Using specialized displays to monitor colorimetry
| RGB Gamut display|
| Composite Gamut display|
| Gamut Analyzer display, with gamut errors highlighted (in red)|
Measuring HD-SDI signals is similar in many ways to SD-both use component waveform monitors and vectorscopes. Eye pattern displays and jitter measurements check the health of the physical layer, with HD being more critical.
Any errors in the Cyclic Redundancy Check (CRC) indicate how close a signal path is to the digital cliff. In HD, CRC is present in each video line with a separate value for chroma and luma components.
Readouts of ancillary data (there's lots more with HD) check if things like closed captioning and embedded audio are where and what they should be.
But there are differences between HD and SD, besides the obvious-number of lines, pixels per line, and frame rates.
A big one is colorimetry. The percentages of red, green and blue that make up the luminance signal are different for HD compared to SD.
Y = .2126R + .7152G + .0722B
While for SD (and NTSC):
Y = .299R + .587G + .114B
So it follows that the color difference signals B-Y (Cb) and R-Y (Cr) will also be different. HD color bars, when viewed on a waveform monitor will indeed look different than the SD color bars waveform most are used to. That is as it should be. Don't try to make adjustments to HD equipment to make color bars look like SD.
Also, some colors that are available in HD don't have an equivalent in SD or analog. While these colors may be legal in HD, they won't be valid when downconverted to SD or NTSC. Or colors generated in RGB space may produce out of range colors in color difference space. This is especially easy in areas like graphics and camera control where RGB levels are readily manipulated.
Invalid colors can manifest themselves as washed out areas, color changes, striping, loss of color detail in an area of the picture, and loss of brilliance. In addition, analog transmitters will clip any signal levels that are too high, like those caused by invalid colors.
Illegal colors are those that fall outside of specified voltage levels in a particular format, and when this happens, a gamut violation occurs. Those colors that are out of range are said to be "out of gamut."
To avoid a run in with the "color police," it's a really good idea to monitor color signals on specialized displays that indicate when illegal and invalid colors occur. There are several tools to help with that task.
The Leader LV5700A rack-mount, LV5750 portable (both with internal displays), and LV7700 and LV7720 (rasterizers) all provide as standard the 5-Bar display for red, green, and blue color signals, plus Y and Y+C (luminance plus chrominance) signals. These bars resemble audio level meters, but here video levels are monitored. If levels are too high this is indicated by a change in color (red) on the bar display.
According to George Gonos, director, sales & marketing, for Leader Instruments, the scope includes modes where the 5-Bars display is seen side-by-side with a waveform display of Y, Cb, Cr as they are encoded from the stream. Three of the bars show the RGB equivalent levels. The Y bar represents the RGB levels converted to luminance using the SD formula. The Y+C bar is used to tell if any HD colors will be invalid in the composite area.
A helpful tool in camera setup is the DSC Labs ChromaDuMonde series of charts. Properly lit, the DSC color chips are at mid-saturation and produce levels of 560 and 280 mV digital (80 and 40 IRE analog).
The HD vectorscope used for camera setup with this type of chart should be taken out of the "CAL" or calibrated position and its gain should be set to 1.875. The camera controls are then adjusted so the color vectors terminate in their boxes on the display.
Once that's done, a DP or director can creatively deviate from the standard setting to create a warmer or cooler look by adjusting the color controls. Gonos said that many DPs, directors, and camera operators use the 5-bar display to help them get the exposure and look they want right in the camera, instead of trying to change things later in post, and also to keep them out of color trouble.
As Gonos point sout, an expert has a lot of control over the camera's performance. The more basic adjustments include lens flair (setting true black), color balancing the camera, and setting the toe and knee points. The more advanced settings involve the camera's color matrix and a read-through of the service manual, but allow DPs and directors greater control of color saturation and the "look" of the camera, Gonos said.
Tektronix developed color-related displays of its own. The Tektronix WFM7100, for example, provides Diamond, Split Diamond, Arrowhead, and Lightning, according to Mike Waidson, video test applications engineer at Tektronix.
The Diamond display is used to monitor the effects of creative adjustments, track gamut violations, and help users make color correction, gray scale tracking and black balance adjustments. The upper diamond displays the green and blue components, and the lower diamond the green and red. When a signal is out of gamut, the trace on the display is outside the boundaries of one or both of the diamonds.
The Split Diamond display just separates the two diamonds so that it's easier to see the black region, where, on the Diamond display, the two diamonds join.
The Arrowhead display is used to determine if a component signal is within gamut after it's been encoded to composite PAL or NTSC signal. This is done without actually encoding the component signal. This display plots luma on the vertical axis and the magnitude of the chroma subcarrier at each luma level on the horizontal axis.
The Lightning display, which shows the luma and color difference components, is used to properly time the component signals together and to correctly set amplitudes.
The Videotek TVM-950 provides the company's Digital Gamut Display to check for out-of-gamut conditions. According to Mike Richardson, director of product technologies for Videotek at Harris, this display is a vector type of plot that maps color space information (based on luminance and chrominance sample values) into a circular diagram. In the composite gamut display, active video samples are converted to a representative composite value before being plotted (see screen captures on p. 12).
In the RGB Gamut display, the vector plot consists of three points, Y, Cb, and Cr sample values that are decoded into RGB values.
The graticule scales are two concentric circles that form a ring. These represent the upper and lower limits of acceptable levels. Inside the ring are legal values, outside the ring are not.
These various color-related displays can greatly help in understanding the intricacies of color space, and as an added bonus, aid in creating better pictures.