On Fri, 2 Jan 1998 16:45:37 +0000 Jim Main writes: >Most modern tv's have no problem in coping with a vertical interval >devoid of equalising pulses - in fact many early videogame consoles >didn't output any (the Sinclair Spectrum for one) This isn't limited to early videogame consoles. Many of the Sony Playstation's NTSC video modes use timing that is a little nonstandard (I think some of them are not even 525 lines). Some TVs with digital horizontal and vertical timing generation have trouble getting a vertical lock. A Zenith circuit is particularly troublesome. The picture will "bounce" up and down about 20 lines at a rate of 2 or 3 times per second. Though the Playstation design is a few years old, it still is in very active production and one of the market leaders. >So long as the field sync is present and correctly timed, the tv >should >lock up to it. Driving the video level to "sync" for 3 full line periods usually works. Another example of nonstandard video comes from a VCR. In normal play mode the sync pulses are read from the tape and are quite standard (though the frequency is related to the speed of the motor and thus isn't perfectly constant). When the pause or fast scan modes are engaged, the VCR applies a simple vertical pulse to the video to improve vertical lock compared to trying to read the vertical sync from the tape. There is also the problem of switching the heads, which occurs at the bottom of the visible part of the picture. Often the picture appears bent or "torn" here because the scan of the next field on the tape (from the other head) doesn't start exactly in phase. I assume the designers of the VCR placed the switching point here, rather than in the vertical blanking interval, to allow as much time as possible for the TV to recover.