well, I´ve found at least fourfive different ways of convert the signals (some are simillar), including the "original" one, in the IBM Color/Graphics Monitor Adapter tech manual, page 32, there is the schematic of the output circuit. The composite is obtained by mixing the RGB, to which is added the synchrony signal that is obtained by mixing the generated HSYNC and VSYNC , using an XNOR. all through a digital buffer and then an array of resistors, with an output transistor....
or inject the RGB directly to a TV
My question is because I have read in some places about this, that some cards put the composite video signal on that connector pin. Knowing that, what they are, expands for those interested the number of cards that can be used with this signal. Due to the difficulty of obtaining a CGA/EGA monitor I have looked for cards with composite output, but either I have had little luck finding them cheap, or the few times I have done so others have beaten me to buying them (the last time was the most painful, an excellent deal, several card from CGA to VGA some 8bit friendly ones and cheap)... those without visible composite connector trend to be cheaper, but some have internal pins or pin 7 output. maybe sellers ignorance make them put lower prices..
This is something similar to making a list of 16-bit, 8-bit friendly VGA cards (I have found a couple lists of this)