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Monitoring |
The waveform monitor and the vectorscope have undergone quite a makeover. In SDI video, there is no sync, burst, or subcarrier. There is no setup (yeah!) and there is no IRE scale, either. Before you take a look at the new monitoring device, we need to look at "gamut".
If you don't remember visiting gamut in the first place, that's because we didn't call it that!
We called it "Ilegal Colours".
The word "gamut" is borrowed from the field of music by many fields. It simply means "the entire range".
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Recall that a 100% Green and 100% Red signal stuffed into
an NTSC matrix results in problem video. There are other troublesome
combinations, but this is the most spectacular. The luminance in this
example is (59% green) plus (30% red) for a total of 89 units, and the
chroma extends to above 130 units.
100% in two colour channels and nothing in the third wasn't encountered in real life. When computers started creating graphics, the problem became real. |
SDI can code and display these fully saturated colours, but SDI usually ends up being converted back to NTSC... either for broadcast, or from a digital set-top box to the TV set. With an NTSC waveform monitor set to display chroma, the problem is easy to see. As there is no chroma in the SDI world, problem colours don't jump right out on their own. To keep compatible with NTSC, SDI waveform monitors include colour-gamut displays.
An SDI Waveform Monitor
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An SDI waveform monitor looks familiar at first glance...
until you try to find one single button or knob that you recognize!
You will learn to love this. |
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This is your new waveform... a parade of the Y
channel, and the two colour channels Pb and Pr.
The yellow circles highlight scale markings. The main scale on the left is calibrated in tenths of a volt. Black is at zero, and white is at .7 (700 millivolts... mv). The green circle is around a "start of active video" SAV spike. Those reserved numbers above peak white decode into a spike above 100%. The monitor can be set to hide these spikes. |
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Well, the vector display looks normal... except there is
no burst, and no phase knob.
The old NTSC I-Axis is displayed. It happens to be where most flesh colour sits. Since there is no Y compontent involved in this display, it doesn't show trouble with out-of-gamut colours. |
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The Lightning display plots the two colour channels
sideways against the Y channel up-and-down.
This display will show errors in timing or levels in the colour channels. |
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The Diamond display plots Pb on the top
against Y, and Pr against Y on the bottom.
Anything outside the diamonds is outside of the NTSC gamut. (Psst... Pb is our old B-Y from the analog days, and Pr is R-Y) |
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The Arrowhead display is another gamut plot of Y
versus the two colour channels, and the easiest to read.
Anything outside the four-sided dashed lines is out of gamut. The red arrow indicates a point that would put the NTSC chroma at 120 on the scale. |
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The Eye display shows the raw SDI pulse train, and includes markings and cursors to measure jitter, and the pulse rise (and fall) times. |
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Surprise! The new waveform monitors can show you what picture you're actually looking at. |
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Pb & Pr rest at mid-scale when there is no
colour, and extend above and below their zero points when there is
colour.
Here's the waveform from the above head-shot. Notice where the face is on the left-hand Y display. There is "downward" video in Pb (it's "anti-blue". Pr has a positive showing where the flesh is. |
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We still have PLUGE!
Pluge was possible in NTSC because of the 7.5 unit setup. In a world where black is at zero, "blacker than black" is not possible, but SDI has reserved numbers below black, used here to create pluge. |
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