Star Trek and Packet Radio?
Posted: 4th December 4, 1989 By: Bob McGwier, N4HY
Several months ago, Harold Price, NK6K, challenged me to demodulate what he thought might be HF packets in Star Trek IV. During the scenes where Scotty is valiantly trying to beam both Chekov and Uhura back from the U.S.S. Enterprise, where they have been stealing Nuclear vessel high speed photons, Scotty is having a hard time hearing them.
One of the sources of interference is what appeared to Harold to be HF packet. Always being one to rise to a challenge, I took on the job of doing some fancy Digital Signal Processing footwork. Almost from the first I was certain that it must be an HF packet since my very first demodulator attempt clearly revealed flags before the start of a frame and end of frame was also clear. I knew it was HDLC of some variety.
Several things impeded the effort, including Scotty’s voice on top of the packets, some SSB from 20 meters was also nearly on top of the signal. All of this had to be filtered out. I spent an hour of time on the Cray-2 at work and used the fanciest FSK demodulator I could write and I finally had noisy baseband signal plotted on paper in front of me. I did my best to get an integral number of samples per baud as the signal was very noisy, and though the bits could be made out by eye, I could tell that it was going to take another hour of Cray-2 time to get the clock recovered and to make good bit decisions. In a couple of places, HDLC showed me what were clearly bit errors, and these could be done by eye as well.
After the filtering, and building a demodulator for the badly mis-tuned signal (it was almost 900 Hz below ormal’), I took the bits to Phil Karn, KA9Q and he decoded the NRZI data and proved beyond a shadow of a doubt that it was indeed an HF amateur radio packet. It was WA8ZCN-0 sending an RR for NR-3 to N6AEZ on 20 meters. I got Bill Harrigill, WA8ZCN on the phone and he agrees that it was probably him.
Thanks Harold for the challenge and Phil for the help. Bob N4HY
P.S. A Cray-2 is about 50% faster than the fastest Pentium-Pro computer available today.