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yb2normal

Automatic Video Switch?

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After hunting around on my new receivers I have come to the conclusion that they do not have an RSSI output. This means I can't use cyber-flyer's nifty diversity receiver.

I recently stumbled across an 8 pin chip that, with very few components, can strip out all sorts of interesting sync information from a video signal. It is the National Semiconductor LM1881. I'm wondering if the sync information from this chip could somehow be used to detect a valid video signal, such that two of these circuits could monitor two video inputs, and switch to whichever one has a valid video signal?

One minor advantage of such a solution is that you would not have to dig into your video receiver to find an RSSI voltage, and could simply plug two composite video sources into the auto-switching device using common RCA cables.

I have seen a few commercial solutions available, but being the tightwad electronics hobbyist that I am, I just have to believe I can do it cheaper/smaller/better. :)

Regards,

Bill

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The sync detection idea was discussed awhile back. It can be used used in this application, but would be much better if it was used to complement the RSSI signal.

The reason I suggest this is that the sync detection cannot be used to predict an impending signal loss. Whereas the RSSI will give you signal strength values, which offer a better idea of which antenna is the best choice at any given time. The sync detection can then be used when interference is affecting an otherwise strong signal.

I have a feeling that you have RSSI inside your Rx. Just open up the can and look up the data sheets to the demodulator IC. Most have an RSSI (or AGC) output of some sort.

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The sync detection idea was discussed awhile back. It can be used used in this application, but would be much better if it was used to complement the RSSI signal.

Too bad I couldn't search for it ;)

Regarding RSSI, I was hoping to avoid popping open the can, it is well soldered around it's perimeter and I fear I might have to dremel it open.

Do you remember if the discussion mentioned sync detection in the heading? If so, I'll start wading through the posts on the old forum

Thanks,

Bill

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Here is the old discussion: Diversity Receivers

In it I talked about using the amplitude of the sync pulse to help determine signal strength, as well as using the video sync pulse to determine valid video. This trickery would work, but would required DSP type signal analysis instead of the simple-brained LM1881 (it will be happy with nearly any "valid" sync it sees).

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Thank you for finding that link. It reaffirms that I should really try to find the RSSI voltage in my receiver.

Bill

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After toying with RSSI based receiver for almost a year I should say it works fine. The best feature is it's redundancy - if one antenna lost signal another one will pick it up. However, multipaths interference is hard to eliminate as it happens with little warning on RSSI pin. I am still waiting for some overseas manufacturer to start incorporating RSSI diversity circuit into a video receiver - it is so much chearper to do it on RF stage. Does anybody know how Wi-Fi access points with two antennas operate? Do they employ diversity algorithm to switch between antennas?

Regards,

cyber-flyer

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It reaffirms that I should really try to find the RSSI voltage in my receiver.
I noticed that the new Lawmate Rx has two unused pins on the RF module. Maybe one of them is used with external circuitry to form the RSSI (buffered AGC) signal? Have you asked Lawmate about the module's pinout?

However, multipath interference is hard to eliminate as it happens with little warning on RSSI pin.
That is where the Sync detect idea would come in. Multipath issues can fool the RSSI signal, but the video sync pulses would be corrupted. A little bit of smarts (low end PIC) could be used to monitor the RSSI and vertical sync to form a very robust diversity system.

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I noticed that the new Lawmate Rx has two unused pins on the RF module. Maybe one of them is used with external circuitry to form the RSSI (buffered AGC) signal? Have you asked Lawmate about the module's pinout?

From what I could tell, there was no voltage change on those pins when I varied the RF power, but perhaps the change would not manifest itself as a simple voltage change. When testing for the voltage change I simply grounded one of my test leads... perhaps I would have seen a voltage differencial between the two pins?

I have not asked Lawmate yet, so I think that is the next step.

Thanks,

Bill

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Does anybody know how Wi-Fi access points with two antennas operate? Do they employ diversity algorithm to switch between antennas?

Those that I looked at used two antennas in a single housing. Two highly isolated feedlines are used. The WiFi's DSP monitors the signal strength and chooses the antenna with the strongest (and valid) RF source.

