VOLUME VARIATIONS

Hawke's Bay Tribune, Volume XXV, Issue 124, 11 May 1935, Page 10

VOLUME VARIATIONS

Difficulties in Radio Control PREVENTING DISTORTION One of the difficulties of broadcasting is that of including the full natural frequency and volume ranges of certain kinds of musical performances within the limits of the transmitting system. In most cases it is necessary to cut off some part of the frequency range and also to compress the volume range so that it is not beyond the safe limits of the system as a whole. Such omission and compression is, of course, a variety of distortion and, depending upon the extent to which it is practised, has a harmful influence upon the reproduction.

The necessity for improving matters in both these respects has long been recognised, and efforts are continually being made to overcome the difficulties. The present aim to produce receivers of high fidelity, extended range, or whatever other term may be applied to indicate improvement and extension of the band of reproduced frequencies is an acknowledgement of one of the shortcomings. The other is also receiving attention. The compression of volume range has no doubt been noticed by listeners who have observed that sometimes the loud passages of a musical performance do not reach the volume which might be expected, while the softer passages are made louder than they should be, or a variation in the intensity of the background noise will perhaps have been observed. A limitation of volume range is at the present time necessary, as there are few broadcasting systems which can handle a volume range in which the louder parts arc more than 10,000 times us powerful as the softest parts without introducing trouble of some kind.

An orchestra may have a variation of volume in which the strong passages contain millions of times the energy of the softest passages. If a reduction of the louder passages is not made there is considerable danger that the signal will be distorted by reason of overloading of certain parts of the transmitter, while if the softer parts are not made proportionately loud they are likely to be overcome by the valve, line, and other noises which are unfortunately associated with transmission and reception.

The problem of dealing with this variation and compressing it so that it may be handled by a system with a much smaller range is obviously a difficult one, and requires considerable skill on the part of the one who is responsible for the operation. Upon the control engineer devolves the whole responsibility of varying the volume not only so that the balance of the performance is preserved as far as possible. The position, therefore, is a difficult one to fill, as not only must he be able to observe the technical requirements, but also understand the musical aspect of the transmission very thoroughly. The problem apparently has become so important in other parts of the world that a system of automatic control has been suggested. According to this scheme the variation of volume is compressed in a strictly proportional and automatic manner before the signal is transmitted, and at the receiving po'nt the compressed range is expanded again.

Thus, the essential requirements of avoiding over-modulation or overloading of the transmitter on the loud passages and the drowning of the soft passages by background noises is avoided, While the original balance is substantially maintained in the final reproduction. The scheme means the introduction of a kind of automatic volume control somewhere between the microphone and the transmitting aerial, and at the receiver a decontrolling arrangement, principle as automatic volume control, but in a reverse direction.

The main difficulty is that there must be a definite relationship between the arrangements at the receiver and the arrangements at the transmitter. In other words, the decreases and increases made at the transmitter must be followed, but in a reverse direction, in exactly the same proportion in the expanding arrangements at the receiver, otherwise the result would be unsatisfactory.