Acoustic coupler

Source: Wikipedia, the free encyclopedia.
The Novation CAT acoustically coupled modem
Sendata Series 700

In

telecommunications, an acoustic coupler is an interface device for coupling electrical signals by acoustical means—usually into and out of a telephone
.

The link is achieved through converting electric signals from the phone line to sound and reconverting sound to electric signals needed for the end terminal, such as a teletypewriter, and back, rather than through direct

electrical connection
.

History and applications

Prior to its

BS 6312
became standardized.

The situation was similar in other countries. In Australia, until 1975 the

Government monopoly, owned all telephone wiring and equipment in user premises and prohibited attachment of third party devices, and while most handsets were connected by 600 series connectors
, these were rare in Australia so imported equipment could not be directly connected in any case, despite the general electrical compatibility.

It was not until a landmark U.S. court ruling regarding the

modems
.

When inventors began developing devices to send non-voice signals over a telephone line, the need for a workaround for the Bell restrictions was apparent. As early as 1937,

telefax machines used by newspapers were using some kind of couplers, possibly acoustic but more likely magnetic for single-directional communication.[2]
Multiplexed bidirectional telephone coupling was not needed by these early fax machines.

teletypewriter to sound that goes into the mouth piece of the telephone handset. His acoustic coupler is known as the Weitbrecht Modem.[3]

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.[4] An early commercial model was built by Livermore Data Systems in 1968.[5] 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 frequency-shift keying encoded 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 134.5 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. 1200 baud endpoints became widespread in 1985 with the advent of the Hayes Smartmodem 1200A, though it used an RJ11 jack and was not an acoustic coupler. Such devices facilitated the creation of dial-up

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 standardization of the dimensions of telephone handsets. Once they were made legal and the

Telecommunications Device for the Deaf) still have a built-in acoustic coupler, which allow more universal use with pay phones
and for 911 calls by deaf people.

Popular culture

An acoustic coupler (a Novation CAT 300 baud model) is prominently shown early in the 1983 film WarGames, when character David Lightman (portrayed 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. The earliest major motion picture depicting an acoustic coupler was probably the 1968 Steve McQueen film Bullitt.

See also

Notes

  1. ^ "Phone Company Upheld in Ban on Hush-A-Phone," The New York Times, February 17, 1951, p. 29
  2. ^ Spot News, film, Chevrolet Motors Division of the General Motors Sales Corporation, 1937; this film is a 9:15 minute dramatic explanation of how newspapers transmitted photographs over telephone lines. The man sending the Fax places the telephone earpiece on the coupler at 3:14 before the explanation. The diagram shown at 6:30 clearly shows the earpiece on the coupler in the data path from fax scanner to receiver.
  3. OCLC 59576008
  4. ^ "Timeline of Computer History". Computer History Museum. Retrieved 2007-02-12.
  5. ^ "Acoustic Modem, with text [Gio] (1968)". Infolab Museum
  6. ^ "Wired 6.04 - Rj-11".

External links

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