The Bell 103 modem used audio frequency-shift keying to encode data. Different pairs of audio frequencies were used by each station:• The originating station used a mark tone of 1,270 Hz and a space tone of 1,070 Hz.
• The answering station used a mark tone of 2,225 Hz and a space tone of 2,025 Hz.
Oh no, immersion broken: how the hell is that ancient thing connected to the internet?!