poynton@vector.Sun.COM (Charles A. Poynton) (08/05/89)
A few recent (unattributed) quotes: > RS-170 and NTSC are very different color signals, RS-170 uses separate > R G and B lines and NTSC puts it all on the same signal. RS-170 refers to 525-line, 60.00 Hz field rate MONOCHROME. > RS-170A refers to an NTSC-encoded composite signal. True. > RS-170 is definately an NTSC signal. RS-170 is definitely MONOCHROME. Pardon my obsession with detail, but video is confusing enough without cascaded nomenclature error and ambiguity. Here are the facts. RS-170, without "A", refers to 525-line, 60 Hz field rate, monochrome video. The horizontal line rate in monochrome systems used to be exactly 15.750 kHz (within some tolerance). EIA RS-170 (now properly denoted "EIA-170") was written by the first NTSC, in about 1943. Upon the introduction of colour by the second NTSC in 1953, the FCC forced the horizontal line rate of broadcast television to be reduced by precisely 0.1%. [It was necessary to change the ratio of the 4.5 MHz sound subcarrier to the horizontal line rate to reduce the visibility of certain sound/picture interference products, and the anecdote goes that the FCC may not have understood picture very well, but they understood FM modulated sound, and THAT was going to stay put, so the raster timing was changed and we have been stuck with dropframe video ever since. This is all now seen to be pretty silly since the sound is FM, so it rarely rests at exactly 4.5 MHz anyway!] This is how we got our current horizontal line rate of nominally 15.734 kHz and a field rate of 59.94 Hz, and even monochrome systems now run at these rates, making RS-170 quite obsolete. The FCC adopted the NTSC broadcast spec in 1953, and the EIA worked for years and years on standardizing a companion studio signal standard which was to be revision "A" of RS-170. This work NEVER achieved the agreement necessary to become adopted as a standard, so on November 8, 1977, the EIA published, get this, "EIA Industrial Electronics Tentative Standard No. 1". This standard, and the associated classic 11-by-17 foldout timing diagram, is what I refer to as "EIA No. 1" and what is colloquially and incorrectly referred to as RS-170-A. [Proper nomenclature would now be along the lines of "EIA-232-C" anyway, if it were a standard.] In the light of the failure of EIA to conclude this work, the SMPTE are now working to standardize a 525/59.94 NTSC studio signal standard, roughly thirty five years later. History is poised to repeat itself in the development of U.S. HDTV standards. Conclusions: NTSC refers to composite (encoded) colour video. "EIA No. 1" is the standard which defines an NTSC colour signal. "RS-170-A" doesn't exist, but implies EIA No. 1 and therefore NTSC. "RS-170-A RGB" is a self-cancelling term. "RS-170 RGB" indicates 60.00 Hz field rate; absolutely nobody uses this. Most computer equipment avoids the complexity of NTSC encoding, but generates RGB signals with the same timing. Until SMPTE publishes a standard, such equipment should be described as "RGB with EIA No. 1 timing". C. ----- Charles A. Poynton Sun Microsystems Inc. <poynton@sun.com> 2550 Garcia Avenue, MS 8-04 415-336-7846 Mountain View, CA 94043 "Solutions, not slogans." -- slogan from Analog Devices, Inc., circa 1985. -----