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NTSC-compatible video signal circuit - ntsc.pdf (0/1)

Discussion in 'Beginner Electronics' started by Rico Rivera, Apr 13, 2004.

  1. Rico Rivera

    Rico Rivera Guest

    I am having a little trouble understanding how this circuit works. I
    got this circuit from one of my microcontroller books in a chapter
    describing how to use a PIC16C71 to generate a NTSC-compatible video
    signal but I assume that any other microcontroller can use the
    attached circuit.

    This is out of the text:

    "The I/0 line RA2 drives Q1, dropping the video line down to 0 volts
    for horizontal and vertical sync. RA0 drives Q2, pulling the video
    line down to 0.25 volts for the blanking level. If neither Q1 nor Q2
    is on, the video line will be one volt (white video pixel). Resister
    networks R3, R4, and R5 generate the proper voltage levels into a
    75-ohm load."

    I have three questions about this circuit and the text. I understand
    why when Q1 is on the video line drops down to 0 volts.

    - What I don't understand is then Q2 is on how is the video line is
    0.25 Volts.

    - Also, why when Q1 and Q2 is off the video line is at 1 volt.

    - And my last question is R5 used for impedance matching?

    I tried to work out several combinations and I didn't get the
    indicated voltage levels described above. Thanks in advance for any
    help.

    If you have trouble opening the attached file, please visit
    http://www.compiledsolutions.com/ntsc.pdf for the same schematic
    online
     
  2. On Mon, 12 Apr 2004 23:31:06 +0000, Rico Rivera wrote:


    R5 is part of the series chain 220R+75R+external 75R which gives you about
    1v when both transistors are off. Note that the external 75R isn't shown
    on the drawing, but it forms a voltage divider with the internal 75R!

    with Q2 on, you have 33R in parallel with (75R+75R), giving about 27R. The
    output voltage, taken across the lower 75R (external) is then half of
    0.546v, which is pretty close to 0.25v.
     
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