DIRECTORATE OF TELECOMMUNICATIONS
THE FIELD ORGANISATION
By: A N Holdstock
page 6 of 8
Figures 1 to 6 illustrate one way in which a speech waveform can be altered or distorted. If we take a simple waveform such as that shown in figure 1 and then a second waveform exactly the same in form but coming along a little later ( ie delayed in phase), they would sound exactly the same, because the ear cannot detect the difference in time factor. If these two waveforms are now added together, or combined, the result would be as shown in figure 3. Again the same shape but a little larger in size or amplitude, and somewhere between the other two in time. Once again, because this waveform is the same shape it will sound exactly the same. In other words we have combined two waveforms not in phase (or in step) with one another but no distortion has been produced.
If we now take a complex waveform as shown in figure 4 and another as shown in figure 5 exactly the same in shape but once more coming along a little later, or delayed in phase, (as in the case above), they will sound exactly the same for the reasons given previously. If we now add together, or combine these waveforms, we shall get the one shown in figure 6, vastly different in shape from the other two, and sounding very different. As speech waves are also complex, the same effect is produced when two which are not in step, (or in phase) are combined, and the result is an inability to recognise the letter or syllable.
These waveforms, when transmitted through space as wireless waves or via telephone wires as electrical waveforms or indeed through the air as sound waves, take a certain fixed time to travel between two points. Hence the greater the distance they have to travel, the longer the time taken and if two such wireless waves, travelling from two transmitters to a mobile receiver, have to travel different distances then one will arrive later than the other.