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TELEVISION AND RADIO TRANSMISSION 635
is amplified by a radio frequency amplifier up to the same power as the sound frequencies— thai is, the maximum power the station is licensed to use. A radio frequency amplifier is simply an ordinary amplifier in which transformers, if used, are of radio frequency, air-core types, and consist of only a few turns of wire.
Modulation
(7) The radio frequency current — which will radiate if impressed upon a transmitting antenna — is the socalled carrier current. The sound frequencies are impressed upon the carrier frequency by a process called modulation. In amplitude modulation the effect is to vary the volume of the radio frequency in accordance with the volume and frequency of the sound current.
UNMODULATED *"F MODULATING MODULATCO
R-r CARRiCR WAV/t R-r WAVE
Figure 225
This is illustrated in Figure 225. The commonest method of effecting modulation is to connect the secondary of the output transformer of the sound amplifier with the primary of the output transformer of the radio amplifier. Thus the sound current flows in the output circuit of the radio amplifier, modifying the plate voltage of the output stage of the radio amplifier in accordance with the frequency and volume of the sound current. The secondary winding of the radio frequency output transformer is connected to the broadcasting antenna. The current in that antenna is illustrated at the righthand side of Figure 225, its radio frequency varying in volume at the frequencies of the sound current, the extent of this variation in r.f. volume depending upon the volume of the sound current.
(8) Another type of modulation is known as fre