stever@videovax.UUCP (04/23/87)
In article <1485@cadovax.UUCP>, Keith Doyle (keithd@cadovax.UUCP) writes: > How do you determine what 'correct' video centering is for video recording? > The Amiga Preferences centering control allows you to adjust for individual > monitors and TVs, but where is the right place for VCR's and television > broadcasting? (any gurus from Tektronix out there?) . . . The controlling specification in this case is RS-170A -- "Color Television Studio Picture Line Amplifier Output." While there are many numbers given in this standard, the "Front Porch" width and "Sync to Set-Up" are the two that count. The diagram below shows the horizontal interval: \ / \ / \ _____________ / \ / \ / \ / \ / \_____ __/ Color Burst \_____/ \ / \ / | | | \ / | | | Horizontal | \_____________/ | | | Sync | | | | | | | | | | | \__________________/ | | | | | | | Front | <---------- Sync to Set-Up ------------> | | Porch | | The negative-going edge of the Horizontal Sync pulse is the reference for all timing in the line. Standard NTSC video has "set-up" on each line, which is just a small positive (upward) voltage added to the signal everywhere except in the horizontal interval. I don't know whether the Amiga composite output (the one you feed to a TV or VCR) has set-up or not -- I've just not looked! RS-170A specifies the Front Porch to be 1.5 +/- 0.1 microseconds, and Sync to Set-Up to be 9.4 +/- 0.2 microseconds. For computer graphic output, you want to center the picture, though -- so put up a signal that has a clearly-defined boundary (e.g., a standard, full-screen CLI window with borders, or the Preferences window) and use an oscilloscope to adjust the centering (using Preferences) until the picture is positioned with equal additional space to the left of the front porch and to the right of the nominal beginning of set-up. As an example, adjust for 4 microseconds (1.5 + 2.5) from the end of video to the leading edge of sync and 11.9 microseconds (9.4 + 2.5) from the leading edge of sync to the beginning of video. In this example, an "extra" 5 microseconds of blue background has been divided equally between the time before the front porch and the time after the nominal position of set-up. This will, in general, produce the best results. You can't use a standard TV set to perform the adjustment, because the TV set almost certainly suffers from overscan. "Overscan" just means that the picture is slightly too large, with some of it being lost at the top, bottom, and sides. TV manufacturers build their sets this way to ensure that there aren't black areas at the edges of the screen -- people complain about pictures that are too small, but not ones that are too large! Neither can you make the adjustment with your Amiga 1080 monitor (which has *much* less overscan, by the way!), because the Amiga monitor has its own centering control. Unless you have previously adjusted the monitor with a known-good signal (and you know for certain the kids haven't messed with it), you are just misadjusting the computer output to compensate for the misadjustment of the monitor. . . Steve Rice ----------------------------------------------------------------------------- Copyright 1987 by Steven E. Rice, P.E. All Rights Reserved. This material may be redistributed only where such redistribution is without charge and without restrictions on further redistribution. Incorporation of this material in a compilation or other collective work constitutes permission from the intermediary to all recipients to freely redistribute the entire collection. All other uses are prohibited. ----------------------------------------------------------------------------- new: stever@videovax.tv.Tek.com old: {decvax | hplabs | ihnp4 | uw-beaver | cae780}!tektronix!videovax!stever