FREQUENCY MODULATION

Evening Star, Issue 23681, 14 September 1940, Page 4

FREQUENCY MODULATION

CRYSTAL-CLEAR RECEPTION Crystal-clear reception, the dream of every radio enthusiast since the establishment of broadcasting, has become a reality at last. The promise of staticfree radio is held out in America by a fast-developing system called frequency modulation. At a recent test of frequency modulation a receiver that had been built for both broadcast and frequency modulation was taken into a big electrical laboratory. When the set was tuned-in on the broadcast band the music was entirely blotted out by the noise of million-volt thunderbolts, but when the set was tuned-in on frequency modulation the music was not interrupted by any crackle, although there were million-volt sparks flying around the laboratory. The sudden change from one system to another would cost a tremendous amount of money, as all present transmitters and receivers would have to be scrapped. The change-over would be a gradual one, and as old sets wore out they would be replaced by new ones. Already in America several manufacturers have been licensed to make the new kind of set. At present the two types are combined so that the listener can hear both frequency modulated and broadcast programmes. In America at present there have been built several stations to broadcast frequency modulation programmes. Owing to the space that they take up on the wave-band, these stations operate on a very low frequency.' The one disadvantage of this system is that its signal can travel no further than the horizon—a distance of 20 to 30 miles. The higher the mast, the better the range. One advantage of frequency modulation is that it can be transmitted from one place to another from where it can he again relayed. This has been tried with great success. Frequency modulation also eliminates the necessity for costly relays from, place to place.

The differences between the old and the new seem to lie in the basic principles of broadcasting. High-frequency, alternating current is used as the source of power in broadcast, and in order that sound waves are changed into radio waves the voltage is modulated. This is called the amplitude modulation system, and is used in present broadcasting, the principle being to vary the strength of the electrical current as the sound enters the microphone. The above system has always been haunted by distortion and static, in one case by operating on a narrow wave band in long wavelengths. If the amplitude station changes down to ultra short waves it runs into static because lightning and other natural electrical disturbances have the sam’e characteristics. An effort has be%n made to defeat static by boosting the power more and confining the signal to a narrow band, so giving static less chance to interfere. Frequency modulation calls for the .voltage to remain constant, the sound being imposed on the waves by wobbling their frequency over a wave band that may be as wide as 150,000 cycles. A sound wave of 5,000 cycles (vibrations a second) would be converted into a frequency wobble of 5,000 times a second; thus the wave band widens or contracts to accommodate the sound. Each station in America is allowed 200,000 cycles, as compared with the_ 5,000 cycles of an amplitude transmitter. While frequency modulation has been proved practical there are some thousands of listeners to frequency modulation stations in America, it will be a long time before it is universally adopted throughout the world.