58.2 Telephone speech functions

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.

58.3 Telephone transmitters

Telephone transmitters have been designed using numerous tech­niques to convert sound impinging onto the transmitter into electri­cal energy.

58.3.1 Carbon granule transmitter

Carbon granule transmitters were very common from the earliest days of telephony until comparatively recently. The principle of operation is shown in Figure 58.2. Sound pressure impinges on the diaphragm causing it to vibrate. The centre of the diaphragm is attached to a carbon electrode which moves, compressing and decompressing the fine carbon granules sealed in the chamber at the rear of the device. A second carbon electrode is fixed to the back wall of the device. The variation in pressure on the fine carbon granules causes the resistance between the two electrodes to vary in unison with movement of the dia­phragm. When a d.c. supply is connected between the two carbon electrodes, and the carbon granules are compressed and decom­pressed, an alternating current is superimposed on the d.c. supply which represents the speech signal.

58.3.2 Rocking armature transmitter

The rocking armature transmitter is shown in Figure 58.3. Sound pressure impinges on the diaphragm causing the armature to pivot, altering the magnetic field and inducing an alternating current in the coils. This alternating current represents the speech signal.

58.3.3 Piezoelectric transmitter

Piezo-electric transmitters work on the principle that certain types of quartz crystals become electrically polarised when subjected to pressure from sound waves. Sound impinges on the disc of piezo material, causing the disc to be stressed, varying the charge on the capacitor within the disc, which in turn varies the gate voltage of the FET impedance conversion amplifier, shown in Figure 58.4. The disc and the FET are all housed in a pressed aluminium casing. The apertures at the front of the transmitter have variable hole patterns which are selected to improve the frequency response of the unit.