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.

Everyone must have heard the effect when listening to two public address loudspeakers which are not the same distance away. Speech becomes difficult to understand and eventually unreadable. In the extreme case when a signal is delayed by one half cycle or 180°, (and this effect can be produced equally by crossing over one pair of connecting wires), there is complete cancellation of all signals and nothing is heard.

Phasing, and the many problems which can arise, is too complex a subject to discuss in a paper of this length, but the hazards are one of the prices we pay for a multi-station wireless scheme which otherwise has many advantages. Care is taken when a scheme is installed to correctly connect and phase all the inter-connections but there are many reasons, which range from deterioration of equipment to a technician at some time making a reversed connection, why the system can get "out of phase". Checking for this "out of phase" condition can be done in one of two ways:

a, check the overall system in one test

b. check each individual piece of equipment in a scheme.

When it is remembered that use of method b, would involve 80 to 100 checks per scheme it will be realised that this type of test could not be carried out very frequently.

Because of the havoc which one reversal can cause to a scheme however, the more frequently this test can be carried out the better. Equipment has now been developed at the Directorate's new test equipment repair and development centre at Weyhill, Nr Andover, Hants, and this will enable quick tests to be carried out by maintenance technicians using method a. There is no reason why such test equipment should not ultimately become a part of routine operator testing, but it is certain that the ability to know that "all is well all the time", will result in a vast improvement in the operational efficiency of the scheme.

Source: Home Office DOT Exhibition 1971 Papers

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