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Prior to market liberalisation in the early 1980s, it was illegal to plug unapproved equipment into the phone system — if you could find a plug — and very difficult to get equipment approved in the first place. A common compromise was the acoustic coupler, which played data tones into the microphone and picked them up from the earpiece of a standard telephone handset.
This was suitable for very slow speeds — 300bps or less — and was also very sensitive to ambient noise, being knocked, poor-quality microphones and many other ills. However, it was very flexible: combined with a Tandy Model 100 portable computer, it could give portable dial-up access from phone boxes, thus creating the very first generation of mobile data access.
Photo credit: Rupert Goodwins