 2Khz BW you got would sound fine for voice apps. I suspect the workers that perform the simple "can you hear me now" tests at the factory would not notice that it was defective.

Well the receivers are not identical. The good one is a "portable" version with a build-in battery, I bought this one 1,5 years ago. The bad one is a "standard" receiver and I bought this one half a year ago. The transmitter was bought at the same time as the the bad receiver, so they should be compatible.

What is interesting is that, while the video looks good, the image from the old good receiver looks "better". It looks brighter while the video levels are the same. My USB video capture device shows the image of the new receiver is "darker", my goggles do not show a difference. Could it be that also the BW of the video is reduced and that this affects the colors?

When I open them I see they are both based on the same RF module. I can easily open the sardine can on one side. For the other side I would have to desolder it from the mother board. Unfortunately, this does not show the components, we can only see that the boards are from a different revision.

post-2786-1215959589_thumb.jpg

Edited by FredericG

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On the other side of the mother board there is a small pot. What would that be for? It is also on the mother board that the switches are to select one of the 8 channels. How does that work?

post-2786-1215960057_thumb.jpg

Would it be a good idea to test with another channel? I know a lot of questions :)

Thanks,

Frederic

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Well the receivers are not identical. The good one is a "portable" version with a build-in battery, I bought this one 1,5 years ago. The bad one is a "standard" receiver and I bought this one half a year ago.

Since they are not identical designs, the performance may indeed vary. Lawmate sometimes does odd things that really make me wonder what they are thinking. For example, your old portable seems to have a video level pot in it. The new one has a location for the pot. But they stuff a fixed resistor. It makes me want to slap them silly.

What is interesting is that, while the video looks good, the image from the old good receiver looks "better". It looks brighter while the video levels are the same.

Are these the same two receivers we discussed in the recent Oracle troubleshooting? If so, then the brightness difference is probably due to a mismatch in the luminance levels. Although you had "1V" video levels, the syncs were stunted and the video luminance was increased to make up the shortfall difference. So, in your comparison, instead of looking at the overall video waveform, compare just the active video region (bottom of black to top of white); Ignore the syncs. This will give you a better method of determining why the two images do not have the same brightness.

Could it be that also the BW of the video is reduced and that this affects the colors?

Color intensity/purity can indeed be related to video bandwidth. But you would definitely notice a difference in perceived image resolution.

On the other side of the mother board there is a small pot. What would that be for?

The big board has two pots. The mid-right one seems to be the video level pot. On both boards the other one seems to be a variable cap for a local PLL. It is near the two ceramic filters and may indeed have something to do with the sub-carrier performance. If you tweak it, be prepared to tweak it back.

By the way, do the two ceramic filters have the same part numbers on both Rx's? These are the three pin orange-brown colored parts.

It is also on the mother board that the switches are to select one of the 8 channels. How does that work?

The DIP switches connect to a microcontroller that I2C communicates the PLL divider codes to the Rx module.

Would it be a good idea to test with another channel?

Yes. Try all the channels.

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Yes, these are the receivers that where used for the Oracle issue.

I did not have a look at the video signal again but I have some info on the audio.

I transmitted a block wave that looked like a triangle on the bad RX. When I turn the variable cap in on or the other direction the amplitude of the audio-signal reduces but the shape does not change. So, I suppose it correctly "tuned".

I opened the other side of the RF-box. No adjustable stuff there.

I changed channels, but with the same result.

I had a look at the ceramic filters.

One of them is identical on both boards, it says "X6.5B delta 7 Q" (the one for video?)

The other one is different. The good receiver has "L6.5C ??? N", while the new one has "L6.5C ??? d" where "???" is perhaps also "delta 7"

I took a picture for the bad receiver. The filter seems cracked...

post-2786-1216221192_thumb.jpg

I tried to find on Internet what these codes could mean, but I did not find any answers.

Thanks,

Frederic

Edited by FredericG

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I had a look at the ceramic filters. One of them is identical on both boards, it says "X6.5B delta 7 Q" (the one for video?)

I'm not totally sure what this one is used for, but my gut feeling is that it is the video signal's 6.5Mhz sound trap.

The other one is different. The good receiver has "L6.5C ??? N", while the new one has "L6.5C ??? d" where "???" is perhaps also "delta 7"

This part is definitely the 6.5Mhz audio sub carrier filter. It is a coincidence that the filter is cracked on the bad A/V Rx, so that may indeed have something to do with your problem. If you are brave, you could desolder the good one and try it. But be careful, these are piezo components and man-handling them will harm them. If you find that the swap is succesful then find a replacement for the broken filter (they are popular components). Beyond that, it's time to replace the bad receiver.

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And replacement parts for virtually any product, Chinese or otherwise, can be found at Allied ElectronicsThe filters you're speaking of probly sell to you for less than 2, $3 each.

Oh heck, you guys all know this....

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And replacement parts for virtually any product, Chinese or otherwise, can be found at Allied ElectronicsThe filters you're speaking of probly sell to you for less than 2, $3 each.

Oh heck, you guys all know this....

