In AM, the carrier itself does not fluctuate in amplitude. It is found that when a carrier is amplitude modulated, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the main carrier. These components are called sidebands. The sideband power accounts for the variations in the overall amplitude of the signal. To see how this happens, let’s take the example of a carrier on a frequency of 1 MHz which is modulated by a steady tone of 1 kHz.
The process of modulating a carrier is exactly the same as mixing two signals together, and as a result both sum and difference frequencies are produced. Therefore when a tone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1 kHz (upper sideband), and a difference frequency is produced at 1 MHz - 1 kHz (lower sideband).
Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present); the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information.
A variant of amplitude modulation when only one sideband is transmitted is called single-sideband transmission (SSB). In this case, the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency. Since the sidebands are mirror images, the choice which sideband to use is a matter of convention.
The problem is that the more of the sideband you filter out, the more distortion you get from the detected waveform. So there is a practical limit to how much you can reduce the bandwidth of the signal. It's about 50% reduction of one sideband. SSB modulation is used for long-distance telephony (such as in the amateur radio bands) and telegraphy over land and submarine cables.
An alternative technique used for signal power reduction is to remove the remaining carrier signal from the AM signal; the signal produced is a double-sideband suppressed carrier (DSSC) signal. A compromise between the DSBSC modulation and the SSB modulation is known as Vestigial Side Band (VSB) modulation. VSB is used in television transmission to save RF spectrum space. If the carrier signal is not completely removed, the signal is called a double-sideband reduced carrier (DSRC) signal.
However, as there now are more efficient and convenient methods of modulating a signal, the use of AM is declining.
14. Ознакомьтесь с дополнительными словами и выражениями:
medium frequency — средняя частота
frequency divider —делитель частоты
picture transmission — телевидение, передача телевизионных изображений
to superimpose one image on another — накладывать изображения одно на другое
15. Ответьте на вопросы, используя информацию текста:
1. What are main reasons to blend data into a carrier signal? 2. Is this form of modulation is a very efficient way to send information? 3. Why is the power required relatively large? 4. Is it possible to demodulate the signal if the carrier isn’t used as a reference? 5. Why is it impossible to filter out the whole sideband? 6. How is the transmission with only one sideband called? 7. What is the reason to remove the remaining carrier signal from the AM signal? 8. Where is SSB used?
16. Вставьте пропущенные слова:
