PAL Video AGC - tips on designing

[davorin]

video agc design

Someone has a good design tip for a PAL type Video AGC?

The problem is that some scrambled TV channels have a sort of triangle wave overlay so a TV or VCR gets confused and adjusts the amplitude automatically. Therefore resulting in an image with some brighter areas in the middle (after descrambling).

The only part of the video signal which can tell me the amplitude is the color burst signal.

So maybe use a rectified/low pass filtered version of the color burst to restore video amplitude?

 

[Wizzard]

descrambler auto inversion

You are better of going in to the tuner section and slow down the agc on the video in there
this will remove the problem you are having

/Wizzard

 

[nandopg]

You need a video clamp not an AGC

Dear friend,

Your problem doesn't address to an AGC circuit it addresses to a video clamp circuit.
By clamping the video signal at the tip of sync at a constant voltage, the low frequency superposed on the video signal will be highly suppressed.
Let me know if you need a video clamp circuit sample.

NandoPG

 

[flatulent]

agc

television sets use synchronized AGC methods where it only reads the peak of the synchronization pulses and holds the value over the frame. The trick for decoding the signal is to be able to generate an external triangel (I suspect a sine wave is used) and subtract it out from the base band video. Another trick is to use a notch circuit at the sine wave frequency.

Some of these systems use a sine wave on the audio channel which can be filtered out and added (subtracted) to the base band video signal.

 

[davorin]

I have tried modifying the AGC circuit of my VCR which has a Sanyo LA7578 chip. Haven't got any luck so far.

The clamping isn't really the problem here (o; Looks like the Cable-TV company send a huge pulse during VBI which confuses the AGC. Maybe the video circuit gets saturated without AGC

Think I'm gonna try today to mask out the incoming Hsync and see if I can use the average level for adjusting the picture...

Strange is that I also have some sort of ghost lines in brighter areas. Like when there is some contrast rich part in the picture...the rest of the line has the inverted value.

Also might be a good idea to post some scope pictures here or on my website (o;

 

[davorin]

Hmm....just a quick thought (o;

Do VCR and TV feed back hsync signal for AGC or do they adjust the level only
based on RF information?


Maybe the brighter bands in the picture arise from the problem that the VCR/TV doesn't recognise the hsync correctly and uses the video signal for gating AGC?

 

[Wizzard]

It would help to know what kind of encryption your system is using

/Wizzard

 

[davorin]

Already solved for the VCR (o;

Manually setting AGC via potentiometers...

The descrambler is based on the 3CHIP design...
modified a little for Finland though (o;

At least auto-inversion detection works now...


Now I need to figure out how to disable AGC in the Blaupunkt TV (o;

 

[sda]

Sounds like the old SAVE system used by the BBC a few years back (analogue).
I solved the problem then, by using a bandpass filter, then amplify the 90~95 khz sine wave that was imposed on the video signal, then added it to the signal again but out off phase, potentialy nulling it out.

What about your audio?
suppressed carrier?


cheers
sda

 

[davorin]

Actually found out wht it is (o;

No sine/triangle wave...

Just shifted during VBI so AGC gets confused...

VCR okay now (o;

Now for the TV (o: