Poor video picture

[Terry 5]

I was doing a test flight the other day and recorded the flight on a VHS recorder with my video glasses plugged in to the output from the recorder. The quality of the video I was flying by was fine but when I played back the tape I found it often had a dark band accross the picture about a 1/3rd of the way down. It seems to be due to an excessive brightness from the sky at times and I assume it is overloading something, but why is it only on the recording ?

Terry

[Kilrah 2]

That looks like what can happen if the video level is a bit too high. Adjusting the receiver might help.

Regarding live/playback difference, you might want to try powering up your setup, get the same conditions and connect your PC capture card to your VCR's output instead of the goggles. Usually video displays crop the image, so the picture might have been the same as here in your goggles, but with the cropping you wouldn't have noticed it. On the PC it should appear too as it displays the entire image.

If not, it would mean the high white level only overdrives a portion of the circuit that serves for recording, while the in -> out path stands it.

[Terry 5]

Yes, I think your last idea is what is happening.

I wonder if I could add a soft clipping circuit to hold back the white level but allow the darker part of the picture to be amplified to reduce the high contrast effect between land and sky. What do you think ?

Terry

[Kilrah 2]

I'd rather use the display's settings that are already here or apply a filter when editing...

[Mr.RC-Cam 126]

Do you measure 75 ohms (or something in that range) on the naked input of your VCR?

[Terry 5]

Yes, 73.6 ohms.

Terry

[Mr.RC-Cam 126]

73.6 ohms, that's plenty good. If this is an Airwave Rx, is its output AC coupled with a big cap (>330uF)? A small cap, or one that is backwards, could cause the tearing.

Beyond that, it would be helpful to put a scope on the signal to measure the bottom of the sync to the top of white level. Should be 1Vpk-pk or so. From what you show, it may be much greater.

[Terry 5]

It's not Airwave, it's a ready made receiver that was reboxed years back. I don't have access to my scope at the moment as my kids are with me this weekend and my work room is now a bedroom until Sunday night. It seems to be a lot to do with the camera but gets worse when recorded for some reason, I think Kilrah was right about my VCR. My diversity receiver suffers the dark line aswell but not the tearing. It happens when the camera goes from dark to a mixed picture of dark and light eg.a bit of bright sky at the top of the picture.

Terry

[Mr.RC-Cam 126]

FWIW, the AGC performance of the video recorder/monitor can mask serious video input level problems. So, one video playback device may display a mess, whereas another one will accommodate the problem and show a normal image.

So, it is best to check the video level and adjust it for standard 1V into 75 ohms. To do this, some systems require tweaks to the Rx, others need tweaks to the Tx. Perhaps there is something in this project that will help: http://www.rc-cam.com/lawmate.htm

[Terry 5]

Well I managed to do some more testing today and I'm more confused than before.

I tested 2 receivers and 2 cameras, both very close to 75 ohms, with all these I get the dark band and tearing even after adjusting to less than 1v with a scope. I get the problem with both an LCD screen and when using my TV as a monitor BUT not when using my EyeTrek glasses. I check the input of the glasses at 75 ohms so I dont know what to try next ? It dose explane why it showed up on the recordings and not the glasses when I was flying though.

Terry

[Mr.RC-Cam 126]

That is very odd. I'm not sure what else to recommend since you've covered the basics. Horrible incoming power supply noise can cause odd problems.

By the way, have you tried a different CCD camera? If it does not clamp the high whites, it could over drive the video. To test for this, measure the output of the camera into 75 ohms. Illuminate the camera with a very right light. Its video output should observe industry standard levels.

[Terry 5]

All my stuff is running off it's own batteries so it's not a PSU problem. I will check the camera levels next.

Terry

[Terry 5]

Nope, it's not the camera, I checked the level with the scope and its good. I then connected it direct to my LCD screen and the picture was great.

It's something that happens in the RF link but the levels are good. Do you think the receivers are set too high from the makers and I have set the transmitters too low to compensate ? It's not the other way round because I set the transmitter is set to minimum and it's still the same.

Terry

[Kilrah 2]

What TX/RX set is it? I always need to lower the video level on the BWAV set 2 receivers to get a correct picture...

[Terry 5]

1 Airwave TX, 1 unknown TX, 1 unknown RX and my 4 way diversity RX with Airwave modules.

Terry

[Kilrah 2]

Okay, then I can't help you, the only Airwave RX (Felsweb actually) I've used ended in the bin for utterly low sensitivity.

But I'd definitely try tweaking both TX and RX level to find a good match. If the TX is clipping it sure won't do any good.

[Terry 5]

Which RX modules do you use Kilrah ?

Terry

[Kilrah 2]

http://www.rc-tech.ch/web/components/com_v...73fbfa70937.jpg

[Terry 5]

Looks very nice but I don't think I can afford 4 of them for my next diversity RX discount for 4 ?

Terry

Edited January 13, 2007 by Terry

[Kilrah 2]

I'm dragging a pair of relatively old ones for myself, I'm not sure if the newest ones (from another manufacturer) are also compatible with the Airwave modules. There have been some compatibility issues between the 2 versions, that actually had something to do with video levels too.

[Terry 5]

I seem to have got an adjustment thats working, I wont be sure until the sun comes up but it looks ok. The funny thing is that turning UP the RX level seems to have done the trick. Fingers crossed.

Thats a shame about the new receivers.

I would like to find some better RX modules than the Airwave ones, they are not great without a pre-amp but they are usable. I have had to start using more power on the plane due to no pre-amp on my diversity RX and local interference.

Terry

[Mr.RC-Cam 126]

I seem to have got an adjustment thats working, I wont be sure until the sun comes up but it looks ok. The funny thing is that turning UP the RX level seems to have done the trick. Fingers crossed.

That's a relief you found a solution. At this point, whatever works is great. BTW, what video level did you end up with?

[Terry 5]

I tested it just now in daylight and it's not a total fix It is better than it was but the change from dark to light still causes the dark band on my LCD screen and when recorded on to video tape. But when viewing with my Eye Trek glasses the picture is always good. I can only asume the glasses process the video better than average. I'm now sure that the output from my receiver is correct and the problem is due to the camera as my other camera now seems ok.

BTW, what video level did you end up with?

I have set the TX very low, about 1/4 of the way from minimum. The receivers are at 0.95v at white, I could push this up but I wanted to be sure I was not over driving anything.

I may need to remove all the receivers from my diversity RX to adjust them as since adding the caps to all the outputs it seems a bit worse, I will have to do a propper test.

Terry

[Terry 5]

Just a thought. I noticed when I was veiwing the scope that when I turned the light on the level of the whole signal (incuding the bottom) would jump up by 0.2v as the light went on and then settled back after a second or two. I think it may be the bottom of the signal that is causing the problem and not the top. My guess is that the glasses are able to follow this jump and the screen and video can not. What do you think ?

Terry

[Kilrah 2]

Mmh, missed or delayed horizontal syncs?

Did you measure that on the camera or at the receiver's output?

Is the camera properly AC-coupled?