dmmartindale@watcgl.UUCP (Dave Martindale) (05/12/84)
Since I'm not familiar with, and don't have the specs for, the SUN's video output hardware, there are several possible answers. I'll try to cover the most likely ones. First, RS-170 (standard television) timing specs specify a horizontal frequency of 15734Hz and a vertical frequency of 59.94Hz, giving 525 lines of interlaced video. Your VTR is designed to record only video meeting these specs. Your SUN may put out video which is too far from this standard for the VTR to record satisfactorily, even if it looks fine on your monitor (many computer graphics products produce video which is pretty sloppy). To find out, try feeding sync from your SUN to your VTR's external sync input and any one of R,G, or B to the video input - it will look like a B&W signal to the VTR. If it records a stable picture, then the timing is close enough. If your VTR and SUN are compatible, then the simplest method of encoding its output is with a one-piece NTSC encoder intended for such use. Such encoders will take RGB and sync signals, generate their own colour subcarrier, burst, and blanking internally, and produce NTSC composite video. The colour subcarrier won't be locked to the horizontal sync, so you may see irregular crawling patterns at edges where colour changes rapidly, but then the VTR will do this to the signal anyway. I don't know what's available in this sort of encoder; we have a Lenco encoder at Waterloo that produces relatively awful video this way. If your SUN will accept an external sync signal and lock its video to it, then there is a better but more expensive way. Buy an ordinary TV broadcast-type sync generator and NTSC encoder. The sync generator will produce composite sync, colour subcarrier, and blanking all locked to each other. Sync, subcarrier, and blanking are fed to the NTSC encoder. Sync is also fed to the SUN. RGB from the SUN are fed to the NTSC encoder, possibly through trimpots or adjustable-gain video amplifiers to reduce the signal level to about 700mV from the 1V output typical of many frame buffers. This approach will generate true broadcast-quality NTSC video, but it isn't cheap. At Waterloo we have a COX 203/222 NTSC encoder and a Leitch SPG-120N sync generator. Canadian prices are about $3600 and $2000 respectively. Finally, if your SUN's output is sufficiently non-standard that the recorder won't lock to it, more processing is necessary. There are devices available which will digitize the output of your frame buffer at whatever rates it runs, and then scan this information back out at standard video rates. The COX CVP100 is one such device. I don't have any prices, but since it contains an NTSC encoder, sync generator, AND a frame store, it is going to be expensive. I realize that some of these answers are probably beyond what the original query wanted, but they're provided to be complete in the hope of helping someone else. There is a moral to all this too: If you want to produce NTSC video, don't buy a framebuffer unless it will lock to an external sync source. Dave Martindale Computer Graphics Lab University of Waterloo