What are the reasons which lead to the use of double side-band amplitude for radio transmission, and single side-band reduce carrier working for point to point transmission?
Thank you for your help.
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Regular double sideband with unsuppressed carrier is used by AM radio stations around the world mainly because the detector in the thousands of listener's radios is as simple as a diode, a capacitor and a resistor. The modulation difficulty is unimportant given that there is only one transmitter but, as it happens, this sort of modulation is fairly easy to implement - it's just 2-quadrant multiplication: -
A refinement of this is DSBSC (double sideband suppressed carrier) and is modulated by using a 4-quadrant multiplier. To receive this type of modulation requires more sophisticated electronics than a simple diode detector and hence it makes receivers of this type more expensive. As can be seen below, at certain points in the waveform the carrier reverses phase and this makes the diode detector demodulate incorrectly.
Sideband modulation is more complicated again but, like DSBSC offers better signal-to-noise ratios than regular AM because the transmit power can be focused into just one sideband. Or look at it another way - the overall transmit power can be reduced significantly. Also because the signal occupies less spectrum, with the appropriate filters in the receiver, the received unwanted noise is less.
Double sideband amplitude modulation is used for broadcast and other radio transmission because it's been grandfathered in since the start of radio, being the first technology used to generate a variable amplitude RF carrier.
Single-sideband reduced carrier modulation is used to reduce the bandwidth of the transmitted signal without affecting its information content while - for the same power input to the final amplifier - increasing the power used to transmit data, both of which serve to increase the signal-to-noise ratio in whatever medium the transmission is occurring.