Barry's Homemade Modem 1981

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I made this modem out of parts from old telegraph equipment. It would take any code (Baudot, Selectric EBCDIC or ASCII) at any level (5, 6, or 8) and parity of Odd, Even or None. It would perform from 50 to 1200 baud, pretty significant for the time considering most modems we either 110, 300 or 1200 baud and cost a small fortune. 

My modem was based on FSK or frequency-shift-keying and I scoffed at Bell 103 and later 212 standards...  There's some info below on FSK. If I remember correctly the FSK frequencies were:
F1 = 1075Hz  space and 1250 mark...   F2 = 2025Hz space and 2225Hz mark.

I used it with my ADM-3 CRT/KYBD and OKIdata 9-pin dot-matrix printer but the bulletin board craze was really ramping up so I needed something better than the ADM-3

Sometime in 1983 I bought my first Atari computer, I now have a museum of Ataris include every ANTIC magazine, every commercial piece of Atari software written and just about every book or manual ever published for an Atari computer.

Here are some interesting things about Atari and also about BBS systems of "the day"...

A brief history of Atari computers  and another that's something from long ago Bulletin Board Numbers

frequency-shift keying (FSK)

frequency-shift keying (FSK): Frequency modulation in which the modulating signal shifts the output frequency between predetermined values. Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the " mark " and "space" frequencies. This is a noncoherent form of FSK. Note 2: Coherent forms of FSK exist in which there is no phase discontinuity in the output signal. Synonyms frequency-shift modulation, frequency-shift signaling.