CRYSTAL CONTROL AT 2YA

Radio Record, Volume II, Issue 5, 17 August 1928, Page 7

CRYSTAL CONTROL AT 2YA

+ MARKED IMPROVEMENT IN TRANSMISSION A FEW weeks ago, when the an"ne icement was made that 2YA had been changed to crystal control, the statement that freqiiency modulation of the carrier wave was detrimental to quality was questioned by a writer in a Wellington paper. He stated: "The accuracy of this argument is open to question. Free cy modulation, if it occurs, certainly cannot be detected where the signal strength is considerable, and at great distances where the wave is very weak its effect on the receiver will not be appreciably, if at all, different from that of amplitude .modulation." [= will be interesting, therefore, to read what other writc:s say on the subject. The following is by Mr. Oar! Dreher, staff. engitieer to the Nationa] Proadcasting Company :--"Tt was found by Bown Martin and Potter, as well as by some investigations in England, that a certain tyne of distortion eould be traced to a slight frequency wobble, inherent in the usual method of modulating broadcast transmitters. This rapid variation within the eycle of es modulating frequency manifests itself in wave. form distortion at the receiver, sov nding somewhat like tube overloading; only wo .. Stabilisation of the radio frequency of the carrier and sidebands helps to eliminate this ‘nieht distortion," as the British call it. The method employed is to use a master ascillator with a 50-watt tube

which may be crystal controlled. The result is that the transmitter holds a constant frequency during modulation, and that distortion is reduced to selective fading which does not hash up the quality as badly as the frequency wobble aforementioned. This is what Messrs. Bown Martin and Potter say on the subject. "It was sus" -cted that present-day radio telephone transmitters leave something to be desired in regard to what we may

call, for lack of a better term, their ‘dynamic frequency stability.’ -A very large percentage of the transmitters in use throughout the world to-day produce amplitude modulation of the carrier by the action of modulating tubes directly upon an oscillating circuit.

If it is to be expected that cyclic changes in circuit conditions occurring at the modulating frequency will have some cyclic effect on the absolute frequency of the carrier, and that this effect will be in the nature of a wob-. bling or rapid shifting back and forth in frequency of the amplitude modulated carrier. In other words, the carrier and side-bands, without change in their relative frequencies, would be subjected to "frequency modulation." What we have called "dynamic instab- | ility" is so rapid that it is difficult to observe by any aural method. The same writers mention the effect obtained by stabilising the wave of a transmitter. The transmitter in this case was almost identical with 2YA. "Using the normal transmitter, nighttime transmission as received at the test stations was sericusly distorted. When the stabilising arrangement was employed; this distortion was apparently eliminated except at the minimum of fading." [= may well be asked why this frequency modulation, since it produces such marked distortion at night in certain places, does not also give rise to distortion by day or in locations where transmission is steady. A full answe to this question would be far from simple. But in brief it is because the carrier and side-bands shift in absolute frequenc, as a unit so that their relative or difference frequencies which determine the audio signal remain unchanged. However, since frequency modulation appreciably broadens the frequency band occupied by the radio signals, it is to be expected that the tuned circuits in the receiver would have some reaction on those louder portions of the signal for which the amplitude modulation, and therefore, the frequency modulation, is large." So far as 2YA is concerned, change to crystal control ha. effected a marked improvement in the quality, and reports received from all over New Zealand indicated that the "night distortion" has now been reduced to an

absolute miniinum.

J.M.

B.