Single-sideband modulation

Amplitude modulation typically produces a modulated output signal that has twice the bandwidth of the modulating signal, with a significant power component at the original carrier frequency. Single-sideband modulation improves this, at the cost of extra complexity.

The best way of thinking of SSB modulation is to first consider an amplitude modulated signal. This will have two frequency-shifted copies of the modulated signal (the lower one is frequency-inverted) on either side of the remaining carrier signal.

To produce an SSB signal, apply a filter that will filter out one of the sidebands, and remove the carrier signal. What remains still contains the entire information content of the AM signal, using substantially less bandwidth and power, but cannot now be demodulated by a simple envelope detector.

When the 'wrong' subcarrier is only partially suppressed, the resulting modulation technique is known as vestigial-sideband modulation (VSB).

To recover the original signal from an SSB signal, the carrier must be replaced with an extra 'false carrier' signal, prior to sending the signal to a standard envelope detector.

For this to work, the false carrier must be accurately adjusted to match the frequency of the original carrier. If the false carrier is mis-adjusted, the output signal will be frequency-shifted, making speech sound strange and 'Donald Duck'-like.

If the wrong subcarrier is selected at IF conversion time, the audio signal will also be frequency inverted. This effect was used, in conjunction with other filtering techniques, during World War II as a method for speech encryption. Radio telephone conversations between the US and England were intercepted and 'decrypted' by the Germans; they included some early conversations between Roosevelt and Churchill. Today (2001), such simple 'inverter'-based speech encryption techniques are easily decrypted using simple techniques and are no longer regarded as secure.

Note: SSB and VSB can also be regarded mathematically as special cases of quadrature amplitude modulation.

See also:

• modulation for other examples of modulation techniques