(Back to the WaveScope User Guide)
AVX Plug-Ins
Primer
When driving, you exit the highway for the city street, and you suddenly feel that you
are going very slowly. You check your speedometer to see how fast you are actually going.
Similarly, when judging video, your perception will be affected by visual desensitization,
your monitor type and setup, room lighting conditions and individual tastes. A waveform
monitor and vectorscope are indispensable to keep you within limits and give you a true
measure of the brightness and colors of the image.
3Prong's WaveScope AVX Plug-In lets you measure video levels in your timeline.
Instrument displays include:
This Primer explains these displays and their possible uses. For
specifics on operation, please see the WaveScope User
Guide.
Instrument Displays
 Waveforms
A video waveform indicates the brightness of the image. WaveScope shows conventional H
waveforms, with the traces of all the horizontal scanlines for the current video field
overlaid atop one another. For example, if you see a bright object on the right side of
the image, you will find its peak on the right side of the waveform. Note that the
waveform does not change with vertical motion in the image.
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Y Waveform
 "Y" refers to Luma, the
brightness component of the image without any color. The Y Waveform measures what a viewer
would see on a black and white screen.
When you measure a standard color bars test pattern, pure black will be seen in the
waveform as a flat step at a height of 0%. Pure white produces a step at 100%. In addition
to the percentage scale on the right, the left side of the Y Waveform shows a digital
level scale in an 8-bit or 256-step range. On the standard digital scale (ITU-R BT.601),
16 is the level for black, and 235 for white.
WaveScope's True Color Option shows the Y waveform in the actual colors of the image.
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YC Waveform
 The YC Waveform shows components encoded as
composite video. Composite video has a Chroma signal (C, derived from Cb and Cr
components) "riding" upon the Y waveform. In WaveScope, the Y trace is shown in
green, and C as a cyan "envelope" surrounding it. Because the C signal usually
has equal positive and negative excursions, you may notice that the cyan bands are at an
equal distance above (Y+C) and below (Y-C) the green waveform.
The left side of the YC Waveform shows a scale marked according to your project
standard, NTSC or PAL.
- NTSC video is measured in IRE units: 0% black is commonly at 7.5 IRE (except in Japan),
and 100% white is at 100 IRE.
- PAL video is measured in millivolts: 0% black is at 0 mV, and 100% white is at 700 mV.
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RGB Parade
 This displays waveforms of the Red, Green and Blue
components side by side. Since video cameras capture in RGB, this display may help to show
camera problems among other things.
Keep in mind that red, green and blue are used together to create all other colors in
video. A white area in the image will be seen as peaks in all three RGB Parade waveforms
at the same relative location. A high red level does not mean a reddish image unless the
corresponding green and blue levels are low.
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RGB Histogram
 A histogram of an image shows which color values
appear most frequently. There are graphs for each of the red, green and blue components.
The darkest red, green, and blue values from the image being measured are shown as peaks
on the left side of each graph, and the brightest values are on the right. The height of a
peak indicates the number of pixels with that value. A wider peak shows that there are a
lot of pixels with similar values. For example, if the blue histogram has a lot of tall
blue peaks at the left side, it shows that there are a lot of pixels with low blue values.
(However, those same pixels might have any red and green values and so could show in the
image as a variety of colors.)
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YCbCr Parade
 Most video devices such as video decks and your
Avid system do not take in or put out RGB signals. Instead, RGB signals are converted
within the camera to an intermediate signal format such as YCbCr, and converted back to
RGB in the receiver or monitor. The Y in YCbCr is the same luma as shown in the Y
Waveform. Cb and Cr are "color difference signals" that represent just the color
information, with the luma removed. (Other names for variants of YCbCr are YUV, YPbPr,
YIQ, and Y/R-Y/B-Y.)
The Y waveform is shown on the left side in white, followed by Cb and Cr shown in the
colors representing those vectors. Cb and Cr values can be negative or positive. You will
see that positive Cb is bluish, and negative Cb is yellowish. Positive Cr is reddish, and
negative Cr is a cyan shade. For images that are entirely black and white, Cb and Cr are
zero, so you will see flat lines halfway up the two right bands. If there is a color cast
to the image, you will find the Cb or Cr waveforms are not centered.
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YCbCr Histogram
 The YCbCr Histogram
shows which Y, Cb and Cr values appear most frequently in the image. The height of a peak
indicates the relative number of pixels of that value.
The Y histogram is on top in white, with the darkest values are on the left side and
the brightest values on the right. An image with good contrast will show a spread of
values from the darkest to the lightest.
A colorful image shows as a wide spread in the Cb and Cr histograms. If they extend too
far from the center, there is too much saturation.
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Vectorscope
 The
Vectorscope shows only the chroma, without the luma. The Cr signal is shown in the
vertical dimension, and the Cb signal in the horizontal dimension. Colors created by
various positive and negative combinations of Cb and Cr show up around the circle. Areas
of the image with little chroma are centered in the circle, and areas with more saturation
are further out from the center. Images with an overall color cast produce a vectorscope
trace that is generally off-center.
Small squares in the Scale mark the location of standard color bar vectors. Inner
squares mark the proper values for 75% color bars, and outer squares are for 100% color
bars.
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Possible Uses
WaveScope is useful as a guide in color correction. You may apply and view WaveScope on
an empty track above, while adjusting 3Prong's ColorFiX
plug-in on the track below. With experience, you will learn to read the instrument
displays to:
- Match scene brightness across a cut in Y Waveform.
- Put your flesh tones along a certain hue axis in the vectorscope.
- Watch the spread of the Y histogram to identify a good contrast range without clipping.
- Watch the top of the YC Waveform to make sure you do not have too much bright chroma.
- Fix white balance and black balance problems by centering the Vectorscope or YCbCr
histogram traces.
The key here is to develop your judgment about what looks good, and not just follow the
rules. A computer could automatically set levels so that luma peaks do not exceed 100% and
the darkest parts of a scene touch 0%. However, it will not consider the objects in the
scene, the contrast ratio, or the desired mood. With backlit subjects, for instance, the
sky or a window in the background may be substantially brighter than the lighting on the
foreground. In such cases, you might want to adjust for the foreground and leave the
background too bright (providing you still meet any safe color requirements.)
Another use for WaveScope is identifying problems with source video, such as
- Excessive levels
- Missing channels in YCbCr or RGB, indicating an equipment problem or a bad connection.
- Clipping in YCbCr, RGB, or YC channels. The trace will appear chopped off at a certain
level below the maximum.
- Graphics imported at the wrong level settings. Those imported at RGB levels of 0-255
that should have been imported at 601 levels of 16-235 will lack contrast. If they have
too much contrast, with levels exceeding the 0% and 100% markings, the opposite is likely
the case.
You can also use WaveScope to align levels of sources using test patterns. If you
capture some color bars from your source footage, you can measure them and set the color
correction needed to restore the video levels to the way the program was created. Import
the Test Patterns from the Supporting Files folder of your Avid system to become familiar
with the proper Waveform, Parade and Vectorscope readings. (Histograms are not so useful
on test patterns.) For example, with 75% color bars, note that the Y+C envelope for the
yellow and cyan bars should match the 100% white level.
(Back to the WaveScope User Guide)
