Television Impedance problems....

Discussion in 'General Electronics Chat' started by PauloConstantino, Jan 9, 2017.

  1. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
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    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
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    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.
     
  3. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
    266
    7
    There is no current going in. The voltage is high as if high impedance. Putting a capacitor won't solve it. Whats happening buddy ?
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
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    Last edited: Jan 9, 2017
  5. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
    266
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    Why does it have to be AC coupled ??? What if it isnt ???? Is it required to be ??????? Omg!
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    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
     
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    4,677
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    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
     
    RichardO and ericgibbs like this.
  8. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
    266
    7

    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 ?
     
  9. PauloConstantino

    Thread Starter Member

    Jun 23, 2016
    266
    7
    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.

    [​IMG] [​IMG] [​IMG] [​IMG]
     
  10. ericgibbs

    Moderator

    Jan 29, 2010
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    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
     
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