Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
The overall brightness of the displayed signal is the average of the video signal levels. The peak brightness is the peak video level. The contrast is the peak-to-peak level of the video. The brightness control on a TV thus moves the displayed average (DC) up or down. The contrast control adjusts the gain of the video amp to change the displayed peak-to-peak video level.
In all the RGB monitors that I've used, "brightness" is the minimum level displayed. Even the cheapest color monitors/televisions had DC restore circuits. When video goes away the raster went as dark as it could go. I set it by removing video and adjusting brightness so the raster just disappears. The black level is set relative to the "back porch" (a short period following the sync pulse) in the waveform, not the sync itself. Black level is supposed to be 7.5 IRE units (Back Porch being 0 and white being 100).
Only with old, old, B&W (before color TV) NTSC televisions without DC restore did "brightness" control the average. You could tell because when the signal went away the raster went gray not black. It was common to see the vertical retrace on the cheapest ones.
Waveform b is brighter than AWhen you say the "minimum" level displayed, what do you mean as far as the signal goes? Do you mean the lowest voltage value in the signal?
For example say you have waveforms (a) and (b) of a single scan line:
**broken link removed**
Would you say that waveform (b) is brighter than waveform (a)?
If so, if the monitor has the signal AC coupled, the DC information about that waveform would be lost, correct? Or did the DC restore actually restore the input DC information? Also, what if the output stage of the video is AC coupled? Then no DC information would get past anyway.
Also, can you give me more information about black level being set relative to the back porch?
The dotted red lines in the chart represent the beginning of each horizontal sync pulse, and are therefore about 64 us apart. (Some extra marks show half lines on the lowest traces.) See the Horizontal timing charts on the same web page for details.But the chart is confusing and there is no timing information
Please point out the back porch; it appears to me that it's the flat spot just to the right of the sync pulse, about the same length as the sync. The brightness is relative to the back porch. A DC restored video should rely entirely on the back porch level to set black. If those two signals are before the DC restore circuits, then the 0V lines are meaningless. The video in the two waves are different, but very similar. One has a lot larger sync, but the DC restore should ignore that. The one on the right looks like it may have a tiny false front porch at 0V which also should be ignored.Would you say that waveform (b) is brighter than waveform (a)?