Apollo Spacecraft S Band Communication System, Part 19—Understanding the Pre-Modulation Processor

The pre-modulation processor can be thought of as the spider in the centre of the web which is the Apollo spacecraft communication system. It is responsible for assembling the downlink signal from everything the spacecraft sent to the ground: astronaut voice, spacecraft telemetry, data from the guidance computer, information from the tape recorder, relays from the lunar module or an astronaut on EVA, television signals, and even the last-ditch Morse code astronauts could send if nothing else worked. Take that, crank in redundancy and a list of alternative modes in case of failures, and you have a box so complicated it’s easy to run out of colours to follow signal paths through the block diagram, no less understand it at the transistor level.