CompuServe Messages

Since we're talking about gray values, the I and Q (Inphase and Quadrature) signal are zero – no color at all. The only reason that black is at 7.5 IRE rather than 0 IRE (0 volts) is to hide the retrace lines on TV sets of decades gone by. The Video Toaster does create the three pluge bars correctly (well, more or less – the Toaster's levels aren't as exact as they could be, especially on the original Toaster). Luminance values below 0 IRE are definately not NTSC/RS-170A legal; they're starting to encroach upon sync territory at that point. Any chrominance riding on a low luminance value (ie- blues & magentas) are allowed to drop below the 0 IRE value since they will be filtered out anyway before reaching the sync circuits. Color bar signals and color bar generators are not all the same. As with any test equipment, I would want to verify what is claimed versus what is generated. Some equipment can generate full field color bars (ie- MX50) that might serve as a reference, but I would much prefer using a proven source of SMPTE color bars. In measuring the color bars, I'm not confident that they can be inspected via framegrab and software techniques with any precision. This would explain the pluge measurement anomally in the Toaster. If you think about it, NewTek made it very difficult (maybe impossible) to generate blacks less than 7.5 IRE (much to everyone's dissatisfaction). Yet we see a 6 IRE pluge bar; obviously they have some trick to load the framestore with such values. Bruce