SMPTE Colorbars

Koz's Calibration Corner

SMPTE Colorbars


640/480 (26.7K Picture)

Well, here we have it. The long awaited 75% colorbars. Or at least as much of it as a pre CCIR-601/SMPTE-RP125M computer can do.

This pattern was created in Corel Draw with reference to SMPTE Engineering Guideline EG 1-1990 Alignment Color Bar Test Signal for Television Picture Monitors , exported to Photoshop and then saved as an almost non-compressed JPEG file for this page. It has enough redundancy of pixels in it to compress to almost the vanishing point. It started out a full 900K Photoshop file and compressed--almost without distortion--to 26.7K.

Where to begin.

I guess I should start with the parts I left out. There are no bluish- purple -I and Q bars flanking the white bar at the bottom left of the picture and the blacker-than-black pluge pulse in the lower right is missing. These three signals are purposely created in the studio by distorting the normal NTSC color encoding process. They do not appear in nature and therefore a conventional graphics computer cannot create them.

The -I and Q bars are produced as color values only and do not have the luminance value normally associated with a colored object. These two colors appear in very specific places on analog NTSC video measuring instruments.

The pluge pulse is two dark gray vertical stripes in the lower right. These stripes are used by the operator to set the "brightness" or black level of a picture monitor. The first one, which I left out, is actually blacker than normal picture black. In computer terms, if picture black is 0, 0, 0 (red,green,blue)and picture white is 255, 255, 255, then the first pluge pulse would have to be -12, -12, and -12. 601 and analog video can deal with this, older computers can't.

This colorbar pattern, or patterns that are close relatives, is standard in the NTSC video world. A video program can be recorded on tape in Boston with the traditional "minute of bars" at the beginning and then flown to San Diego. The operator in San Diego plays the colorbar pattern and adjusts the machine so the bars are recreated with the least distortion. This insures that the following program will look the same in San Diego as it did in Boston. This adjustment process is done with all those knobs behind the flip-down panel on professional and broadcast tape machines.

Computer folk can enjoy the same advantages. Any time you put a computer generated graphic on video tape, put at least 30 seconds of this pattern at the beginning of the tape. This will do three important things. It gives everyone the impression you know exactly what you're doing, it puts something pretty at the beginning of the tape where most of the noise, scratches, speckles, and dropouts are, and if the tape is played back in either a professional or broadcast house, the pictures can easily be made to look as good there as they did when they left you.

That is, the pictures will look good if they are NTSC legal. It's perfectly possible for a computer to create pictures with colors and object brightnesses that don't fit within the NTSC guidelines. This is one reason why generating graphics for video isn't as easy as it looks.

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