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the commonest technique for recovering the information (sound or picture) from carrier waves of a range of frequencies, transmitted by different broadcasting stations. The circuitry, devised by Edwin H. Armstrong during World War I, combines the high-frequency current produced by the incoming wave with a low-frequency current produced in the receiver, giving a beat (or heterodyne) frequency that is the difference between the original combining frequencies. This different frequency, called the intermediate frequency (IF), is beyond the audible range (hence the original term, supersonic heterodyne reception); it can be amplified with higher gain and selectivity than can the initial higher frequency. The IF signal, retaining modulation to the same degree as the original carrier, enters a detector from which the desired audio or other output signal is obtained.