The SMPTE color bar pattern is a standard test pattern used in video production and broadcasting to calibrate colors and adjust levels properly. SMPTE stands for the Society of Motion Picture and Television Engineers, the organization that originally developed this pattern. The color bar pattern displays a series of bars in different colors at specific luminance levels that allow technicians to adjust a video monitor or broadcast encoder to ensure accurate color reproduction.
History of the SMPTE Color Bars
The original color bar pattern was developed by engineers at RCA in the early days of color television broadcasting in the 1950s. It was officially adopted as a standard by SMPTE when the organization published their first Television Studio Standards in 1961. The color bar pattern has gone through a few revisions over the decades but has largely remained the same in order to preserve compatibility with older video equipment.
Early color bars used the three primary colors of light – red, green, and blue (RGB) – and secondary colors yellow and cyan. In the original RCA pattern, color saturation was very high to enable easy identification of each color. Later SMPTE bars reduced saturation to more closely match the range used in actual broadcast programming. Black and white bars were also added to test contrast and luminance.
The most recent version, known as SMPTE RP219-2002 or “EBU-SMPTE bars,” includes horizontal bars, vertical bars, and a pluge pattern to test signal parameters. Pluge refers to “Picture Line-Up Generation Equipment” used to generate test patterns. This standard color bar pattern is recognized worldwide for aligning video signals accurately.
Composition of the SMPTE Color Bars
The standard SMPTE color bar pattern contains the following sequence of vertical bars from left to right:
- White – At 100% luminance level
- Yellow – At 75% luminance level
- Cyan – At 75% negative luminance level
- Green – At 50% luminance level
- Magenta – At 50% negative luminance level
- Red – At 25% luminance level
- Blue – At 25% negative luminance level
- Black – At 0% luminance level
In addition, there are bars for -I and +Q that are used for adjusting chrominance levels. There is also a pluge pattern that contains black, gray, and white areas for calibrating black levels and contrast.
The color bars are displayed over a grayscale background that corresponds to a standard luminance level of 7.5 IRE units. The varying luminance levels of the bars allows the colors to be adjust for proper brightness and intensity on the video display.
Using SMPTE Color Bars
There are several ways video engineers use these standard color bars:
- Set up camera exposure and color balance – The color bar pattern provides a quick way for camera operators to adjust exposure, white balance, and color before shooting a video project.
- Calibrate video monitors – Engineers use the color bars to adjust the settings like brightness, contrast, saturation, and hue on video monitors for accurate color reproduction.
- Check video signal paths – The pattern can be used to diagnose issues with video cabling, switchers, distribution amplifiers, and other signal processing equipment through the video chain.
- Adjust encoder/decoder settings – The bars provide a reference to calibrate hardware and software compression and encoding systems to ensure colors translate properly during encoding/decoding.
- Set audio reference tone – The EBU-SMPTE bars include an audio tone of 1000 Hz at -18 dBfs that serves as a reference for audio level alignment.
By setting all video equipment and software to match the standard color bar pattern, it ensures that colors and luminance will be reproduced accurately down the signal delivery chain during broadcasting or video post-production.
Analyzing the SMPTE Color Bars
There are some key things to look for when examining the color bar pattern on a waveform monitor or vectorscope:
- On a waveform monitor, check that the luminance levels of the bars match the standard percentage values relative to the blanking level.
- Confirm transitions between the bars are crisp and distinct without blurring.
- The pluge pattern should show distinction between black, gray, and white levels.
- A vectorscope will plot colors at precise angles and intensities when calibrated properly.
- The white bar will be at 100% luminance centered vertically on the vectorscope graticule.
- Audio test tone should hit -18 dBfs at 1000 Hz.
If the color bar display deviates from the standard pattern, it indicates an issue needing correction somewhere in the video chain. The bars act as a troubleshooting tool to help align equipment and signals to maintain color accuracy.
Generating SMPTE Color Bars
There are several ways to generate the SMPTE color bar pattern:
- Dedicated test pattern generators – Specialized hardware devices designed specifically for calibrating video signals and equipment.
- Video test pattern generators – Software that runs on computers that can display a variety of video test patterns, including color bars.
- Editing software – Most professional non-linear editing systems include color bar pattern generation within the software interface.
- Broadcast master control switchers – High-end production switchers used in studios and mobile trucks offer internal color bar generators.
- Camera test patterns – Many professional video cameras have a built-in SMPTE bar pattern that can be displayed to set exposure and color.
Working from a standard reference pattern is essential for matching all video signals throughout a broadcast facility or remote truck. Portable test pattern generators offer flexibility for fieldwork. Software-based generators provide convenience for post-production applications. And cameras with internal test patterns accelerate initial setup.
Color Bar Resolution
The number, width and position of the color bars varies based on the video resolution. Some examples include:
| Video Resolution | Number of Bars | Bar Width | Bar Position |
|---|---|---|---|
| 720×480 NTSC | 7 | 52 pixels | 20 pixels from left |
| 720×576 PAL | 7 | 52 pixels | 20 pixels from left |
| 1920×1080 HD | 8 | 190 pixels | 95 pixels from left |
| 3840×2160 UHD | 8 | 760 pixels | 380 pixels from left |
As video resolutions get larger, more pixels are available to devote to wider color bars while maintaining the proper luminance levels. For any resolution, the colors bars should occupy the full vertical frame height.
Variations of Color Bar Patterns
While the standard SMPTE color bars are recognized worldwide, there are some variations used in different regions:
- EBU Bars – Developed by the European Broadcast Union, these are technically identical to SMPTE bars but numbered from left to right in the opposite order.
- BBC Bars – Used by the BBC, these have the same colors but a different sequence of bars.
- Philips PM5544 Bars – Philips color bar pattern with somewhat different colors including background gray stripes.
- FuBK Bars – Specified by the Institute for Broadcasting Technology in Germany, similar to SMPTE but with differently colored gray bars.
While local variations exist, the standard SMPTE color bars defined by RP219 remain the most widely adopted worldwide. Different patterns are generally compatible as long as the color targets are adjusted properly.
Limitations of Color Bars
While the SMPTE color bars are extremely useful for video calibration, there are some limitations:
- Only represents colors at 100%, 75%, 50%, and 25% saturation levels. Doesn’t test tints at lower saturation levels.
- Limited number of colors tested whereas video includes a much wider color gamut.
- Bars are static whereas actual video contains color in motion.
- Does not reveal all possible artifacts such as banding, smearing, dot crawl, etc.
- Does not test audio fidelity, stereo imaging, phase, or frequency response.
For these reasons, additional test patterns and media are also used to thoroughly calibrate an end-to-end video system. Multiburst patterns, zone plates, sweep patterns, and motion tests reveal other aspects not shown by SMPTE bars alone. But color bars remain a quick first step for initial alignment of video signals and equipment.
Conclusion
The SMPTE color bar pattern has proven itself as an indispensable tool for broadcasters, video engineers, and production teams for over 60 years. The standardized sequence of colored bars provides a quick reference for calibrating cameras, monitors, encoders and decoders to maintain color accuracy through an entire video workflow. As video resolutions and color reproduction continue to evolve, this standard test pattern remains a staple for video professionals across all generations of technology.
Understanding the composition, intended use cases, and analysis methods for these color bars enables any video technician to properly set up and adjust equipment for optimal quality. The various software and hardware options for generating color bars offers flexibility for all types of production environments and budgets. While originally developed for the TV studios of the 1950s, this classic test pattern remains core to delivering great looking video in the 2020s and beyond.
