# composite signal

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

1. ### WongGuest

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 GarthGuest

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. ### BaphometGuest

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 MyersGuest

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.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Continue to site