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Single Sideband Modulation
With AM-modulated transmission, "sidebands" are created just below and just above the carrier frequency known as the upper and lower sidebands (USB and LSB, respectively).
AM radio stations typically transmit on both sidebands. That is to say, they use the full available bandwidth. However the two sidebands are copies of each other.
This is not technically necessary, however. It is possible to transmit by just using a single sideband. This is more efficient and means you are focusing your power to transmit a single frequency.
The formula for the generation of the full audio signal is:
\text{USB: } f_c + f_m \
\text{LSB: } f_c - f_m \
\text{Carrier: } f_c
The standard process of transmitting AM:
- Audio signal multiplies with carrier in accordance with formulae
- Two sidebands are created (USB, LSB)
- All three components are transmitted
With SSB:
- Audio signal multiplies with carrier in accordance with formulae
- Two sidebands are created (USB, LSB)
- A filter removes the carrier and one sideband
- Only one sideband (USB or LSB) is transmitted
Crucially, this is possible because the carrier wave frequency is not needed for transmission and receiving. It is only needed in order to get the audio signal (f_m) up to the transmitter frequency ready to transmit. But it it is kept in during normal AM transmission.