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composite signal

Discussion in 'Electronic Basics' started by Wong, Oct 16, 2003.

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  1. Wong

    Wong Guest

    Hi,
    When we look into composite video signal, the electrical spec: 1Vpp
    @ 75 Ohm, unbalanced, terminated.
    What does this means ?

    Thanks in advance.
     
  2. Lord Garth

    Lord Garth Guest

    The video signal will not exceed 1 volt peak to peak amplitude. The 75
    ohm part is the impedance of a typical video system. Unbalanced means
    it is not a differential signal, one side is referenced to ground and
    terminated means there is a matching (75 ohm) load at the receiver.
     
  3. Baphomet

    Baphomet Guest

    The sync signal is .3 vpp. The video signal is .7 vpp. From the bottom of
    the sync signal to the top of the video signal is .3 + .7 = 1 vpp. 75 ohms
    is the load impedance that the video signal is fed into (terminated) and
    unbalanced means that the load goes from output to ground.
     
  4. Bob Myers

    Bob Myers Guest

    Very close, but I have to point out one item that might otherwise
    lead to some confusion when you're viewing this stuff on a
    'scope.

    The 1 Vp-p amplitude for a composite signal is for the most
    negative-going portion of the signal (the tip of the sync pulse)
    to the reference white level. A composite signal which includes
    color information (using any of the standard encoding methods,
    i.e., NTSC, PAL, or SECAM) may actually exceed the white
    level by some amount, making the overall signal somewhat
    greater than 1 Vp-p when properly set. In other words, the
    1 Vp-p actually applies only to the "black-and-white" (luminance,
    or Y) part of the signal. In the worst case condition for the
    "100%" NTSC signal (NTSC 100/7.5/100/7.5), a fully-saturated
    yellow will take the signal to a bit over 30% above the white
    level at its peak. (As this goes over the overload limit for a broadcast
    video transmitter, such signals are NOT seen in those applications.)
    You may also run into situations in which the chrominance
    level is intentionally limited for saturated yellows and cyans,
    such that the signal does NOT significantly exceed the white
    level ever; this is done specifically to avoid the overload problems.

    Per the original U.S. version of the 1Vp-p standard, white is
    supposed to be 0.714V above the reference level (blanking),
    while the sync tips are 0.286V below this level. Standard
    European practice simplified this to +0.700V/-0.300V.

    Bob M.
     
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