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Composite video signal

Discussion in 'Electronic Basics' started by MRW, Nov 14, 2007.

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  1. MRW

    MRW Guest

    Good day! I am reading some application notes regarding Video Basics:
    http://www.maxim-ic.com/appnotes.cfm/appnote_number/734

    I cannot seem to understand Figure 6: Composite Video Waveform. I
    understand that the IRE levels represent the brightness of the
    picture. What I cannot comprehend is the representation of the color
    bars. It shows yellow from 140 to 50 IRE, red from 100 to -20 IRE. The
    application note also mentions this:

    "Color information is added on top of the luma signal and is a sine
    wave with the colors identified by a specific phase difference between
    it and the color-burst reference phase."

    How does this look exactly on an oscilloscope assuming that the
    oscilloscope readings are accumulated thru time?

    I'm not understanding that if a sine wave "rides" on top of the luma
    signal, then shouldn't the brightness of the picture also change with
    the amplitude variation of the sine wave?

    Please explain in very basic terms. I find sometimes that explanations
    are laced with technical terms, and the actual basic explanation is
    lost. I'd like to get the big picture first before delving into the
    technical details.

    Thanks!
     
  2. Bob Myers

    Bob Myers Guest

    What they're showing on that page is one line of a standard
    "color bars" test pattern, with color added so that you can see
    what section corresponds to which bar.

    When the NTSC color encoding system was added "on top of"
    the existing monochrome ("black and white") broadcast standard,
    the practice of referring to video level in terms of "IRE units" had
    already been established. This came about because of at least two
    different signal level standards, which had the same RELATIVE
    values between such things as the sync, blank, black, and white
    levels, but differed in the absolute voltages that corresponded to
    each level (one was a 1.0 Vp-p standard, while the other, older
    standard was 1.4V). It became the norm to refer to the blank-
    to-white difference as "100 IRE," with everything else in the signal
    scaled to match regardless of which standard you were speaking
    of in absolute-voltage terms. (Note that this refers to the U.S.
    standards; the European standards developed somewhat later
    than the U.S., and did away with the difference or "setup"
    between the blank and black levels.)

    The color information appears as high-frequency bursts added
    to the original luminance-only signal. If you showed only the
    luminance ("Y") part of the signal in the diagram on the page
    you referenced, you'd still see the same "stairstep" sort of
    shape, as the "color bars" pattern now turns into a "grayscale"
    pattern of eight levels going from white to black. (The ordering
    of the color bars - White, Yellow, Cyan, Green, Magenta, Red,
    Blue, and Black - was done so that this would be so.)
    "Turning on" the color, though, adds this additional high-frequency
    information (high-frequency because it's transmitted on a 3.58
    MHz subcarrier, and so is "above" the Y signal - actually, intermixed
    with it, starting from above- in the channel). You see a whole lot
    of really high-frequency stuff added to the "stairstep," and if
    you're looking at it on a scope that's set to show the entire line
    (as shown in the diagram), the color stuff is a high enough frequency
    so as to simply appear as "blurry bands" roughly centered on the
    original luminance stairsteps.

    There was a problem with this, though, in that the amplitude of
    this additional color information could, if left unmodified, have
    made the resulting combined signal too large to be transmitted
    (under the original modulation standards) without overmodulating
    the video carrier. As a result, the amplitude of the overall signal
    (and therefore esp. the "higher-brightness" colors, namely the
    yellow and cyan) was explicitly limited, which is why, for instance,
    the bar corresponding to the yellow is not as wide as the others.
    Note that at the other end of the pattern, the blue is similarly
    limited so as to prevent it from extending "down" into the region
    reserved for the sync tips.

    Bob M.
     
  3. MRW

    MRW Guest

    Thanks for the reply, Bob! I have a few more questions, or
    confirmation questions, so I can understand better. Am I interpreting
    this right?

    The IRE level of the lines (example: http://i4.tinypic.com/6pz05xk.jpg
    (line circled in red) ) in the diagram are completely independent from
    the colors? The height of the color bar (highlighted with red arrows:
    http://i16.tinypic.com/6u9pnj9.jpg ) is also independent of the IRE
    level?

    Based on what I've read so far, I can easily see the IRE levels as
    representative of the brightness at that point in time. What I am not
    too clear about as of now is the following:

    - Let's say the IRE level is at 100. Then, is the color information
    biased along this level? In other words, I am picturing it like a sine
    wave with a DC offset. If so, how does the NTSC decoder differentiate
    the amplitude of the sine wave from the brightness value? My problem
    is that if a sine wave rides along the IRE level, then I'm picturing
    it as the IRE level increasing and decreasing in time, so in essence
    the brightness of the color should also change, right?

    Thanks again!
     
  4. Nobody

    Nobody Guest

    Correct; however, the chroma carrier has a relatively high frequency and
    low amplitude. It can be filtered out without noticable loss of
    resolution, and even if it isn't filtered out, you would have to be
    sitting with your nose pressed to the screen to notice it.

    From another perspective, if you look at a fine pattern of stripes or
    dots (e.g. someone wearing tie with a fine chequered pattern), it often
    has a rainbow "moiré" pattern. This occurs when the frequency is close to
    the chroma carrier frequency.
     
  5. Bob Myers

    Bob Myers Guest

    Re the first question, right - what those lines are in the diagram
    are the levels of the luminance (Y) part of the signal only. Again,
    if the color ("C", or "chroma") parts of the signal weren't there,
    you'd be looking at a gray-scale pattern whose brightness
    descreased from white to black as you looked left to right on
    the screen.

    The height of the color bar is independent of the basic "Y"
    signal level, yes; you can still express all of the amplitudes
    in terms of "IRE units," though.

    More correctly, the level of the Y signal represents the "brightness,"
    or luminance, of the image independent of the color information.
    Don't confuse that with "IRE units," which are just ways to
    express the amplitudes of ANY of the parts of this composite
    signal.

    Let's say the Y signal by itself would be at 100 IRE. In
    that case, you're already at the "white" level by definition,
    and there will be no color information "on top of" the signal
    at that point, ever. But now look at the green bar in that
    diagram. The "Y" level (the underlying line) is at roughly
    60 IRE, and what that is saying is that the brightness of
    luminance of this bar is 60% that of full white. The additional
    information that "rides on top of" the Y signal - the chroma
    information - is a high-frequency signal (it's actually a couple
    of signals, combined) which tells the system two things.
    First, the phase of the signal identifies the "hue" of the
    color (is it red, green, blue, purple, yellow, what?). The
    amplitude of that signal - how far it extends above and
    below the "Y" signal level (which yes, is sort of behaving
    like a DC offset in this diagram) is the "saturation" of the
    color - how "pure" it is. To better understand saturation:
    pink is a low-saturation red. A pure, bright red is an
    example of high saturation. And together with the Y,
    you now have a way to completely describe the color in
    terms of its hue, saturation, and value (intensity or
    "brightness").

    Bob M.
     
  6. MRW

    MRW Guest

    Thank you, Bob! That made so much more sense.
     
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