AUDIO FREQUENCY AMPLIFICATION are illustrated by examples from modern practice, but it must be remembered that this practice is changing from year to year, almost from month to month. Put broadly, radio reception means the reception of signals from a certain distant transmitter with a reasonable degree of loudness and a minimum of interference from other sources of radio transmission. To accomplish this, certain conditions must be recognized and complied with.
The sensitivity of a radio telephone receiver depends upon the aerial system and upon the amplification due to the valves. An open aerial is much more sensitive than a frame or loop aerial, unless the open aerial is unusually small and situated indoors. However, in modern receiver design there is no difficulty in secur ing adequate signal strength when a frame aerial of but small dimensions is used. The frame is more selective and it also per mits—in certain cases—a greater immunity from jamming, owing to its directional properties. There is a limit to the sensitivity of a receiver, and this is fixed by the promiscuous interference due to atmospherics, various radio transmitters, local noises due to tramways, electric motors, and the like. Interference of this nature can be collectively classed as "noise." When, therefore, the "noise" is sufficiently loud to interfere with the reception of a distant station, the limit of sensitivity for that station has been reached and no useful purpose is gained in increasing the ampli fication.
The quality of reproduction from a radio receiver depends upon a number of factors. One of these is the response characteristic of the receiver, apart from the loud-speaker or telephones. The receiver should respond equally to any frequency from 20 to cycles—this being the compass required to accommodate all musical instruments—but there are few, if any, receivers which attain to this standard. Assuming that a receiver complies with this condition, when a series of notes of equal loudness are played in a broadcasting studio, they should all have the same loudness at the receiver provided a satisfactory loud-speaker is used. In listening to distant stations it will, in many cases, be found impera tive—due to interference—to make the receiver so selective that the band of frequencies covered is much smaller than 20 to io,000 cycles. When the upper frequencies (above 3,50o cycles) are re duced too much the reproduction becomes muffled and lifeless. Thus when interference is rife, high quality cannot be obtained from distant stations. For with a sensitive receiver set to cover the whole musical scale, the signals are swamped by "noise," whereas with a selective receiver the essential musical character istics are stifled by super-tuning. In general, distant reception and quality are mutually exclusive.
Where telegraphy is concerned, the selectivity can be much greater than in broadcast telephony. To get clear cut signals in the Morse code, it is necessary to receive a band of frequencies whose width increases with the speed of transmission. For example, to receive good signals at a speed of ioo words per minute, the receiver should respond equally to a band of frequencies 200 cycles on each side of the tune point. When the received band is narrowed down, the signals are ill defined, and instead of being square they become rounded. This is detrimental to relaying the signals and subsequently printing them at a central office. More over, when it is imperative to narrow the receiver frequency band to avoid interference, the more economical procedure is to reduce the speed of sending, thereby obtaining square signals which can be printed direct on a paper tape, instead of being laboriously transcribed and typed by an operator.