Jump to content
ThomasScherrer

Audio track as modem data NMEA

Recommended Posts

hi , my DLZ working fine on table with airwave module ,need to test with 900 mhz TX from long range video cause of using a spektrum radio ....

thank for the kit

:D

Edited by tazdevil3000

Share this post


Link to post
Share on other sites

Thomas, which configuration do you recommend for the EM406/411?

RMC, VTG, GGA, all once per second? Is that ok, or should I leave more space?

Share this post


Link to post
Share on other sites

Hartwig, I recomment you fint out what messages you need,

and only turn on them,

and then you measure the data and delays using the sound card methode,

I know I only need VTG and GGA for my project,

but other things like PDA with tomtom and so might need more or less messages,

Edited by ThomasScherrer

Share this post


Link to post
Share on other sites

Today I tested a modem in combination with my Lawmate system. I understood that the modem I received worked well on a an Airwave video system but for me it did not work at all.

I did some tests with a function generator and this is what I found:

- The cutoff frequency is about 20kHz

- The input started to saturate from 600mV (p-p) for a sinus.

- For a square-ware the signal looked best below 400mV (p-p)

- Even after filtering the signal a bit, above 300mV some interference in the video-channel started to show (I used a uniform blue screen; probably very sensitive). So I decided to use a signal of 300mV (p-p)

I decided to use a voltage-divider to lower the 5V signal to 300mV. After that a resistor of 1K, combined with the capacitive input of the TX, cleaned-up the sharp edges.

I have very little experience with this. Does this all look sensible and a good approach?

Thanks,

Frederic

post-6-1197830196_thumb.jpg

Edited by FredericG

Share this post


Link to post
Share on other sites

Your output waveform looks very faithful to the original. So, you are on the right track. If your audio drive continues to cause video interference, then try filtering the input waveform so that it is more sinusoidal (soften the square edges -- they are evil).

Share this post


Link to post
Share on other sites

Thanks for the feedback and the advise!

Something I forgot to ask. I am surprised the audio signal is amplified by 10...

From a capture of Thomas, I recall that the amplitude of the signal supplied to the TX was the same as the audio signal coming out of the RX.

Is this not surprising? Could it be that my system is designed to connect a microphone? (Don't know what comes out of a microphone)

Thanks,

Frederic

Share this post


Link to post
Share on other sites
From a capture of Thomas, I recall that the amplitude of the signal supplied to the TX was the same as the audio signal coming out of the RX.

ThomasS used Airwave A/V modules and they require a preamp when a typical microphone is used. So, that made his modem integration a bit easier.

Could it be that my system is designed to connect a microphone?

Some A/V Tx's have an internal preamp so that cheap electret condenser microphones can be directly connected. Those will need attenuation when a modem is connected.

Share this post


Link to post
Share on other sites

Gentlemen,

It was with great interest I read through this thread, but why did it suddenly stop?

Did anyone come to a conclusion? Was is possible to transmit the NMEA signal and was the received signal good enough to use?

If a product came out of this, I would really like to buy a couple :D

Jan

Share this post


Link to post
Share on other sites
Some A/V Tx's have an internal preamp so that cheap electret condenser microphones can be directly connected. Those will need attenuation when a modem is connected.

Might consider an inline capacitor as well to keep the electret cartridge voltage from feeding into the output of the modem unless there's a voltage block built into the modem..

Edited by W3FJW-Ron

Share this post


Link to post
Share on other sites
Gentlemen,

It was with great interest I read through this thread, but why did it suddenly stop?

Did anyone come to a conclusion? Was is possible to transmit the NMEA signal and was the received signal good enough to use?

If a product came out of this, I would really like to buy a couple :D

Jan

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

Share this post


Link to post
Share on other sites

Hi Hartwig,

An update of 1 Hz would be more than good enough and the most important thing to transfer to the ground is the position of the RPV.

I have been offered a couple of sets from one of the members in this forum already at almost no cost :-) , but thanks anyway.

Regards

Jan

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

Share this post


Link to post
Share on other sites

I too read this discussion front to back getting more excited for a final product... but alas... there is none... or is there? I would be more than willing to pay for one of these kits but i use the lawmate tx on 900mz is this going to be a real difficult thing to fix if I can convince someone to sell me one???

Thanks for all your hard work guys and I hope to hear from someone. PM is preferred.

Steve

Share this post


Link to post
Share on other sites
I too read this discussion front to back getting more excited for a final product... but alas... there is none... or is there? I would be more than willing to pay for one of these kits but i use the lawmate tx on 900mz is this going to be a real difficult thing to fix if I can convince someone to sell me one???

Thanks for all your hard work guys and I hope to hear from someone. PM is preferred.

Steve

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/~s0350672/catalogo/ind...fc89f8cc84db486

Cheers

Dimitris

Share this post


Link to post
Share on other sites

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/~s0350672/catalogo/ind...fc89f8cc84db486

Cheers

Dimitris

Share this post


Link to post
Share on other sites

>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/projects/electronics/modem/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 :-)

Share this post


Link to post
Share on other sites

I integrated a DZL modem in my Oracle diversity receiver:

post-2786-1215873951_thumb.jpg

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... :angry: 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.

post-2786-1215874481_thumb.jpg

Share this post


Link to post
Share on other sites
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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites
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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...