Single-sideband modulation
In AM transmitters this mixing usually takes place in the final RF amplifier (high level modulation).
The lower efficiency of linear amplification partially offsets the power advantage gained by eliminating the carrier and one sideband.
SSB reception requires frequency stability and selectivity well beyond that of inexpensive AM receivers which is why broadcasters have seldom used it.
[4][5] SSB first entered commercial service on January 7, 1927, on the longwave transatlantic public radiotelephone circuit between New York and London.
The high power SSB transmitters were located at Rocky Point, New York, and Rugby, England.
Single-sideband has the mathematical form of quadrature amplitude modulation (QAM) in the special case where one of the baseband waveforms is derived from the other, instead of being independent messages: where
The method was popular in the days of vacuum tube radios, but later gained a bad reputation due to poorly adjusted commercial implementations.
This method, utilizing the Hilbert transform to phase shift the baseband audio, can be done at low cost with digital circuitry.
Another variation, the Weaver modulator,[11] uses only lowpass filters and quadrature mixers, and is a favored method in digital implementations.
In Weaver's method, the band of interest is first translated to be centered at zero, conceptually by modulating a complex exponential
However, in order for a receiver to reproduce the transmitted audio without distortion, it must be tuned to exactly the same frequency as the transmitter.
Since this is difficult to achieve in practice, SSB transmissions can sound unnatural, and if the error in frequency is great enough, it can cause poor intelligibility.
In other cases, it may be desirable to maintain some degree of compatibility with simple AM receivers, while still reducing the signal's bandwidth.
This produces an ideal CSSB signal, where at low modulation levels only a first-order term on one side of the carrier is predominant.
When voice is conveyed by a CSSB source of this type, low-frequency components are dominant, while higher-frequency terms are lower by as much as 20 dB at 3 kHz.
The various Kahn systems removed the hard limit imposed by the use of the strict log function in the generation of the signal.
Earlier Kahn systems utilized various methods to reduce the second-order term through the insertion of a predistortion component.
Later, the system was further improved by use of an arcsine-based modulator that included a 1-0.52E term in the denominator of the arcsin generator equation.
An additional audio processing device further improved the sideband structure by selectively applying pre-emphasis to the modulating signals.
While CSSB is seldom used today in the AM/MW broadcast bands worldwide, some amateur radio operators still experiment with it.
The front end of an SSB receiver is similar to that of an AM or FM receiver, consisting of a superheterodyne RF front end that produces a frequency-shifted version of the radio frequency (RF) signal within a standard intermediate frequency (IF) band.
If the BFO frequency is off, the output signal will be frequency-shifted (up or down), making speech sound strange and "Donald Duck"-like, or unintelligible.
These effects were used, in conjunction with other filtering techniques, during World War II as a simple method for speech encryption.
Largely to allow secure communications between Roosevelt and Churchill, the SIGSALY system of digital encryption was devised.
To conserve bandwidth, SSB would be desirable, but the video signal has significant low-frequency content (average brightness) and has rectangular synchronising pulses.
ACSSB also offers reduced bandwidth and improved range for a given power level compared with narrow band FM modulation.
The standard SSB envelope peaks are due to truncation of the spectrum and nonlinear phase distortion from the approximation errors of the practical implementation of the required Hilbert transform.
It was recently shown that suitable overshoot compensation (so-called controlled-envelope single-sideband modulation or CESSB) achieves about 3.8 dB of peak reduction for speech transmission.
This requires that the standard SSB radio's modulator be linear-phase and have a sufficient bandwidth to pass the CESSB signal.
If a standard SSB modulator meets these requirements, then the envelope control by the CESSB process is preserved.
