Narrow bandwidth modulation of a radio frequency sinusoidal carrier commonly used by mediumwave (MF) and shortwave (HF) broadcasters.
Amplitude modulation is illustrated in the diagram above: the top trace shows the sinusoidal radio frequency carrier; the middle trace is the audio signal to be modulated; the bottom trace is the product (filtered) of the carrier with the modulating waveform. The bottom trace is the waveform that is transmitted.
Spectrogram of DSBAM showing carrier
and symetrical sidebands
Spectrogram obtained using software by Richard
S.Horne and AOR AR7030 Communications receiver.
The efficiency of this modulation technique is revealed by studying the spectrum of the transmitted signal. The spectogram shown above is showing a double-sideband full carrier AM signal modulated with speech. We see immediately that the energy on one side of the carrier is an exact duplicate to that on the other.
The diagram above represents the power spectrum averaged over time. It shows that most of the power is concentrated in the carrier frequency and only some of the total is transmitted in the two symmetrical sidebands that appear of either side of the carrier frequency. The sidebands vary in shape according to the instantaneous to the instantaneous condition of the modulating wavefore and have the same power spectrum.
This is inefficient because power is wasted in the carrier and duplicated in each sideband. An improvement can be obtained by suppressing the carrier to obtain the following power spectrum:
This imposes extra complexity in the receiver because the carrier must be re-inserted into the receiver's detector in phase with the original signal for it to be demodulated properly. A conventional AM detector will produce a distorted output if the carrier is absent.
Some high performance radios have a synchronous detector which is used with fading double sideband full carrier transmissions: this phase locks to the carrier and re-inserts a local oscillator in phase with the original carrier. Synchronous detection prevents the bouts of distortion commonly heard when listening to AM broadcast stations when the carrier fades but one or both sidebands remain.
A further improvement in transmitter efficiency can be made by also suppressing one of the sidebands. This is called single sideband suppressed carrier (SSB).
