Spectrogram obtained using software by Richard S.Horne and AOR AR7030 Communications receiver.
Optimum efficiency narrowband amplitude modulation commonly used for speech and data communications on the HF bands.
The spectogram below shows a voice signal transmitted using single sideband modulation. A comparison with the spectogram for AM shows that the carrier and duplicate sideband are now removed and we are left with only the speech energy being sent. No power is being wasted in sending the carrier or the duplicate sideband.
Spectogram of voice transmitted using
single sideband
Spectrogram obtained using software by Richard
S.Horne and AOR AR7030 Communications receiver.
The power spectrum for a single sideband transmission looks like:
This is the most efficient way to transmit a speech signal because the bandwidth of the transmission exactly matches that of the speech and all the signal power is available for the intelligence in the signal.
This modulation technique demands extra complexity in the receiver over a conventional AM receiver because the carrier frequency must be re-inserted in the receiver's product detector (although it does not need to be in phase with the original carrier). The receiver also needs to have narrow filters fitted to make use of the saving in bandwidth that can be made. There is a need for high frequency stability and accurate tuning (well within 100 Hz) for optimum clarity.
The advantage of better communications efficiency outweighs these disadvantages so that SSB is widely used. Applications include speech and data communications (such as FSK and HF FAX).
