Despite what it looks like in the movies, it is hard to communicate with astronauts from Earth. There are delays, and space vehicles don’t usually have a lot of excess power. Plus everything is moving and Doppler shifting and Faraday rotating. Even today, it is tricky. But how did Apollo manage to send back TV, telemetry, and voice back in 1969? [Ken Shirriff] and friends tell us part of the story in a recent post where he looks at the Apollo premodulation processor.
Things like weight and volume are always at a premium in a spacecraft, as is power. When you look at pictures of this solid box that weighs over 14 pounds, you’ll be amazed at how much is crammed into a relatively tiny spot. Remember, if this box was flying in 1969 it had to be built much earlier so there’s no way to expect dense ICs and modern packaging. There’s not even a printed circuit board. The components are attached to metal pegs in a point-to-point fashion. The whole thing lived near the bottom of the Command Module’s lower equipment bay.
The processor, or PMP, played a key role in multiplexing different streams in different configurations and passing them to (and from) the onboard S-band transmitter. Inside the box, [Ken] found four subassemblies nicely labeled and connected to a thin backplane. Along with discrete components, the modules also employed off-the-shelf assemblies that predated ICs and offered functions like filters or oscillators in one convenient package.
One thing that further complicated the design was the need for redundancy. For example, there are two switching regulators inside — yep, a switching regulator in a piece of gear from the 60’s — an ..
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