The acoustic coupler is a device for the transmission of digital data via an analog subscriber line . Acoustic couplers were used from the 1970s to the late 1980s and allow data to be transmitted through the receiver of telephone desk sets . An acoustic coupler does not require an electrical connection to the landline connection because this was not allowed in many countries and with many network operators.
Acoustic couplers are used when no analog modem is available or an electrical connection between the modem and the telephone network is not possible or permitted. Acoustic couplers use the handset of an existing telephone to send and receive the modulated audio signals. To do this, they have a microphone and a loudspeaker which are attached to the corresponding counterparts in the telephone receiver.
There are two types of acoustic couplers:
- Acoustic couplers with an integrated modem such as the Dataphon s21d are connected directly to the serial interface of the computer . They take over the digital data from the computer and convert them into analog audio signals. Simple digital keying or modulation processes are used as modulation methods, such as binary frequency shift keying (FSK). The parameters of the modulation, such as the frequencies used, are specified in standards such as ITU-T V.21, V.23 or in the US according to Bell 202 and must be supported by both acoustic couplers involved in the communication. With these models, the remote station must typically be dialed by hand using a dialing device such as the rotary dial for pulse dialing or the telephone keypad .
- Acoustic couplers without an integrated modem, such as the Road Warrior Telecoupler II, are connected to an existing analog modem and only form the hardware interface to the telephone network, i.e. they only convert the audio-frequency electrical signals supplied by the modem into audible tones or those received by the remote station Converts tones to analog electrical signals for the modem to use. Since analog modems are typically capable of multi-frequency dialing , these acoustic couplers can also be used to dial the remote station under software control.
Due to their design, acoustic couplers are susceptible to interference from external noises and are dependent on the quality of the telephone or the telephone receiver. Due to the inevitable loss of signal quality due to the acoustic transmission step, acoustic couplers do not achieve the data transmission rates of the direct electrical connection between the modem and the telephone network. With older acoustic couplers, the transmission rates only range from 300 to 2,400 bit / s . Later models, for example Konexx Coupler or Road Warrior Telecoupler II , achieve data transmission rates of up to 33,600 bit / s in practice; This means that e-mails can be picked up from a telephone booth in a reasonable time.
In the 1980s, the operation of self-made acoustic couplers in the telephone network of the Deutsche Bundespost was illegal and subject to heavy fines. Nevertheless, the number of self-made devices increased significantly after the Chaos Computer Club published a relatively easy-to-implement construction manual for a do-it-yourself acoustic coupler - the so-called “ data loo ” - in the Hackerbibel in 1985 .
Parallel to the acoustic couplers, the first modems directly connected to the telephone line became available. Here, too, the connection of freely available devices to the German telephone network was prohibited; Swiss Post only allowed the use of Post's own modems in its network, which either had to be rented monthly or - compared to freely available devices - had to be bought from Post at higher prices.
In the German-speaking world, acoustic couplers are sometimes referred to as data hairdryers , after the once popular S21 series of Dataphones from Woerltronic from Cadolzburg .