Help on composite video signals

I have problem in converting composite video signal to black and white signal. I used sallen key low pass active filter to remove color information. Its output is amplified, and fed to window comparator, its output are 1Vpk for white, 0.3Vpk for black, and 0V for sync pulses. When output is attached with TV, only large size objects are displayed correctly, small size objects aren’t displayed correctly, instead of small objects TV screen is completely white.
Thanks for help………..

What frequency filter did you use? How many poles and what type?

You would be better off notching out the colour burst, I would have thought.

Keith

I am using two pole filter. Which completely removes color reference burst and color information in active video.

But what is the corner frequency? It sounds like it also completely removes the detail!

Keith.

I used corner frequency 447 KHz, because it attenuates up to -12dB high frequency color signal (which has corner frequency 3.58MHz).

That is very low. Normally it would be around 2.5MHz if using a low pass filter. It would be worthwhile looking at the signal on an oscilloscope to see what is happening. Also, you need to be sure that your filter has the bandwidth you think. If your opamp doesn't have a wide bandwidth your filter frequency could be lower than you think. A 2 pole Sallen Key Butterworth would require around 50MHz bandwidth opamp.

http://www.intersil.com/data/an/an9/an9644/

Keith.

I think you are right, my cut off frequency is really too low,
Thanks for help

There is an alternative way.
The color decoder is locked to a burst of 3.58MHz sent during the horizontal blanking period. The burst is required for the decoder to operate and if missing, the TV/monitor will disable it's color decoder. If you strip out the sync signals using an LM1881 or similar device which is very inexpensive, and use the blanking output to gate the video signal you can 'chop out' the color burst without having to do any filtering at all.

Brian.

If you use a "color carrrier trap filter" ( 3.58MHz or 4.43MHz or depending on your TV system..) the signal won't carry the color information anymore..