Am I getting multipath distortion?

...and if so what can I do about it?

I have a cobbled-together setup that almost works. I coach swimming, and I have a small video gadget that links to a Tivo and a TV via one of those video transmitters that you can get from ebay. I think it is a 1.2 gHz

50mw version.

The kids swim and then watch, and having the Tivo means there's a continuous flow of action. The problem is that the picture quality is bad. I'm only 20 metres away and there is a direct line-of-sight from transmitter to reciever - ocassionally with a body or two in the way; but even without, there is considerable jumping and colour distortion and generally a bad picture. If I stand still at just about any distance I can find a place that gets a good picture, but moving - even turning - makes it change.

The ceiling and the upper half of the inside walls of the pool are panelled with strips of aluminium sheathing - about 6 inches wide and 20 or more feet long. These are separated by about a 1-inch gap along the long edges, and mounted to a framework (which is mounted to concrete) with some sort of clip arrangement. This is a box about 220 feet long and 25 feet high.

What's the best guess for improving the picture? More power? Different frequency? Antenna mounting (I could wear a beanie with it sticking up from the top - that'd be cool)?

Reply to
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Hello,

When the multi path effect only affects received signal strength, you can find many places with good reception and many with bad reception.

Best thing is to try it in an empty swimming hall. Your environment is stable in that case. Move slowly < 1"/s and see what happens. At 1.2 GHz (WL about 10", 0.25m), the distance between bad reception and good reception can be as low as some inches.

50mW is more then sufficient power for such an indoor application, so assuming reasonable receive and transmit antenna, increasing power will not help. You might switch to another channel or look to the picture with transmitter off to check for interference from a similar or other system sharing the same frequency.

In case of severe multi path effects bad and good reception may alternate within a yard (1m). When in the good places the reception is noise free, signal will have sufficient strength.

With severe multi path, where path via reflection can be many yards longer then the direct path, not only signal strength, but also received signal quality is affected (delay spread). In this case you will find few places with reasonable reception, even under good signal strength.

Given your equipment (probably Frequency Modulated system with one receive antenna), a directional receive antenna may give improvement. The disadvantage is that you need a second person to have the antenna aimed at the transmitter (in case of moving setup).

If this all is not possible and it must be wireless, you have to divert to multi antenna systems (space diversity) in combination with channel equalization or real wide band systems based on OFDM or other multi path resistant modulation scheme.

Hope this helps you a bit.

Wim PA3DJS

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Reply to
Wimpie

yes it seems so...I have seen the same thing with those devices.

it is very difficult to solve, you would need better antennas...

in your case it may be more practical to use a coax wired connection

For electrical saftey around water...be VERY careful that all equipment is properly grounded, that the outlet grounds are wired properly and I would also use GFI circuits.

Mark

Reply to
Mark

Well that's the first problem. Those cheap video transmitters are barely worth what you paid for them. There is a huge quality and price difference between models. Pay $50 and you get $50 worth of video quality.

Ok, so not really a perfect environment for wireless. :)

Get a better unit. Can you possibly run cable? Either use a better grade of video cable, or use a video-over-copper extender if the cable path is pretty long. Oh, and stay away from cheap video baluns too.

If you have true line of sight, you are probably ok with the power.

Is this really something you can change? I mean more than just a frequency within a range? If you can change bands, then maybe --- that usually takes dual-radios etc.

The other poster Wimpie is right on. If you think it's multipath, then either use antenna diversity by using two antennas (which of course your radio has to support), or try using a directional antenna with a tight beamwidth. The stronger (higher dbi gain) antenna will have a smaller horizontal/vertical beamwidth. And so the chances of multipath will be lessened.

Keith

Reply to
Keith M

No beanie but,

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Mike

Reply to
amdx

Are you holding the transmitter while you're moving?

I am picturing you walking along the side of the pool as the kids swim from end to end. It's quite possible you are detuning the transmitting antenna when you step.

This could be electromagnetic coupling, capacative, or who knows what??. The fact that you can stand still and get a good picture is a clue. - one that would seem to eliminate the multi-path thoeries....

I generally agree you get what you pay for. But at 1.2GHz, these are going to be very small wavelengths. My first inclination is that this is not a multipath problem, though it is impossible to tell without further experimentation, and knowing a lot more about exactly what type of modulation scheme is being used.

I second the thought about electrical safety around kids and water.

-mpm

=2E

Reply to
mpm

Oops, another thought just came to me. If you're moving the camera / transmitter, it might have a real chicken-s^it timebase (crystal?) or whatever and could be sensitive to motion and going slightly off frequency with every bump (step taken). Maybe you can eliminate this possibiliity by walking very softly. Like that Karate guy on TV used to do on the onion paper... :)

Reply to
mpm

It is almost certainly multipath: The frequency, mobility, and environment are wrong for what you're trying to do. If you can lower the frequency - the lower, the better - then this will help. For example, 72 MHz. You an make it legal by applying for an experimenter's permit (if in the USA, see FCC form 442).

I'm not sure if you can find any commercial transmitters below 900 MHz, so you may need to DIY.

There may be wireless products for DVB-H, DVB-T which would theoretically be an improvement.

As one poster mentioned, your best bet may be a cable.

Frank Raffaeli

Reply to
Frank Raffaeli

What about a plain old modulator? They make some decent frequency agile ones for whole-house distribution and whatnot,some have up to ~1W of output. (CATV modulators)

Reply to
PhattyMo

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