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mosfet

Brightness / contrast modification

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Hello,

I am looking for a method to adjust black level (brightness) and white level (contrast) of a video signal.

Ideally, I would plug a device between a video receiver and video devices (LCD display / video glasses, etc), and be able to change brightness and contrast.

Any ideas about how I could proceed ?

Regards,

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lots of video amps made for car dvd systems have a brightness control.

Terry

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No sorry I was talking about ready made units from car hifi shops not DIY.

Terry

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And those amps have a signal amplitude control - which is not a "true" brightness control. Yes changing the overall signal amplitude does change brightness, but also changes things that it shouldn't... a true control would separate the image data from the sync in the composite signal, change the signal black and white levels, then reconstruct the composite signal... far from easy. The best way is to use a monitor/goggles with contrast and brightness control, which most have :)

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Thanks Kilrah.

Sure, I could adjust brightness/contrast on the display devices. That's what I'm doing right now, but on some devices (ex: iTheater goggles) the contrast is too strong, and brightness too low, even on max settings.

I had a search before about this topic on the forum, and I noticed that the Oracle Diversity controller has an ic that can match the brightness of the two video receivers. Have you got any idea about what IC is used ? It the system doing amplitude control on the two signals ?

Regards,

And those amps have a signal amplitude control - which is not a "true" brightness control. Yes changing the overall signal amplitude does change brightness, but also changes things that it shouldn't... a true control would separate the image data from the sync in the composite signal, change the signal black and white levels, then reconstruct the composite signal... far from easy. The best way is to use a monitor/goggles with contrast and brightness control, which most have :)

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There's someone here that can answer much better than anyone else any questions about the Oracle, but AFAIK the adjustement potentiometer is simply a resistive attenuator that you can adjust to reduce amplitude of the stronger signal to match the weaker one.

As the need for equalizing levels precisely comes from transmitters and receivers from most manufacturers not having their overall video amplitude levels properly calibrated and can thus vary from unit to unit, this kind of compensation does the job perfectly, no need for a true brightness/contrast handling.

Edited by Kilrah

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Kilrah's comments are correct on all counts. :)

... on some devices (ex: iTheater goggles) the contrast is too strong, and brightness too low, even on max settings.

The first thing that comes to mind is that your native video signal is incorrectly calibrated and the display device is not able to compensate it. So before you build/buy anything, calibrate the video levels and then re-adjust the goggle settings; That alone may cure the problem.

Another thing that comes to mind is that you are ganging two (or more) video display devices onto one video signal, a common mistake. That's easy to solve.

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Mr.RC-Cam:

The source is a KX171 camera, connected to an airwave receiver through an OSD (own design).

It seems I can't adjust the video level on airwave receivers.

Every input/output is 75 ohms, I'm not using the Y cable trick.

See here, the RX (airwave receiver with 4 buffered output): http://www.wiredhouse.fr/2010Projects/video_splitter/splitter.html

I will check the camera video signals on a scope, they may be a bit low.

Regards,

Kilrah's comments are correct on all counts. :)

The first thing that comes to mind is that your native video signal is incorrectly calibrated and the display device is not able to compensate it. So before you build/buy anything, calibrate the video levels and then re-adjust the goggle settings; That alone may cure the problem.

Another thing that comes to mind is that you are ganging two (or more) video display devices onto one video signal, a common mistake. That's easy to solve.

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The video levels on the Airwave modules are not precise and will vary. In cases like this one it is best to measure the full white level on all the buffered Rx outputs while they are terminated with 75 ohms. If you find the levels are out of spec then you might be able to modify the gain on your 1x4 buffer circuit.

Your video coupling caps seem physically small in the photo. What is their uF?

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I did the measurements, and I got 1.96Vpp when camera not connected to the receiver, and 900mVpp when connected to the goggles (full white level).

So the signals seem to be allright.

I noticed a voltage drop of 200mV when going through the OSD, I will have to inspect this part, this is not normal.

But still in direct connection to the goggles the picture is very dark. I suspect the goggles, but don't worry I'm definitely not happy with these and I am about to order some fatsharks.

Coupling caps: they are 470uF.

Regards,

The video levels on the Airwave modules are not precise and will vary. In cases like this one it is best to measure the full white level on all the buffered Rx outputs while they are terminated with 75 ohms. If you find the levels are out of spec then you might be able to modify the gain on your 1x4 buffer circuit.

Your video coupling caps seem physically small in the photo. What is their uF?

Edited by mosfet

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