RS-170 is the United States standard that was used for black- and-white TV, and defines voltage levels, blanking times, the width of the sync pulses, and so forth. The specification spells out everything required for a receiver to display a monochrome picture. The output of those little black-and-white security cameras hanging from ceilings conforms to the RS-170 specification. Now, SMPTE 170M is essentially the same specification, modified for color TV by adding the color components. They modified RS-170 just a tiny little bit so that color could be added (RS-170A), with the final result being SMPTE 170M. This tiny little change was so small that the existing black-and-white TVs didn't even notice it.