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ModulationWhen information is transmitted by radio or microwaves there has to be a carrier wave. The sound information is combined with the carrier wave during modulation at the radio transmitter. The signal is then demodulated at your radio so that the sound information can be sent to the speaker and the sounds emitted into the air. Previously all radio stations were 'analogue'. Tuning a radio into an AM (amplitude modulated) signal or to an FM (frequency modulated) signal identified the carrier wave's wavelength (or frequency) that your radio had to recognise and pick up.
Amplitude Modulation is the superposition of the sound information
onto a high frequency carrier wave. This creates a wave with a varying
amplitude. The radio subtracts the high frequency carrier wave (demodulates)
and then feeds the sound signal to the output (loudspeaker). The problem with amplitude modulation is the noise or interference. The electromagnetic waves from a nearby electrical machine can be picked up by the radio which then outputs them as sound (hissing or buzzing as it's a continuous low frequency wave) which masks the programme being transmitted. Frequency Modulation also requires a high frequency carrier wave but its frequency is altered - the high voltage period of the information wave increases the frequency of the carrier wave and vice versa. Frequency Modulation is preferred as noise or interference is much less of a problem. Pulse
Code Modulation: Digital radio doesn't use AM or FM, it uses
pulse code modulation. There's still a carrier wave so the radio still
has to be tuned to pick up a particular frequency. However the sound
wave has to be encoded as a series of numbers. To do this, it is sampled
at regular intervals and the amplitude measured. The value of the amplitude
is converted into binary (base 2); so 2 is 0010, 4 is 0100, 6 is 0110
and 8 is 1000 etc. The 1s and 0s are now combined with the carrier wave
as 'ons' and 'offs'. The sampling has to be at least twice the frequency of the highest frequency being transmitted. If it is not, the higher frequencies are missed and the wave cannot be reconstructed accurately from the available information. As most people hear up to a maximum of 20 kHz, the sampling rate for perfect transmission of music, such as on a CD, is 44kHz. The big advantage of pulse code modulation (or digital signals) is that it suffers much less from interference (noise), and mistakes caused by interference can be rectified by using error correction codes. Also, the signal can be checked and 'boosted' if it is beginning to fade. Digital signals can be sent directly to computers which use digital systems. Frequency Shift Keyed Modulation: Satellites use a modulation system called 'frequency shift keyed'. It's slightly more complicated than simple frequency modulation: the frequency of the carrier wave is changed slightly to represent the 0s and 1s of the digital signal. In this case, the carrier wave (frequency 430 MHz) is shifted up and down by 10 kHz; the receiver detects these higher and lower frequencies, interprets them as 1s and 0s and reads the digital information. |
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