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Composite video signal

M

MRW

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!
 
B

Bob Myers

MRW said:
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:

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.
 
M

MRW

<snip>
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.

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!
 
N

Nobody

- 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?

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.
 
B

Bob Myers

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?

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.

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:

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 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?

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.
 
M

MRW

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.

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