Acoustic coupler

The Novation CAT acoustically coupled modem

In telecommunications, the term acoustic coupler has the following meanings:

1. An interface device for coupling electrical signals by acoustical means—usually into and out of a telephone instrument.
2. A terminal device used to link data terminals and radio sets with the telephone network.

The link is achieved through acoustic (sound) signals rather than through direct electrical connection.

History and applications

Prior to its breakup in 1984, Bell System's 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 phones not made or sold by Bell to the network, and for a long time the phone itself was owned by the phone company and leased to customers. In many households, telephones were hard-wired to wall terminals before connectors like RJ11 and BS 6312 became standardised. The telephone network was essentially a closed system wholly controlled and owned by Bell end-to-end. Interconnection of outside phones or other terminal equipment to the telephone system was not allowed.

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.

Earlier, any third-party terminal equipment had to be acoustically, rather than electrically, connected to the phone system. Acoustic Modems or Couplers were developed around 1966, a.o.. by John Van Geen at Stanford Research Institute, 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.

Design

Usually, a standard telephone handset was placed into a cradle that had been engineered to fit closely (by the use of rubber seals) around the microphone and earpiece of the handset. A modem would modulate a loudspeaker in the cup attached to the handset's microphone, and sound from the loudspeaker in the telephone handset's earpiece would be picked up by a microphone in the cup attached to the earpiece. In this way signals could be passed in both directions.

Acoustic couplers were sensitive to external noise and depended on the widespread standardisation of the dimensions of telephone handsets. Direct electrical connections to telephone networks, once they were made legal, rapidly became the preferred method of attaching modems, and the use of acoustic couplers dwindled. Acoustic couplers are still used by people travelling in areas of the world where electrical connection to the telephone network is illegal or impractical. Many models of TDDs (Telecommunications Device for the Deaf) still have a built-in acoustic coupler, which allow more universal use with pay phones.

An acoustic coupler is prominently shown early in the 1983 film "WarGames", when character David Lightman (depicted by actor Matthew Broderick) places a telephone handset into the cradle of a film prop acoustic modem to accentuate the act of using telephone lines for interconnection to the developing computer networks of the period, in this case, a military command computer. Although this device was obsolete even at the time of filming, it was felt that an acoustic coupler would be easily identifiable and understood by the audience who would then know what was happening.

See also

• Carterphone
• Federal Standard 1037C
• MIL-STD-188

External links

• 1964 Livermore Data Systems Model A acoustic coupler modem surfs the web
• U.S. Patent 3,553,374 Digitronics acoustic coupler patent, January 5, 1971