|
The Electron Multiplier, described in our issue of November 22, 1935, is primarily attractive because it offers more magnification with less noise.
Just as geologists amuse themselves by dividing up the last few hundred million years into various periods Eocene, Miocene, Pliocene, Pleistocene, etc. (finishing up in the modern nursery with Plasticene) - so we may find it instructive to classify the few years covered by broadcast radio according - to the trends of progress therein.
First, we went all out for sensitivity. The main thing was to receive anything at all. The means available were so poor, and the transmitters so few and weak, that the possibility of receiving too much simply did not occur to one. The idea of a volume control seemed at one time to be fantastic, for there was never any excess volume to control. And it was so wonderful to hear any sound, be it music, Morse or atmospheric that nobody had the heart to grumble about the quality. That was the next thing to be considered, when the first novelty had worn off.
When we had learned to capture transmissions originating hundreds of miles away, and to reproduce programmes with a fair degree of fidelity to the original, and stations sprang up everywhere and increased their power, the emphasis shifted from bringing things in to keeping things out. Selectivity was the cry. The superhet, born in the interests of sensitivity, and killed in the interests of quality, was brought to life again because it was and is the most practicable means of obtaining high selectivity. With the introduction and (more or less) perfecting of variable selectivity, the last phase of this era closes. We are now - unless I am greatly mistaken - well embarked on the fourth period; that of anti-noise.
In the first three directions the closeness of approach to perfection is almost entirely a question of how much one is prepared to spend. But the greater the range, and to some extent the better the quality, the more likely are one's efforts to be marred by noise. Selectivity, though theoretically helpful up to a point, does not solve the problem.
Wanted: A Better Name
There are several quite distinct species of noise. This is so even if the term is used in its specialised technical sense, excluding such nevertheless serious types as those generated by children, itinerant 'musicians', and pneumatic drills. There is the type, much discussed for some time past by committees and others, for which there appears to be no other brief term than the hateful expression 'man-made static'. As the name suggests, this is a variety to which was non-existent at the dawn of creation, but is more or less proportional to modern civilisation with its multifarious applications of electricity. These need not be named in detail, but a large proportion of them cause interference with radio when they are in good working condition, and all of them are liable to interfere when they are not. Even the common electric lamp, when it is on the point of burning out, can simulate a massed machine gun attack on the loudspeaker front.
There are two main lines of attacking this form of noise: to silence the interfering source with an appropriate 'suppressor'; or to exclude the interference from the receiver. The former is by far the most satisfactory and effective. In an earlier age the appropriate suppressor would no doubt be deemed to consist of a well-wielded battle-axe, but in these more enlightened days a few capacitors, chokes and screens serve the purpose without loss of life. The difficulties are almost entirely economic. The owner of an interfering appliance tells the radio listener to go to blazes before he will spend money on suppressors which do nothing for him (the owner) except slightly increase the risk of breakdown. It seems reasonable to hold that it is the listener who should see to it that his receiver doesn't receive the noise. But, unfortunately, all he can possibly do may not in every case overcome the trouble. And day by day the use of electrical appliances grows. And what manufacturer of them is voluntarily. going to increase their cost by fitting suppressors unless all his competitors are compelled to do likewise?
Atmospherics, on the contrary, were here before radio or even man himself had been thought of. And although numerous patents have been filed for the complete conquest of atmospherics, we still seem to be pretty much where we stood at first with regard to them. Fortunately in this country it is only during unfavourable conditions - which are least likely to coincide with long winter evenings - that they are a serious bar to broadcast reception.
Undisciplined Electrons
The aforementioned noises originate outside the receiver. There are others for which the receiver itself is responsible. I remember that even in the early days novices were instructed in such things as noisy HT batteries, valves and switches. And these evils are still with us. They are, however, the province of that person who, in America, is picturesquely referred to as the 'trouble-shooter'. So far as the serious investigator is concerned they do not rank as a problem. When all has been done to put a set into perfect working order there are still sources of noise in it that no science has succeeded in eliminating. They are due to the fact that a stream of water in a pipe, or an electric current in a wire, is not so smooth and continuous as it may seem. A column of infantry on the march, viewed from a distant aeroplane, might seem to be one continuous stream; a closer view reveals that it consists of many separate units. But infantry is at least orderly and disciplined. Imagine an immense crowd of people in a street, jostling hither and thither. In spite of the random movement there may be a constant drift of, say, 500 an hour from east to west. Although at a great distance the crowd may appear, a continuous mass of humanity, closer investigation would reveal the separate individuals causing instantaneous fluctuations in, the flow. That, represents an electric current in a wire. It may average out at so many milliamps, but if the voltage it sets up along the wire is amplified sufficiently the irregularities due to the crowd nature of electricity may give rise to a rushing sound in a loud speaker.
That would be so even if a perfectly 'quiet ' amplifier were used. But valves themselves are necessarily noisy. The flow of current across the vacuum in a valve may be likened to the silent flow of flour down a chute the separate particles are too small to produce audible impacts. But just as a microscope makes grains of flour look like boulders the effect of high amplification is to make the valve current sound like boulders!
There is no longer great enthusiasm in professional circles when a means of getting greater amplification is invented. That is easy. But it is also useless, when it brings up the noise in the same proportion as the desired signal. What matters now is an increase in the signal/noise ratio. That is why Zworykin made a hit at the Institute of Radio Engineers when recently he demonstrated his 'electron multiplier'. An amplification of several millions in one valve was accepted with mild interest. But an estimated improvement of 20 dB in the signal/noise ratio captured attention all over the world.
At the same time Armstrong (of superhet fame) demonstrated his new system of frequency-modulation. A novel system of transmission and reception successfully demonstrated - very interesting. But 30 dB better signal /noise ratio - Ah! now he's talking!
There is no real difficulty in receiving and reproducing at any desired volume signals of less than one microvolt - corresponding to very weak and distant stations indeed. But even in the most favourable conditions such reception is submerged in noise. Increased range depends now almost entirely on our ability to overcome noise.
|
