Adjusting video level on DOSD

While lines
I’ve recently purchased DOSD. The first I wanted to fix was these white lines on the screen. They appear if black borders are enabled in the settings and signal is fed through RF video link. When DOSD is directly connected to the goggles or the computer, this problem is not noticeable.

As I’ve noticed on youtube clips this problem is not rare one. Few of my friends has bought Dragon OSD and had the same problem.

The problem is in the black level in the composite video signal

As you may notice on the scope view, very short bursrs of signal means white letters being drawn on the screen with black border. The lower amplitude is, the darker the view is. But it should stay above reference black level, which goes right after the sync signal. Here we see, that it goes under. That probably confuses video TX or maybe RX AGC. You may hide this problem by loading video signal with parallel resistors on the composite cable, but this might degrade your contrast, colors or even a synchronization. THIS IS NOT A MAXIMUM VIDEO LEVEL PROBLEM!!!

When I’ve looked at the DOSD PCB I’ve found that two signals comes from microcontroller. One of them draws white pixels, another is for black ones. White pixels right now probably is not the part of the problem, so I’ll draw only the black color part . This scetch shows how the parts are connected (guess only, based on observation):

Accidentally I’ve damaged onboard transistor, and tried to replace it with pin-to-pin compatible BC847 (100MHz). This general purpose NPN transistor seems too slow for this task, the image was very shady. So I’ve searched and found very good and kinda cheap transistor: BRF93A. It is able to work up to 6GHz (!). It worked really nice. Not sure what was the original part number, but text in the picture now is crystal clear.

To limit voltage being shorted to ground by transistor, I’ve inserted 100 Ohm resistor on the emiter, and it solved a problem. You can see in the video how I’ve tried adjustable resistor to find optimal blackness level in the picture. There was no slot for the load resistor, so I had to lift transistor and solder 100 Ohm resistor nearby. Not very elegant, but .. it works.

As you may see, I have great picture in the screen. With solid black border and no white shading ;) .

embedded by Embedded Video

YouTube Direkt

Hope this will help someone to improve the image!

_________________

Never tried, but maybe there is even easier way to solve this, by increasing marked resistor:

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Comments

  1. On November 18, 2011 ilovefpv says:

    How do you like the Dragon OSD? I’ve been putting together a comparison article of all OSDs on the market and would like your feedback on this one:

    http://www.ilovefpv.com/2011/11/choosing-fpv-osd-system/

    Comment system working better now?

    -Steve

  2. On November 18, 2011 ilovefpv says:

    Clarification – “Blog comment system working better now?”

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