The Program Coordinator for Rockville Science Center invited me to exhibit at Rockville Science Day. This was supposed to be a repeat of the demo I did at last year's Rockville Science Day but with the equipment fully working. This year everything worked until the last hour when the oscilloscope died. Turning it off for a while would get it to run for a short time. I went back to the demos from previous year. My primary demo was using signal processing to show how touch tone phones encode the buttons. Touch tone phones were introduced in 1963 two years before the PDP-8.
I had kids push a button on a touch tone phone and remember it. A speaker allowed them to hear the funny tones the phone generated. I used the PDP-8 to capture 1024 samples of the waveform. You couldn't tell anything from the waveform display. Part of that was the PDP-8 just drew a dot for the amplitude of each sample. Modern scopes draw lines and can interpolate between samples to make clearer display. I then had the PDP-8 do a Fast Fourier transform (FFT) which took several seconds. That clearly showed two frequency peaks. I had them use the knobs on the AX08 to measure the frequency of each peak. I then used a Dual-Tone Multi-Frequency (DMTF) frequency chart to determine what button corresponded to the frequencies. The kids liked it when the number matched what they pushed. I don't know how well they understood what was going on. Didn't draw as large crowds as punching paper tape but kept me busy most of the day. Some remembered the previous years demos and asked for them so I ran them.
The knobs on the AX08 are potentiometers which generate a voltage that the DAC can convert. The software uses one for X (frequency) zoom, one for Y (amplitude) zoom, and one for X scrolling. The X scroll displays the frequency of the left most location on the screen. This is a slightly modified version of DECUS 8-250.
Did have some adults do the signal processing demo. Several of them said FFT before I did when saying I was going to generate spectrum of the signal.
Did try using a tone generator to play a note and have kids try to hum/sing same note. I used the FFT to see how close they were. Used the phone as a microphone. The touch tone decoding seemed to go over better.
Since finally got it to run what I intended I'll likely bring a different machine next year.
The following picture links also have descriptions of what is shown in the pictures.
Computer and peripherals ( 55K)
Ephemera ( 53K)
Reading signs ( 54K)
Doing my spiel ( 87K)
Doing my spiel 2 ( 91K)
Feel free to contact me, David Gesswein djg@pdp8online.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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