When using multi-band or even full-band radios, we usually need to choose the working mode, which includes USB and LSB, which means the Upper Side Band and the Lower Side Band. they are the SSB signals, and the DSB is the sum of the USB and LSB.
DSB is a normal AM signal. SSB comes from AM, but it is different from normal AM. The DSB signal needs to transmit three frequency components, including the carrier, the USB, and the LSB signals. For them, the carrier does not carry useful information, but it accounts for most of the power in the transmit power; the USB and LSB signals contain the same information, so the utilization efficiency of the power by the DSB signal is relatively low.
In order to improve the efficiency of transmit power, only one sideband signal can be transmitted to complete the information transmission, and one of the sidebands and the carrier are eliminated. This process is called SSB modulation. The generation of SSB signals generally uses a balanced modulator. After modulation, the audio frequency and the carrier wave are eliminated inside the modulator, so the signal at this time only contains the frequency components of the USB or the LSB, which are close to the 0 point in the spectrum. The two envelopes are the LSB signal, and the one farther away is the USB signal.