Well no, I did not kwow this :) I did a quick search on farnell for example but did not find anything. Even with your link, I have trouble finding my way to the right component :huh:

I suppose that replacing the component is the most obvious route. However, Thomas mentions that these components do not like to be handled manually. :(

Buying another receiver might not be a good solution as a friend mentioned that "it is a well-known issue that some lawmates have a low audio-bandwidth"

Do you guys think that the trailing "N" and "d" could indicate a different bandwidth? Did they change this to have less noise?

Thanks,

Frederic

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I can't speak to the Lawmate bandwidth, however I probably should have mentioned that Allied has over two million components listed on their website and their catalog has over 2,000 pages of tiny print and weighs about 12 pounds. The catalog is free upon request and they also pay the postage.

With all of this, it is hard to find a particular component until you get used to how their search utility works. I think Thomas was referring to the static sensitive parts on the board. As long as you ground yourself to drain any static electricity from your body first, and don't walk across any Nylon rugs, and then pick up the board or part you'd have no problems. At any rate, a filter is not a static sensitive component.

As far as Lawmate may have a bandwidth problem, I should think any receiver would work with their Tx as long as it can be put on the same frequency.....

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As far as Lawmate may have a bandwidth problem, I should think any receiver would work with their Tx as long as it can be put on the same frequency.....

Actually, the given frequencies are for the video channels. Even if receivers brands can pretty often be swapped when they sport identical frequencies (not always the case though!), audio subcarriers are VERY often different so audio doesn't work without retuning, if that can be done on the device.

I wouldn't advise buying a different brand to try, especially not if the goal is to have good audio characteristics as that's where most incompatibilities are found.

Edited by Kilrah

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And replacement parts for virtually any product, Chinese or otherwise, can be found at Allied Electronics

I checked Allied and did not find any 6.5Mhz filters.

However, Thomas mentions that these components do not like to be handled manually. :(

Sorry for the English slang. "Man-handling" means being too rough. Just treat the filter with care, such as you would when installing a crystal.

Do you guys think that the trailing "N" and "d" could indicate a different bandwidth?

I believe the "C" indicates the bandwidth.

These filters are found in VCR's and A/V receivers and are VERY popular. That does not mean they will be easy to find by a hobbyist. You should look at any broken A/V receivers you might have in your junk box to see if you can find a parts donor.

Ceramic filters for TV sound applications come in various frequencies (5.5, 6.0, and 6.5 Mhz are commonly used for sound sub-carrier apps). They are offered in passband and trap configurations. To replace the bad L6.5C, you want a 6.5Mhz passband type.

Honestly, if it was me I would not bother to search for a replacement part until I confirmed that the cracked part is bad. Perhaps try comparing the filter's signals on the good Rx and bad Rx using your o-scope. The ideal way is to substitute the part. That means "borrowing" the one from the working Rx. The danger is that if you are too rough you may break the good one. If you are not prepared for that, then just order another Lawmate receiver that is the correct match for your A/V Tx. For sure, don't randomly choose a 2.4Ghz A/V receiver since not all use the same freq set, audio sub carrier, and video de-emphasis techniques.

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Any progress on fixing the problem? If so, how's the DZL modem working out with the diversity arrangement?

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So what is the verdict on these things? Are they still available for sale? Did anyone get thiers working? I was watching this since the begining and thought about being a beta tester, but I don't have the time anymore to trouble shoot things for hours. I would really like to get away from using a TinyTrak and AGWTracker and use google earth without resorting to maxstream or aerocomm radio modems. Any suggestions or thoughts?

-dave

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I didn't build the device, but it was reported to work. It requires an A/V system with good audio bandwidth. Jeff originally was going to build/sell it, but the last rumor I heard was that he transferred the project over to Mark/Intelligent Flight (but I could be wrong).

That was many many moons ago. I haven't seen any major posts about it since then. Perhaps try a PM to Jeff to see if he has any kits available from his early involvement. And like most things in this hobby, plan on tinkering with it. :)

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Any progress on fixing the problem? If so, how's the DZL modem working out with the diversity arrangement?
Sorry, I missed your question.

I reassembled the receivers again so that I can fly. I think I will wait until winter before looking into the issue again, the problem is not important enough for the time being. In Belgium you need to fly when the weather is good :)

Frederic

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So what is the verdict on these things? Are they still available for sale? Did anyone get thiers working? I was watching this since the begining and thought about being a beta tester, but I don't have the time anymore to trouble shoot things for hours. I would really like to get away from using a TinyTrak and AGWTracker and use google earth without resorting to maxstream or aerocomm radio modems. Any suggestions or thoughts?

-dave

As Mr.RC-Cam mentions it depends on the BW of your setup. Perhaps you could check the BW before you buy something.

My modems come from Eladio and I think they are on sale here: http://alai.h3m.com/~s0350672/catalogo/

Frederic

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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.

:(

Hi all,

I am looking for the simple DIY circuit to use my video transmitter audio channel as GPS data channel for GCS and I used these circuits from http://ieee.uow.edu.au/~daniel/projects/radio_link/.

I just added IN4148 diode, 220 resister and 5.6V diode in to TX input and assembled full RX circuit, but I didn’t get good result.

