Slight problem with PAL video generation (Circuit done)

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PauloConstantino

Joined Jun 23, 2016
266
Hi friends,

I have finished my circuit to display composite monochrome video signals onto a TV. I can see that the TV is synchronizing to the signal, however the TV shows a white screen. I am not sending any pixels to the TV yet. I am only sending synch pulses. The problem is that I wanted to be able to at least make it display a black screen at will, but I am having problems with it doing it. No matter how small the voltage that I send to it, it always shows white... My synchronization pulses are correct.

The signals are monochrome PAL standard... Basically I have the output signal that goes to 0V at synch times, and then goes to around 0.4V when its not synching. As far as I know, 0.3V is black, so it should be almost black on the TV, but its not. I don't have a schematics for it.

But my output signal is either a logical High or Low, then I attached a 1K resistor to it, and connected to the TV. The strange thing is that when I connect the signal to the TV, and attach the scope to it to see whats going into the TV, the signals are OK but the sync no longer reaches 0V. But when I remove it from the TV, the sync goes to 0V.

What should I do ?
 

bertus

Joined Apr 5, 2008
21,312
Hello,

What is the output impedance of your circuit?
Usualy the video input of a TV is 75 Ohms.
When your driving ciruit has a to high impadance, the signal will be influenced.

Bertus
 

DickCappels

Joined Aug 21, 2008
7,244
Congratulations on getting your monitor to sync with your monitor!

From memory, the sens of modulation in the PAL RF Envelope has the sync tips corresponding to minimum carrier amplitude and white corresponding to maximum carrier amplitude. NTSC is the opposite with sync tips corresponding to maximum carrier amplitude and white corresponding to minimum carrier amplitude.

So...if you are coupling via RF check and see whether your modulator is for PAL or NTSC.
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
Congratulations on getting your monitor to sync with your monitor!

From memory, the sens of modulation in the PAL RF Envelope has the sync tips corresponding to minimum carrier amplitude and white corresponding to maximum carrier amplitude. NTSC is the opposite with sync tips corresponding to maximum carrier amplitude and white corresponding to minimum carrier amplitude.

So...if you are coupling via RF check and see whether your modulator is for PAL or NTSC.

Hi Dick. My TV is PAL. And I am not using RF, but composite video. The yellow connector.
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
Can you post your video output circuit's schematic?

I dont have schematics for it. I built it from my mind just to see how it would work. I got it to synch but cant control the shade. I just dont understand what the output is doing :( I know that TVs have 75Ohm to ground so I figured a 1K ohm in series with my output would generate 0.3/0.4V to the output, which is black level, but it doesnt work and I dont know why ? I noticed on the scope that when I connect it to the TV the voltages jump higher than what they were before...
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
Congratulations on getting your monitor to sync with your monitor!

From memory, the sens of modulation in the PAL RF Envelope has the sync tips corresponding to minimum carrier amplitude and white corresponding to maximum carrier amplitude. NTSC is the opposite with sync tips corresponding to maximum carrier amplitude and white corresponding to minimum carrier amplitude.

So...if you are coupling via RF check and see whether your modulator is for PAL or NTSC.


My TV is PAL Dick. Also, I have even tried inverting the signals with an inverter, and the funny thing is that when I invert it, then I can see the horizontal sync periods as little white lines on the screen, and the vertical sync as a thick white line at the bottom. This means that my original signals are correct and the inverted ones are incorrect...

However when I try to lower the voltage of the display by attaching a resistor, all voltages become 200mV higher than before, and that is bad because then my syncs are no longer 0V but 200Vm and my whites are even higher voltage.

I just dont know what to do to lower the voltage.
 

AnalogKid

Joined Aug 1, 2013
9,151
What is the make and model number of the monitor? There might be a schematic online that can explain what you are seeing.

The monitor input might be AC coupled followed by a tip clamp, a back porch clamp, both, or neither. It the signal is unclamped, that might explain the on screen brightness. Are the brightness and contrast controls turned all the way down?

ak
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
What is the make and model number of the monitor? There might be a schematic online that can explain what you are seeing.

The monitor input might be AC coupled followed by a tip clamp, a back porch clamp, both, or neither. It the signal is unclamped, that might explain the on screen brightness. Are the brightness and contrast controls turned all the way down?

ak

It's an old monochrome chinese CRT tv. MTEC, model AT0424. If that helps :D

I tried AC coupling it to the TV also to remove the DC I guess, but it didn't work.

The controls are down yes....
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
I don't have schematics for this.... All you need to know is that my signal is going to 0V at times, and then back to around 4.5V. And I need this signal to become a signal from 0V to 1V with impedance good for the composite CRT TV.

I've read somewhere though that the sync signal is actually -0.3V, and that black is 0V. If this is the case my signal will not work because my sync is 0V...

I need then some circuit to convert 0V to -0.3V for my sync.... Something like a transistor circuit ?

Can anyone help me with this please buddies ?
 

bertus

Joined Apr 5, 2008
21,312
Hello,

How do we know what you need without ANY schematics?
I am almost 40 years busy in eletcronics, but always have used and made schematics of what I created.

Bertus
 

Thread Starter

PauloConstantino

Joined Jun 23, 2016
266
I will solve my own simple problem again guys.

My simple problem was that the sync level is actually -0.3V rather than 0V.

Thanks everyone for your awesome help :)

Bertus is a nice guy though
 
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