(continuing)

were more or less sitting around idle because of course business is happening during the day. So General Electric Network Internet Exchange, or Genie, was launched.

Priced at $6 per hour at night and $36 per hour during the day, Genie had all the best online games at the time and lots of sci-fi writers ended up using the service, including J. Michael Straczynski, who announced some TV show called Babylon 5 on there. And Babylon 5, one of my favorite TV shows, just celebrated its 30th anniversary of the series debuting.

Anyway, back to Apple Link and the shuttle email. On STS-43 the communications experiment was designated DTO-799, PGSD-slash-PADM Air Ground Communications Demonstration. So PGSC is the Payload General Support Computer, the aforementioned Grid Compass, and PADM was the Portable Audio Data Modem. And the modem was of course key in communication back then, right? This would be talking back to Earth. The space shuttle already had amazing communications capabilities compared to all previous space programs. It had been launching the Tracking Data and Relay Satellites, or TDRS. In fact, the primary payload on STS-43 was TDRS-5. These satellites sat out in geostationary orbit and they enabled communications with the shuttle when the shuttle didn't have a line of sight to the ground stations.

The shuttle actually supported digital communications protocols at bit rates that absolutely eclipsed the lowly dial-up connections that us earthbound plebs generally had to deal with. Like the telemetry and the voice communication, all that stuff would take up about 216 kilobits per second. But the shuttle could also send data like a real-time video as much as 50 megabits per second, so it was able to scream out data.

So yeah, there wasn't also an existing teletype communications system on board, so from the shuttle they could send up text to the teleprinter to provide written instructions and messages to the astronauts on their trip. And sometimes these printouts could be huge. They would spill out of the printer in zero-g, making for some comical sights as astronauts tried to make sense of the massive instructions flowing out of this device. So a big part of the air-to-ground communications experiment was to provide a two-way equivalent of this, and maybe one day eliminate the paper copy if it wasn't needed.

So now the question is, how do you connect a Mac to this space-age digital communication system so it can take advantage of all that bandwidth? Well, it turns out that modifying the digital communication system to support a data link to a device on the flight deck was not possible with resources they had on hand, right? So they just hooked up an old school modem, the portable audio data modem, to an audio channel. And let's be clear, the space channel communications were all digital, including the analog audio, which was converted to a digital format for transmission and then decoded and converted back to audio.

So the modem on the flight deck, it would convert the digital data on the computer to analog audio and vice versa. So the binary data on the computer was converted to an analog audio signal by the modem, fed into the shuttle's audio processor, which could convert it to a digital signal, and then beam that to the TDRS satellites, which would then send it back to the Earth at White Sands, and then that would be sent across the country on data links to the Johnson Space Center in Houston. And there, the data would be decoded, including the shuttle audio. And some lucky person at JSC could have picked up the wrong phone and, you know, sure, no doubt have listened into the modems screeching at them, right?

But anyway, according to the documents that I found, the protocol they used was V27TER, half-duplex. That's a fax protocol that uses 1200 symbols per second at 2 bits per symbol, making a total data rate of 2400 bits per second. And the half-duplex part is significant. It means that this is a one-directional transmission. And when a pair of modems share the line, only one can be transmitting while the other is receiving. And I think this may be down to the fact that shuttle audio was like a push-to-talk system, where you would push, talk, and then you would release and you would listen. And so running a modem over this had to run with the same constraints.

So, you know, you might think the obvious next step is just to plug that audio from