英语翻译
Amplitude Modulation
Amplitude modulation (AM) is a modulation technique in which the amplitude of a high frequency sine wave (usually at a radio frequency) is varied in direct proportion to that of a modulating signal.The modulating signal carries the required information and often consists of audio data,as in the case of AM radio broadcasts or two-way radio communications.The high frequency sine wave (the carrier) is modulated by adding the modulating signal to it in a mixer.A simplified AM radio transmitter system is shown below.A simple form of amplitude modulation was originally used to modulate audio voice signals onto a low-voltage direct current (dc) carrier on a telephone circuit.A microphone in the telephone handset acts as a transducer,and uses the sound waves produced by the human voice to vary the current passing through the circuit.At the other end of the telephone line,a second transducer (in the form of a small loudspeaker mounted in the remote handset) uses the varying voltage to produce sound waves that are close enough to the original speech patterns to be recognisable as the voice of the caller.Although the human voice is composed of frequencies ranging from 300 to approximately 20,000 hertz,the public switched telephone system limits the frequencies used to between 300 and 3,400 hertz,giving a total bandwidth of 3,100 hertz.This bandwidth is perfectly adequate for purely voice transmission,since the higher frequencies in the human voice (i.e.those above 3,100 hertz) are not really needed for recognisable speech reproduction.The use of a limited bandwidth also makes the telephone system much simpler from an engineering perspective.Whereas telephone signals can be transmitted at audio frequencies,the same is not really a practical proposition for radio transmissions.The main reason for this is that the optimum length of a radio antenna is a half or a quarter of a wavelength.Since a typical audio frequency of 3,000 hertz has a wavelength of approximately 100 kilometres,the antenna would need to have a length of 25 kilometres to be effective - not a realistic proposition.By comparison,a radio frequency of 100 megahertz would have a wavelength of approximately 3 metres,and could use an antenna 80 centimetres long.It becomes necessary,therefore,to use a radio frequency carrier signal in order to transmit audio