Acoustic Coupler - History and Applications

History and Applications

Prior to its breakup in 1984, Bell System's legal monopoly over telephony in the United States allowed the company to impose strict rules on how consumers could access their network. Customers were prohibited from connecting equipment not made or sold by Bell to the network. The same set-up was operative in nearly all countries, where the telephone companies were nationally owned. In many households, telephones were hard-wired to wall terminals before connectors like RJ11 and BS 6312 became standardised.

It was not until a landmark court ruling regarding the Hush-A-Phone in 1956 that the use of a phone attachment (by a third party vendor) was allowed for the first time; though AT&T's right to regulate any device connected to the telephone system was upheld by the courts, they were instructed to cease interference towards Hush-A-Phone users. A second court decision in 1968 regarding the Carterfone further allowed any device not harmful to the system to be connected directly to the AT&T network. This decision enabled the proliferation of later innovations like answering machines, fax machines, and modems.

Robert Weitbrecht created a workaround for the Bell restrictions in 1963. He developed a coupling device that converted sound from the ear piece of the telephone handset to electrical signals, and converts the electrical pulses coming from the teletypewriter to sound that goes into the mouth piece of the telephone handset. His acoustic coupler is known as the Weitbrecht Modem.

The Weitbrecht Modem inspired other engineers to develop other modems to work with 8-bit ASCII terminals at a faster rate. Such modems or couplers were developed around 1966 by John van Geen at the Stanford Research Institute (now SRI International), that mimicked handset operations. An early commercial model was built by Livermore Data Systems in 1968. One would dial the computer system (which would have telephone company datasets) on one's phone, and when the connection was established, place the handset into the acoustic modem. Since the handsets were all supplied by the telephone company, most had the same shape, simplifying the physical interface. A microphone and a speaker inside the modem box would pick up and transmit the signaling tones, and circuitry would convert those audio shift-key encoded frequency binary signals for an RS232 output socket. With luck one could get 300 baud (~bits/second) transmission rates, but 150 baud was more typical. That speed was sufficient for typewriter-based terminals, as the IBM 2741, running at 147.5 baud, or a teleprinter, running at 110 baud.

The practical upper limit for acoustic-coupled modems was 1200-baud, first made available in 1973 by Vadic and 1977 by AT&T. It became widespread in 1985 with advent of the Hayes Smartmodem 1200A. Such devices facilitated the creation of dial-up bulletin board systems, a forerunner of modern internet chat rooms, message boards, and e-mail.