Please look at these circuits and give me advice to get GPS data trough video TX audio channel. ( I check the http://dzl.dk/projects/electronics/modem/modem.html, but I haven’t any experience to program ATTINY chips)

Regards

Samanpriya

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I just added IN4148 diode, 220 resister and 5.6V diode in to TX input and assembled full RX circuit, but I didn’t get good result.

My suggested scheme requires that the A/V link audio circuit have the bandwidth and characteristics to handle the raw GPS signal. Besides the attention that is needed on the Tx side, the Rx' output would need to have conditioning circuitry to provide clean logic levels for the MCU (e.g., amp->filtering->comparator). The required solution would need to be designed to work with your A/V parts. On the other hand, the DZL modem project should reduce the required complexity and relax the performance requirements of the A/V system (its NRZ modulation should help in this regard). I've never built the DZL modem, so someone else will need to chime in and offer advice on it.

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Hi all,

I'm having a look at this project........just wondered if there was any out-come/suggests.

I'm building my own RC antenna tracker using a PIC at the basestation on the ground, but I want to keep the additional electronics on the plane as simple as possible.......i.e. I don't want a PIC in the plane. I also want to use the audio channel as the video channel is tied up with an OSD unit, and although I could piggy back it's just getting to messy. I don't use the audio anyway.

I have looked at varying designs but came across the following conclusions thus far:-

FX614 - Would work great but requires a PIC at the plane to interface to the 614. The 614 is limited to 1200bps and so would definitely require a PIC to repackage the comms data for the 614.

MX469 - Not easy to get a hold off......a pity because it looks like it could tie straight to 4800bps GPS module.

I am currently in the midst of building a test design based on ModemTx from ElectronicaRC.com. I've order one but there's no info available there as to exactly how/what is does. All it says is GPS in one end and audio out the other. So what I need at the basestation I don't know yet.

Failing the above......I was thinking off just simply converting the GPS signal to something audio compatible, a couple of op-amps etc and shoving it down the audio input of the transmitter then op-amp/comparator at the base-station.

Any other suggestions? Look slike the audio approach is harder than the video method!!!

Ian.

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MX469 - Not easy to get a hold off......a pity because it looks like it could tie straight to 4800bps GPS module.

Actually, I would suggest the CMX469A instead since it is much friendlier to 4800 baud apps. I've been thinking about trying these out to see if there is merit in them, but the chips are expensive. I think a pair of them would cost me $50 or more. Ouch!

I don't want a PIC in the plane.

I don't think you will be able to escape the need for a PIC or other microcontroller when you use a modem IC. As you have recognized, the FX614 needs one to configure its operation. But, the MX469 series would also need one to generate the required training preamble. Other modem IC's would have similar needs that would be solved with a microcontroller. But, it's not a huge deal to include one and shouldn't be a show stopper at all.

I was thinking off just simply converting the GPS signal to something audio compatible, a couple of op-amps etc and shoving it down the audio input of the transmitter then op-amp/comparator at the base-station.

For low data rates my simple bench test seemed to suggest that would work. But, you'll need a RF link with extraordinary bandwidth and good fidelity. So, be sure to measure your system's audio path to see how it performs before considering this method.

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Hi,

Yes, I just read the datasheet for the MX469 and sadly came across the pre-amble requirement. Dang! so it looks like I'll need a PIC. But whoa!,........maybe it's possible to add a pre-amble without a PIC and by just being a bit inventive with the hardware design...........hmmmmm, will have to think about this one.

Other than that, it'll be interesting to see how this device operates..........with it's RX mate which although is designed for connecting to a PC should hook up to a PIC easily enough. I'd rather have my own design in it's entirety.........however!

http://alai.h3m.com/~s0350672/catalogo/pro...products_id=231

MondemTX.JPG

http://alai.h3m.com/~s0350672/catalogo/pro...products_id=232

ModemRX.JPG

Ian.

Edited by IanJ

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Thomas, tanks for the tip !

Edited by Gil

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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.

Why not encode the GPS data along with other data like airspeed, temp, current draw etc. It would seem to be a waste, streaming the GPS data by itself.

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Simplicity... This modem project is something extremely cheap and simple... Of course you can start adding more and more, but it just complicates to a point that many people don't need :)

There are other devices made to have the full-blown thing.

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Is there a schematic or circuit diagram available anywhere for the modem? (I'm not able to reach dzl.dk)

I like to build rather than buy :-)

thanks,

Paul

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Hello Jan,

I tried this and it works fine. The only thing is that you have to limit the NMEA output of your GPS receiver to the sentences which you definitely need, and at a reasonable rate. The circuit needs some "recovery pause", as Thomas mentioned before. I limited it to RMC and GGA, both at 1Hz which seems to be fine.

Jeff (jparisse) offered the kit a while ago. Maybe he still does. You might like to send him a pm and ask.

Cheers,

Hartwig

2 questions:

How do I limit the output of a GPS Rx?

How do I measure the audio bandwidth of my video Rx? I have an oscilloscope, not sure how to use it for this purpose though.

Thanks,

David

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