Double Sideband-Suppressed Carrier Modulation

Transmitter

Recall that AM is power and bandwidth inefficient, DSB-SC attempts to be more power efficient.

Note: Similar to AM, bandwidth of the message for DSB-SC () is expected to be much smaller than , that is: This is so that, in the frequency domain, when the cosine doubles the message, they don’t overlap

The Envelope doesn’t follow the message

Notice: We get phase reversals because the original message not everywhere positive, so the envelope (dashed line), doesn’t follow the message. As a result, we can’t use our previous envelop detector.

Receiver

where is a scaled version of the message called the demodulated signal.

Note: Low-pass filter removes high frequencies in a signal

Frequency Domain

As we can see, although the signal doesn’t waste power in transmitting a carrier, it still wastes bandwidth since

Synchronization

Up until now, we’ve assumed that the frequency of the the transmitted signal () is equal to the signal frequency of the multiplied by the receiver (, but in practice, they are never equal due to uncertainty.

Taking into account phase difference

If is small and constant then , and we wouldn’t have to worry about it, this is not often the case, so we need a circuit to track . (WHAT IF IT’S CONSTANT BUT LARGE? CAN’T WE JUST INCREASE THE AMPLITUDE?)