Back when I built my first vacuum tube TU with 88 mH loading coils, I knew
that mark was 2125 Hz and space was 2975 Hz (the 850 Hz shift days). But,
what is the history of these tones? Why were they chosen? why do we have
space high on audio and space low on RF? I've read that tones in the 2.1
kHz area would turn off the echo suppressors on long distance telephone
networks. Were these tones brought over from telephone? Bell 103 does not
use these tones. Where did they come from?

Harold
https://w6iwi.org




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