Here is a look at what is inside a typical Wi-Fi antenna: http://www.ethertronics.com/assets_f/MPCI%...20Datasheet.pdf

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The WiFi's DSP monitors the signal strength and chooses the antenna with the strongest (and valid) RF source.

Thanks! The next natural question is - can this diversity circuit be used for video broadband signal switching? May be the best way is to find a used WiFi access point and hack it out ;)

Regards,

cyber-flyer

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The key is to only switch the antenna if it has a valid signal on it, which may conflict with the strong signal detect methods. I think that your RSSI method, assisted with video sync detection, would be the lowest cost (effort wise) to obtain what you want.

I assume that the Wi-Fi solutions just look for a carrier that has valid data (there are ID preambles that help this task out). In our app, all we have is the ability to detect a RF carrier (which may or not belong to our video Tx) and maybe also key off the audio sub-carrier. Once the carriers are found, we can then look to see if there is valid video by analyzing the presence of authentic video syncs. This should help the switch to ignore video signals that are strong, but corrupted.

It could all be done to the basebanded RF domain using DSP methods, but the s/w project would be immense. Using the basic RSSI (AGC) signal, and checking for valid video, would be much easier. At least for me. :)

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Mr. RC-Cam,

I noticed on the old forum you posted picture where you managed to find a RSSI signal on the Lawmate.

The link is: http://groups.msn.com/RCCAM/telemetrysyste...oto&PhotoID=115.

I am interested in a diversity receiver project based on the Lawmate recievers. I was hoping you could give some guidance about finding the RSSI output on the Lawmate. Do you have a pin-out or some sort of picture of how you connected and find a signal strength signal?

Much thanks in advance,

- Happy Electron

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RSSI location on the "older" Lawmates Rx's:

900MHz 500mW is second pin from the antenna side of tin sardine can.

Signal Level: 3.90VDC = Strong, 0.13 to 0.15VDC = weak, 0.11VDC = Tx off

2400MHz 100mW is third pin from the antenna side of tin sardine can:

Signal Level: 0.90VDC = Strong, 1.70VDC = Snow, 2.20VDC = noise only, 2.60VDC = Tx off

Voltages are approximate.

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I found those voltages on third pin from antenna side, wich means I got one of the "older" 2,4 GHz receivers i guess.

I will use it for a RSSI meter to start with. I think I will use a differential amplifier and convert it to 0..5V, 0 V being "No signal" and 5V "Strong signal"

Thank you very much Mr. RC-cam.

Best Regards,

Bosse

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Yes, thank you RC CAM for this helpful information. It will save me a lot of time fishing around trying to figure out which pins were the RSSI outputs.

Several other questions:

1) is it the 2nd& 3rd Pins from the Antenna *directly* coming out of the metal box? The reason I am asking is that there is a small PC board that is connected to the metal box pins, and which in turn has wires connected to it. I want to make sure we are talking about the actual pins coming out of the metal box and the not the PC board wire pins.

2) I saw on your signal strength indicator project that your RSSI ribbon cable had 4 wires, not two. I was curious what the other wires were used for.

Again, much thanks for your very helpful information!

- HappyElectron

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1) is it the 2nd& 3rd Pins from the Antenna *directly* coming out of the metal box?

They are the pins coming out of the RF module (sardine can). These pins solder to the PCB. If you are not sure what is the correct choice, then just put a meter on them and look for a DC voltage that varies with signal strength.

I was curious what the other wires were used for.

1) +12V for low battery alarm sensing.

2) +8V from XCam's LM7808 Vreg for FSM's circuitry

3) Raw AGC (RSSI)

4) Gnd

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I've just read this post again as I want to make a 'valid video detector', did anyone make one ?

Terry

UK

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There are several examples of missing sync detectors on the internet. None that I have seen really excite me. So, I cannot recommend one. You might want to Google a bit to see what is out there.

I have thought about coding a PIC to do the job, but I have not had the need as yet (but it is something I would like to do). The old LM1881 that Yb2normal mentioned would be a good front end to a PIC based solution that also monitored RSSI: http://www.national.com/ds/LM/LM1881.pdf

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