Hi friends,

in my video generation circuit, when I send the signal onto the TV, and measure the voltage into the TV, I measure 4.5Volts, which is rather a lot. I've known that TVs composite input is 75Ohm impedance, and I connected a 1K resistor before that, so it shouldnt take 4.5V. Why is this happening? Also the TV isnt drawing any current. This is weird. The TV is working perfectly though as I have tested it with a console.

Please help :I

 

With RS-170 (same for other standards) the average voltage is supposed to be very close to zero. The main reason is to limit the current through the receiving end termination resistor. A capacitor is sometimes used in series with the output of the output amplifier to get a low average DC level.

 

There is no current going in. The voltage is high as if high impedance. Putting a capacitor won't solve it. Whats happening buddy ?

 

hi PC,
Watch this vid and note the video input circuitry. AC coupled.

E

EDIT:
You may also find this link a useful info source.
http://www.epanorama.net/links/videocircuits.html

 

hi PC,
Watch this vid and note the video input circuitry. AC coupled.

E

EDIT:
You may also find this link a useful info source.
http://www.epanorama.net/links/videocircuits.html

Why does it have to be AC coupled ??? What if it isnt ???? Is it required to be ??????? Omg!

 

Why does it have to be AC coupled ??? What if it isnt ???? Is it required to be ??????? Omg!

hi PC,
If I understand correctly from this Thread and the others you have started, you are planning to design and build a TV video sync/modulator driver circuit..???

Hope you do not mind me saying this, but if you do not understand the fundamentals of basic electronics you have set your self an almost impossible task.

Post a diagram showing the driver circuit you are using and an idea of the data you intend to display.

Eric

 

Why does it have to be AC coupled ??? What if it isnt ???? Is it required to be ??????? Omg!

I think the standard video protocol is to DC couple the output and to AC couple the input in the video processor chain.

http://electronicdesign.com/analog/get-grip-clamps-bias-and-ac-coupled-video-signals

Despite these complications, video signals must be ac-coupled at the point where the voltage domains change. Connecting two different power sources through a dc connection is dangerous, and usually forbidden by safety regulations. Consequently, video-equipment makers have a tacit agreement to ac-couple the input of their equipment, and dc-couple the output—requiring the next stage to re-establish the dc component (see EN 50049-1 for PAL/DVB \[SCART\] and SMPTE 256M for NTSC, which allow a dc output level). Failure to establish such a protocol leads to "double coupling," where two coupling capacitors appear in series, or to a short where there are no capacitors. The sole exception to this rule is battery-powered equipment, such as camcorders and still cameras, which ac-couple the output to minimize battery drain.

 

hi PC,
If I understand correctly from this Thread and the others you have started, you are planning to design and build a TV video sync/modulator driver circuit..???

Hope you do not mind me saying this, but if you do not understand the fundamentals of basic electronics you have set your self an almost impossible task.

Post a diagram showing the driver circuit you are using and an idea of the data you intend to display.

Eric

I dont mind you saying that. But I never give up and this is how I achieve things. I do know the basics of electronics, but I know more about digital than analogue. I have the circuit almost completed, but I dont know what the TV is doing to the signal. I have a signal that goes from 4.5V to 0V. From what I know the TV has a 75Ohm resistor to ground. So I put a 1k resistor after my signal and before the TV, in order to get a maximum of 0.3V going in. However, the voltage is always 4.5V in.

I tried attaching a bipolar transistor. Then the voltage did lower, however I cant tell if the TV is syncing to the signal or not....

When I invert my signal, the TV goes black, and I can see a few white lines on the screen, rolling upwards and leftwards. When I remove the inversion, the TV goes nice and white. I cant tell which is the correct signal. Is the white TV screen syncing? Or is the black TV screen syncing badly ?

 

Please, here are some photos if what the TV is showing.... I really need help

When I invert the signal, and 0V becomes 4.5V, then the TV shows these small little white lines. When I do not invert it, the TV is kinda white...

I am using ripple counters to count the lines and columns, and then using logic gates to decide when to output the SYNCs. The signals are all over the place. When I look at my scope, the signals are a mess. But when I probe individual signals either H or V sync, each is fine.

 

Please, here are some photos if what the TV is showing.... I really need help

Morning PC,
We are keen to help you, it's what we do.
You must help us by posting a circuit diagram of the circuit you have built for driving the TV input.
Post the make/model type of the TV monitor
E