the sound level that produces a feeling of discomfort being known as the threshold of feeling. There are three ways in which a digital signal may be transmitted to Figure 10.6 illustrates the threshold of feeling and audibility by a line : plotting sound pressure against frequency. It can be seen that the most sensitive region is between 1000Hz and 2000Hz although 1. A single (unipolar) current. response is capable with the 30Hz to 16500Hz range. 2. The presence or absence of a voltage. 3. Two voltages (bipolar) one positive and one negative with Threshold of feeling respect to earth. The use of digital transmission instead of analogue gives the following advantages 1. A better signal to noise ratio. Noise and interference are accu- mulative with distance on an analogue system, but not on a digital system. Threshold of audibility 2. Improved signalling capability. 3. Time division multiplexing, used with digital signals, is simpler than frequency division multiplexing, which is used with anal- ogue signals. 4. Digital switching is more straightforward to implement. 5. In digital systems different kinds of signals, e.g. data, telegraph, 31.25 32,000 telephone speech and television, have the same representation Frequency (Hz) on the medium and therefore can be treated in a similar manner during transmission and switching. Figure 10.6 Audability Examples of signals 10.4.2 Telephone and telegraph 10.4.1 Voice and music By international agreement the audio frequency for a commercial The sounds produced by a human voice are variations of air pressure quality speech circuit routed over a multi-channel telephone system above and below normal pressure. Such sounds are alternating in is restricted from 300Hz to 3400Hz. The suppression of all frequen- nature and have a complex waveform that is different for each voice. cies above 3400Hz reduces the quality of sound but does not affect The voice contains fundamental frequencies and associated har- the intelligibility. 10/6 Signals and noise A telegraphy system is one that pa