How to Understand Video Waveform Monitor
Video waveform monitors are used to calibrate video equipment to provide accurate and consistent reproduction of video signals using standard calibration test patterns. A video waveform monitor is useful to ensure that video from a particular source conforms to Federal Communications Commission broadcast standards. Video waveform monitors are designed according to the specifications outlined by the National Television Systems Committee and are designed to use a video display known as color bars designed by the Society of Motion Picture and Television Engineers. Knowing how to read a waveform monitor requires understanding the composition of SMPTE color bars.
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Instructions
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Familiarize yourself with the waveform monitor display. The display consists of a graph, a vertical scale that measures video luminance (brightness) from minus-40 to 100 units.
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Generate SMPTE color bars using an SMPTE-calibrated video test generator. If using professional video editing software, create an SMPTE color bars test file.
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Select the color bars test pattern as your input to the video waveform monitor.
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Examine the color bars display in the video waveform monitor. A properly calibrated test pattern should display as a series of stair-stepped waveforms, with the highest portion of the test pattern display touching the 100-unit cross bar. Two solid lines should appear before and after the stair-step pattern at exactly 75 units.
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Locate the calibration point for pedestal black; 7.5 units of video is known as video black, or "pedestal." Black generated below this level is known as "super-black" or "negative black" and is used for nonbroadcast luminance key effect generation by analog video switchers and title generators. The 7.5-unit mark is indicated in the waveform monitor with a dashed line. Portions of the stair-step pattern should touch just above this dashed line.
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Tips & Warnings
Luminance test patterns are most easily viewed in waveform monitors by removing the chrominance (color) information.
FCC broadcast regulations require NTSC video content to not exceed 100 units of brightness; they prohibit black levels from being below 7.5 units.
Video luminance that exceeds 100 units can cause "clipping," or a shaving off of the video waveform. This can cause image distortion depending on limitations of the video equipment used. Think of audio distortion that occurs when the volume of the audio signal exceeds equipment capabilities.
While luminance is not allowed to exceed 100 units, chrominance (color) levels above 100 units are permissible as long as they do not exceed the capabilities of the video equipment to provide accurate color reproduction.
A vectorscope frequently accompanies waveform monitor displays. This calibration device is designed to align color components of the SMPTE color bar pattern to insure accurate color reproduction. Color is properly aligned by placing the dots that represent each of the six colors from the test pattern (excluding white) in the designated target zone of the vector display.
