Despite what it looks like in movies, it is difficult to communicate with astronauts from Earth. There are delays, and spacecraft usually don’t have much power. Plus everything is running and moving Doppler and rotating Faraday. Even today, it’s clever. But how did Apollo return TV, telemetry and voice 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 in a spaceship are always at a premium, such as energy. When you look at the pictures of this solid box, which weighs more than 14 pounds, you will be amazed at how much is stored in a relatively small space. Remember, if this box was to fly in 1969 it should have been made earlier so there is no way to expect dense IC and modern packaging. Not even a printed circuit board. The elements are attached to the metal peg in a point-to-point fashion. The whole thing lived at the bottom of the bay of the lower equipment of the command module.
The processor, or PMP, has played a key role in transmitting (and from) to different streams multiplexing and onboard S-band transmitters in different configurations. Inside the box, [Ken] Four subsembles were found, nicely labeled and attached to a thin backplane. In addition to isolated components, the modules also employ off-the-shelf assemblies that offer functions such as filters or oscillators in previous ICs and a convenient package.
One thing that made the design more complicated was the need for redundancy. For example, inside there are two switching controllers – yes, a switching controller on a part of the 60’s gear – and the crew can choose between two power supplies.
[Ken] Each module takes us through. The voice and data detector module emits voice over a 30 kHz FM subcarrier. There is also a two-phase modulator, voice clipping and a relay module that transmits signals back to Earth from the lunar module.
If you want to get a closer look at the Apollo less system, [CuriousMarc] There is a series about it and was part of the group and snatched this PMP. The radio signal is certainly funny, but the best footage is back as a film. However, modern technology has sharpened some of that old footage.