Modulation

This text gives very detailed information about the modulation of the carrier wave. The modulation of the carrier wave may carry impulses is performed either at low level or high level. In the former case the audio-frequency signal from the microphone is used to modulate the output of the oscillator. And the modulated carrier frequency is then amplified before it is passed to the antenna. In the latter case the radio-frequency oscillations and the audio-frequency signal are independently amplified, and modulation takes place immediately before the oscillations are passed to the antenna. The signal may be impressed on the carrier either by frequency modulation (FM) or amplitude modulation (AM).

The simplest form of modulation is keying, interrupting the carrier wave at intervals with a key or switch used to form the dots and dashes in continuous-wave radiotelegraphy.

The carrier wave may also be modulated by varying the amplitude, or strength, of the wave in accordance with the variations of frequency and intensity of a sound signal, such as a musical note. This form of modulation, AM, is used in many radiotelephony services including standard radio broadcasts. AM is also employed for carrier current telephony, in which the modulated carrier is transmitted by wire, and in the transmission of still pictures by wire or radio..

In FM the frequency of the carrier wave is varied within a fixed range at a rate corresponding to the frequency of a sound signal. This form of modulation, perfected in the 1930s, has the advantage of yielding signals relatively free from noise and interference arising from such sources as automobile-ignition systems and thunderstorms, which seriously affect AM signals. As a result, FM broadcasting is done on high-frequency bands (88 to 108 MHz), which are suitable for broad signals but have a limited reception range.

Carrier waves can also be modulated by varying the phase of the carrier in accordance with the amplitude of the signal. Phase modulation, however, has generally been limited to special equipment.

The development of the technique of transmitting continuous waves in short bursts or pulses of extremely high power introduced the possibility of yet another form of modulation, pulse-time modulation, in which the spacing of the pulses is varied in accordance with the signal.

The information carried by a modulated wave is restored to its original form by a reverse process called demodulation or detection. Radio waves broadcast at low and medium frequencies are amplitude modulated. At higher frequencies both AM and FM are in use; in present-day commercial television, for example, the sound may be carried by FM, while the picture is carried by AM. In the superhigh-frequency range (above the ultrahigh-frequency range), in which broader bandwidths are available, the picture also may be carried by FM. Experiments have also been conducted in which sound as well as pictures are transmitted digitally at these high frequencies. Such transmissions may some day replace current analog broadcasting techniques.