doublesideband

Double sideband (DSB) refers to a class of amplitude-modulated signals in which both the upper and lower sidebands produced by the modulation process are transmitted. There are two common variants: double sideband with a full carrier (DSB-FC), also known as conventional AM, and double sideband suppressed-carrier (DSB-SC). In DSB-FC, the transmitted signal can be written as s(t) = [A + m(t)] cos(2πf_c t), where m(t) is the baseband message and A is a constant carrier amplitude. The carrier enables simple envelope demodulation but wastes transmitter power when m(t) is small. In DSB-SC, the carrier is omitted and s(t) = m(t) cos(2πf_c t). This is more power-efficient and avoids a visible carrier in the spectrum but requires coherent detection at the receiver.

Spectrally, a baseband signal with bandwidth B produces two symmetric sidebands around the carrier: f_c ± f for |f| ≤ B. The total occupied bandwidth is 2B. The carrier frequency f_c appears only in DSB-FC.

Demodulation: DSB-FC can be demodulated with a simple envelope detector provided the modulation index is not excessive. DSB-SC requires a coherent demodulator or a product detector with a locally generated carrier, or a carrier reinsertion scheme at the receiver.

Compared with single-sideband (SSB) and vestigial-sideband (VSB), DSB uses more bandwidth and, for the same baseband signal, generally more transmitter power than SSB or VSB. DSB-FC is widely used in traditional analog AM broadcasting, while DSB-SC and related forms have been employed in certain communications links and digital modulation contexts where bandwidth and power efficiency tradeoffs are managed differently.