Telephone speech functions

Telephones

UNIT 4 (58) Telephones and headsets

PART 1 (58.1 – 58.4.2)

When we speak our voice sets up sound vibrations which disturb the surrounding air, and travel through the air to be detected by the listener's ear drums. Sound will travel through most media, air, water, wood, plastic, etc. and at different speeds through the differ­ent media. For example in air sound travels at approximately 1100 feet per second (335m/s) and approximately 4300 feet per second in water (1311m/s). Electrical signals travel at the speed of light 3 x lrfWs.

The telephone, patented by Alexander Graham Bell in the USA in 1875-77, was an apparatus named the 'Electrical Speaking Tele­phone'. It was a means of transmitting sound (especially voice) over a distance, by converting sound vibrations into electrical signals which passed through wires as electrical signals, and were then reconverted to sound at the distant end. This will provide one direction of communication. In a practical telephone bothway com­munication is necessary, so in a simple telephone system each end is provided with means for transmitting and receiving sound.

In addition some form of mechanism is required to signal to the distant end to attract the distant party's attention to the fact that the caller wishes to talk to the distant end. Numerous forms of signall­ing schemes have been devised and they vary depending on the type of telephone system to which the telephone is connected. Some form of power supply is required to generate the electrical signals and many forms of circuits exist to provide power to the telephone. These consist of local batteries to power only an individual tele­phone or central batteries in the telephone exchange (Central Office or CO in the USA) where the power is sent down the individual telephone line to the telephone, where circuits exist to extract the power from the line and feed the telephone circuits. Cost has and is very important in telephone equipment, so the first practical tele­phones economised on the wiring from one telephone to the other by combining the pair of wires from the transmitter and from the receiver onto one single pair of wires, to connect both telephones together. The basic functional model for a telephone is as shown in Figure 58.1.

In speech communications we are primarily concerned with intel­ligibility i.e. the percentage of voice signals transmitted from one telephone and correctly received at the telephone at the distant end. We do not have to transmit every single part of the sound generated, since the listener acts as an error correction mechanism and can fill in any missing elements and still understand completely what has been transmitted. The prime intelligence in the human voice is contained in quite a small segment of the bandwidth of the hear­ing/voice spectrum. The human ear can detect sounds from 16Hz to 20000Hz and the human voice can generate sounds from l00Hz to l0000Hz.

Most of the energy in an average male voice is contained within the band from 125Hz to 2000Hz and the average female voice from 400Hz to 2000Hz. CCITT recommend a bandwidth of 300Hz to 3400Hz as being adequate for telephony and provide an acceptable level of intelligibility on a speech connection.

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