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zactommo

Improve Video Signal Quality

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Thanks to the help i've got from this forum, I've got to the stage where my TX is working, which is good news, but I have some video quality issues that i'd like to sort out.

What i'm seeing in my picture outputting from RX is what looks like horizontal lines the the picture, that are moving up and down the picture, and some noise in the picture.

It gives the picture a cheap VHS sort of look to it.

I'll attach a couple of still frame from the output of the RX, but it is more obvious seeing it in a video, which I will try to upload that too.

I'll also attach a photo of my TX so you can see the layout (don't laugh) and provide some feedback about what I could improve to get better picture quality.

roomshot.png

barsz.png

img5326zv.jpg

I had a couple of thought about what it could be, tell me what you think

1) Pin 14 which is labelled Ground on the schematic is connecting to the -VE of the Video Input BNC plug, instead of the common ground like all the other GND Pins

2) Maybe i've stuffed up how i've installed the smoothing capacitor on the 3.3V line, or the capacitor on the bypass line?

Thanks in advance for your help!!

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Can you add some details of the rest of your setup?

eg TX, power supplys ect.

Terry

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It looks like power supply noise to me. Are you sure the problem is in the Tx? Have you investigated the Rx too?

Have you directly connected the camera to your video display to confirm the problem is the wireless link?

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Details of the power supply. Its a LM317 voltage regulator, I was running 12v in, but it gets too hot, so now its running 5V input. The AC/DC plug pack is 5v DC regulated, rated at 1A.

The output of the LM317 has two capacitors on the output, for smoothing I would guess. 1) 10uF 16v, 2) 0.1uF. Also has a 25V 470uF cap on the input.

The RX is a prebuilt one from DPCAV, so I assumed it would be okay, but I could be wrong.

http://www.dpcav.com/xcart/product.php?productid=16243&cat=249&page=1

And the camera is outputs a clean signal. Its a Sony PD-170, and I uses it regularly for Pro work, and stuff for broadcast, so the noise isn't coming from there.

I'm recording the output on a Sony GV-D900 Portable DV Deck. When I connect the camera directly to that it looks good!

I hope thats enough detail, let me know if you need to know more!

Thanks

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Nothing jumps out from the photos, so I won't be of much help here.

BTW, a LM317 set for 3.3V, that is powered by only 5V, is quite close to its drop out voltage. So, any noise on the 5V supply may get into the 3.3V.

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Plug pack... seen many things with those. It would be indeed wise to first have a try with a battery (2s lipo would be fine) so you have a known clean supply.

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The AC/DC plug pack is 5v DC regulated, rated at 1A.

I missed the comment about using the plug pack supply (I assumed batteries), so I think Kilrah nailed it.

I suspect it is a DC-DC switcher type. A switcher's noise can make its way through a simple linear Vreg with inadequate filtering. Plus operating close to the VReg's drop out voltage will invite this to occur. As he mentioned, try a battery and see what happens.

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I tried it out using a standard 9V Alkaline as I didn't have anything beefier on hand, and it fixed the horizontal lines issues. There still was some noise, but it was a vast improvement! (Was still using a AC/DC PLug pack on the RX)

You mentioned filtering; if I add a DC filtering circuit in there, will it allow me to power the TX and RX from the plug packs?

I was having a read of this page: http://www.play-hookey.com/ac_theory/ps_filters.html

Is that the sort of thing I need to do?

Thanks for your help!

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You need LC filtering (example is in the article you mentioned). You also need to power the LM317 from something higher than 5V, perhaps 6V.

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Okay, so I found a couple of online tools for calculating what components I need in the LC Filter circuit, but some of the values i'm not to sure about.

I was looking at:

http://www.wa4dsy.net/filter/hp_lp_filter.html

I'm not quiet sure what poles means, and what is the frequence I am trying to filter? The 2.4ghz of the TX, or the frequencies of the video signal? I guessed it was 50 Ohms for the other value.

I found another calculator: http://www-users.cs.york.ac.uk/~fisher/lcfilter/ but that had a whole lot of values I didn't understand!

The actually circuit itself looks very simple, its finding out the values of the componenets that seems difficult

Thanks for you help

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Your online filter search needs to be reworded. Try: Switching Power Supply LC Filter

In your application, these are starting (minimum) values:

L: >220uH, rated at about twice the Airwave module's operating current.

C: >470uF

Increase the values if necessary.

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