"If it can be written, or thought, it can be filmed..." STANLEY KUBRIC

SMPTE color bars

From Encyclopedia PRO

Jump to: navigation, search
The SMPTE color bars
Enlarge
The SMPTE color bars
SMPTE color bars look somewhat like this on an NTSC vectorscope.
Enlarge
SMPTE color bars look somewhat like this on an NTSC vectorscope.

The SMPTE color bars are a type of television test pattern, and is most commonly used in countries where the NTSC video standard is dominant, such as in North America. The Society of Motion Picture and Television Engineers (SMPTE) refers to this test pattern as Engineering Guideline EG 1-1990. The components of this pattern are a known standard, so comparing this pattern as received to that known standard gives video engineers an indication how an NTSC video signal has been altered by recording or transmission, and thus what compensation needs to be applied to that signal to bring it back to original condition. The pattern is also used for setting a television monitor or receiver to reproduce NTSC chrominance and luminance information correctly.

In an SMPTE color bar image, the top two-thirds of the television picture contain seven vertical bars of 75% intensity. In order from left to right, the colors are white, yellow, cyan, green, magenta, red, and blue. This sequence runs through all seven possible combinations that use at least one of the three basic color components of green, red, and blue, with blue cycling on and off between every bar, red cycling on and off every two bars, and green on for the leftmost four bars and off for the rightmost three. Because green contributes the largest share of luminance, followed by red, then blue, this sequence of bars thus appears on a waveform monitor in luminance mode as a downward staircase from left to right. The graticule of a vectorscope is etched with boxes showing the permissible regions where the traces from these seven bars are supposed to fall if the signal is properly adjusted.

Below the main set of seven bars is a strip of blue, magenta, cyan, and white castellations. When a television receiver is set to filter out all colors except for blue, these castellations, combined with the main set of color bars, are used to properly adjust the color controls; they appear as four solid blue bars, with no visible distinction between the bars and the castellations, if the color controls are properly adjusted.

The bottom section of the test pattern contains a square of saturated (100% intensity) white and a rectangle of saturated (7.5% intensity) black, for use in setting the luminance range. More modern versions of the pattern feature a "pluge pulse." The pluge (short for "Picture Line-Up Generation Equipment") pulse is positioned within the black rectangle, below the red bar (it is present in the illustration but may be hard to see). It comprises three small vertical bars, a rightmost one with intensity just above the saturated black level, a middle one with intensity exactly equal to saturated black, and a leftmost one with intensity just below saturated black (or "blacker than black"). The pluge pulse aids in adjusting the bottom of the luminance range to avoid either washing out the black tones into grays or collapsing picture information into the signal clipping that occurs a small distance below the saturated black level (known as "crushing the blacks"). Also in the bottom section are two sections that contain -Inphase and +Quadrature signals (see YIQ), centered on black level and having the same gain as the color burst signal; these show up on the pattern as two squares of very dark blue and on a vectorscope as two short lines ninety degrees apart. These are used to ensure that the television receiver properly decodes the 3.58 MHz color subcarrier portion of the signal. The vectors for the -I and +Q blocks should fall exactly on the I and Q axes on the vectorscope if the chrominance signal is demodulated properly.

These bars give rise to the former portion of the casual term, "bars and tone". Typically, a television network, TV station, or other originator of video programming transmits SMPTE color bars together with a continuous 1000 Hz audio tone Template:Audio before sending program material, in order to assert ownership of the transmission line or medium, and so that receiving stations and intermediary telecommunications providers may adjust their equipment. Likewise, producers of television programs typically record "bars and tone" at the beginning of a videotape or other recording medium so that the playback equipment can be calibrated. Often, the name or callsign of the TV station, other information such as a real-time clock, or another signal source is graphically superimposed over the bars.

Although color bars were designed to calibrate analogue NTSC equipment, they remain widely used within modern digital television facilities. Currently, all display monitors use some form of voltage to illuminate or activate a level of opacity in a pixel. Using color bars will allow video, RGB, LCD and Plasma displays, as well as duplication, television and webcast facilities, to maintain the intended chroma and luminance levels.

Calibrating a Monitor using SMPTE Color Bars

Grayscale version of SMPTE color bars viewed in blue channel only.
Enlarge
Grayscale version of SMPTE color bars viewed in blue channel only.

To begin, the color bar pattern is sent to a monitor. The color bars may be generated in a mixing desk or sent from a camera fitted with a test pattern generator. Non-Linear Editors such as Apple's Final Cut Pro or Adobe Premiere contain bars and tone generators that can be placed on the timeline.

Next, the monitor is set to show only the blue channel of the picture, which on a professional monitor is done by selecting the Blue Only option. On an older monitor without a Blue Only option, the red and green color channels can be turned off to achieve the same effect. The resulting picture will appear in different shades of blue or gray.

The aim is to make the color and brightness of the alternating vertical bars match with the smaller rectangular bars underneath them. The phase and chroma controls on the monitor are adjusted until as close a match as possible is obtained. Once adjusted, the red and green bars are turned back on.

Brightness (or luminance) is adjusted by the "pluge" pattern of three small grayscale bars which is located beneath the main red bar. The picture is too bright if all three bars can be distinguished, and too dark if none of the bars can be distinguished. Ideally, the brightness should be set so that the left hand bar (which is blacker than black) is just invisible.

The contrast control should be adjusted so that the 100% white reference square in the lower left is bright enough to appear as white. If the dark squares on either side appear to glow, the contrast is set too high.

If the monitor has a Degauss button, it is then used to reset the magnetic field on the monitor.

For best results, the monitor should be allowed to warm up before calibration and should not have too much direct light falling on to the screen.

See also

Views
Personal tools