190 replies to this topic

[saabguyspg]

Hi there, thanks greatly for the link I am very excited to hear I can buy it but.... I can only see a link to the GPS unit itself. I have the new 5hz GPS from I.F. and was hoping to use it....

If there is a place to purchase this and I can't find it please help!! lol...

cheers!

Steve,

you can order Airwave and Lawmate TX versions of this modem and the RX side (which includes a RS232 port) here:

http://alai.h3m.com/...fc89f8cc84db486

Cheers
Dimitris

[ThomasScherrer]

>I have the new 5hz GPS from I.F. and was hoping to use it....

great it is also a really impressive GPS

>If there is a place to purchase this and I can't find it please help!! lol...

Purchase what ??
I assume you mean this :
http://dzl.dk/projec...odem/modem.html
a few people in this forum have make ready soldered constructions of that project and sold,
please read the whole thread to find out more :-)
also that project uses ONLY 4800 baud !! from the GPS, so you must select that and also go into 1Hz mode and turn off
all unused sentences, you need a few holes in the GPS datastream so the system can re-sync,
if you dont have some time to spare (check with a scope) the system will not re-sync after a tiny noise pulse or data glitch will jam some bytes that is also normal, there will always be some minor drop outs on all RF links.

add a battery backup to the GPS, so it will keep your settings after you disconnect it from the PC configure software,
if no battery it will go back to default setting at next powerup :-)

[wisistem]

Hi,

thanks. for this Project.

questions =

pmr radio bandwidt = ?

for pmr radio max. modulation audio signal = ?

600 - 1200 - 2400 baud = auidio signal frquency = ?


http://www.rc-cam.co...=...0306&st=140
Edit Post

Edited by wisistem, 09 July 2008 - 03:55 AM.

[FredericG]

I integrated a DZL modem in my Oracle diversity receiver:


The problem is now that the DZL modem does not seem to work in combination with my new 2nd lawmate receiver.
I experimented with adjusting the level of the audio signal to the TX, but I never get a signal that is good enough for the modem to decode... It looks to me as if the signal heavily filtered.

These are the signals that the receivers are generating, both signals 1V/div, the bottom signal is from the old receiver.

[FredericG]

Here I have dramatically lowered the signal amplitude: (1V/div)


And here I have increased the signal: (2V/div)


Frederic

[Mr.RC-Cam]

The problem is now that the DZL modem does not seem to work in combination with my new 2nd lawmate receiver.

As a sanity test, perform a audio bandwidth measurement on both receivers. Try both sine and squarewaves to see where the "bad" receiver goes bad. I bet the new Rx is just a couple KHz and the old working one goes nearly to 10Khz. Even on a good A/V Rx the sinewave performance will be better than squarewave due to the needed filtering, so some signal rounding is unavoidable.

It looks to me as if the signal heavily filtered.

It is probably just a defective receiver. From what I can tell, the factory does not test each one using real measuring equipment. So, a bit of distortion on the audio would probably get past the earball test that some tired human is required to do.

[ThomasScherrer]

audio carrier must be adjusted correctly on the transmitter so it is centrered,
also the amplityde needs to be correct to achive best audio bandwith,
this modem is designed for near hifi style audio links, many video links support audio with 10khz or more bw
you can also add a schmit-trigger and adjust it so your data signal is restored as good as possible,
adding max peak and min peak holds, and centre trig system might also be a way
this is easy to design using an opamp.

[FredericG]

As a sanity test, perform a audio bandwidth measurement on both receivers. Try both sine and squarewaves to see where the "bad" receiver goes bad. I bet the new Rx is just a couple KHz and the old working one goes nearly to 10Khz. Even on a good A/V Rx the sinewave performance will be better than squarewave due to the needed filtering, so some signal rounding is unavoidable.
It is probably just a defective receiver. From what I can tell, the factory does not test each one using real measuring equipment. So, a bit of distortion on the audio would probably get past the earball test that some tired human is required to do.

It looks you are absolutely right. I measured the bandwith with a sinawave and the cutoff-freq (-3dB) of the new receiver is only 2KHz, while the older one goes upto 20KHz. I was assuming that he limited BW was due to a design-change, but I suppose 2KHz is not acceptable for audio and should be seen as a defect. Right?

Is there something I could check/fix or do I just send it back?


Thanks a lot,
Frederic

[FredericG]

audio carrier must be adjusted correctly on the transmitter so it is centrered,
also the amplityde needs to be correct to achive best audio bandwith,
this modem is designed for near hifi style audio links, many video links support audio with 10khz or more bw
you can also add a schmit-trigger and adjust it so your data signal is restored as good as possible,
adding max peak and min peak holds, and centre trig system might also be a way
this is easy to design using an opamp.

Thomas, what do you mean with "audio carrier must be adjusted correctly on the transmitter so it is centrered"? Does this relate to the audio signal that is provided to the TX or is this related to the TX-RX link (the carrier on which the audio is modulated perhaps)?

To be sure there is no misunderstanding. I use one TX and the signal is received by the two receivers simultaneously.... On the -scope graphs you see the audio signals of both receivers.

Does the fact that the audio signal is reversed tell us something?

Thanks,
Frederic

[Mr.RC-Cam]

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.

Does the fact that the audio signal is reversed tell us something?

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 was assuming that he limited BW was due to a design-change, but I suppose 2KHz is not acceptable for audio and should be seen as a defect. Right?

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.

[FredericG]

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.

Edited by FredericG, 13 July 2008 - 06:42 AM.

[FredericG]

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?



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

Thanks,
Frederic

[Mr.RC-Cam]

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.

[FredericG]

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...


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

Thanks,
Frederic

Edited by FredericG, 16 July 2008 - 07:17 AM.

[Mr.RC-Cam]

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.

[W3FJW-Ron]

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....

[FredericG]

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

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

[W3FJW-Ron]

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.....

[Kilrah]

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, 17 July 2008 - 05:10 AM.

[Mr.RC-Cam]

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.