The single-sideband ( SSB, today more common than SSB, English:. Single-sideband modulation ) is a spectrum - and energy-efficient modulation method for voice and data transmission on wireless connections, including in the short wave band for mobile radio systems ( marine radio, aviation radio on long journeys, Military, amateur radio ) will be used. It was developed in the 1930s by the telecommunications administrations and initially used for wired transmission of telephone calls over long distances, and later for transcontinental radio links. In the field of wireless communication, the previously common double-sideband amplitude modulation ( DSB -AM) has been almost entirely supplanted by the single-sideband modulation in the course of the 1960s.
The shape of the discrete value Einseitenmodulation within the digital signal processing is also called VSB modulation (English Vestigial Sideband Modulation).
Compared to traditional amplitude modulation, which was prior to the introduction of SSB in general use, no redundant signal components (second sideband, carrier ) are emitted at SSB. The total transmit power is used solely for the information content of the signal, thus longer range and better signal to noise ratio can be achieved for a given transmit power. SSB has compared to the amplitude modulation with suppressed carrier key advantages:
- SSB halved the bandwidth requirement
- The amount of carrier frequency in the synchronous demodulator must be exactly in double-sideband modulation, therefore, vote in perfect phase. When SSB is sufficient (for language ) a frequency accuracy of a few Hertz.
In SSB the susceptibility is reduced by other radio signals. The SSB signal has proven especially in the shortwave range to be particularly robust against interference from there typical selective fading and atmospheric disturbances.
Transmitters operating with Strapless single-sideband modulation can be bad bearings taken. This can be during an emergency call from an unknown location (eg from a ship in distress ) of disadvantage. In this case, no strapless single-sideband modulation should be used. The bad Peilbarkeit but will be called by some users (military ) as an advantage.
The top image shows the effects of ( low-frequency ) modulation signal whose trace is shown on the left, to the transmitted frequency spectrum of an amplitude modulated transmitter.
By the modulation signal produced symmetrical to the carrier frequency (English: carrier) two additional frequencies, the distance indicating the current modulation frequency. Any change is immediately reflected in the position of this companion frequencies with respect to the carrier frequency. If the modulation frequency varies for example between 300 Hz and 4000 Hz, a frequency band of the overall width of 8000 Hz is generated and needs. Squeezed to this area a, distortions arise, in particular, lack the high modulation frequencies. Occupied the upper frequency range is known as USB (English: Upper Side Band ); occupied the lower frequency range is called LSB (English: Lower Side Band ), both contain exactly the same information.
Changing the amplitude ( " volume " ) of the modulation signal, will not affect the amplitude of the carrier, but only the amplitude of the satellite frequencies, thus energy is wasted.
In the picture carrier frequency and the lower sideband is suppressed, and therefore ( in this simple modulation signal), the transmitted signal can not be distinguished from an FM signal ( application at slow scan television, radio and packet radio teletype ). With a reduced amplitude of the modulation signal and the transmit power decreases.
Advantages over AM: The required bandwidth in SSB is only half as large as that of a symmetric double-sideband wave with carriers to required transmit power is significantly lower due to the lack of permanent carrier wave in the mid-band, it varies between 0% and 12.5 % and is therefore only depends on the what of modulation signal level on the mixing frequency relative to the band center itself remains (so-called " differential modulation "). The disadvantage is that no information is sent where the (necessary for demodulation ) carrier frequency is and whether it is USB or LSB. If language sent, can be very easily determined by trial error because this is incomprehensible at a deviation of more than 100 Hz. In SSTV regularly sent sync pulse facilitates finding the necessary carrier frequency. But music sounds inharmonious already at frequency deviations of a few hertz and is thus not transferred by SSB.
An SSB signal can be generated in several ways:
- The filter method starts with a mixer, which suppresses the carrier frequency. Usually a Gilbert cell is used for this purpose, which generates an amplitude modulation with a suppressed carrier. Then a very narrow band and therefore expensive quartz filter ensures that only one of the two sidebands can be further enhanced. Because the frequency of this band-pass filter can not be changed, another mixer must bring the frequency of the SSB signal at the desired transmission frequency.
- In the phase method, which corresponds to the IQ process eliminates the expensive filter, for two balanced mixers are used, the input signals are out of phase with both the low as well as at the high frequency side by 90 °. It is very difficult to generate this value in the entire speech frequency range of 300 Hz to 3500 Hz with the components of the analog equipment ( capacitors and resistors). Any deviation will result in poor suppression of the unwanted sideband. Thus SSB can be generated at the transmission frequency.
- Today, the Weaver method ( " third method " ) is almost always used, which allows digital components with a very inexpensive treatment of the SSB signal. Here, the speech signal is digitized, and produced by Hilbert transformation and the IQ - modulator. After D / A conversion is the SSB signal at the desired frequency.
Although the amplitude of the SSB signal has some similarity with the modulating low frequency, SSB can not be demodulated with an envelope detector. The reason is the absence of the reference signal ( " carrier frequency ").
Demodulation of the SSB signal is performed by the frequency offset to the original position. In a typical SSB receiver, the received signal is mixed in a mixing stage to a fixed intermediate frequency, and with a steep-edged filter free of all interfering signals in the adjacent radio frequencies. A further mixing stage, called for historic reasons, " product detector ," then takes the final frequency translation to the original position.
Also in the receiver technology, the analog signal processing, which requires expensive electronic components, has been replaced gradually since the mid-90s with lower-cost digital signal processing. Up to the reception area and the mixing stage now fully digital receiver concepts are common, which are referred to as software defined radio. Depending on the design of digital signal processing or whose configuration can thus be realized and be adapted to different requirements and changed software changes various demodulation.
Other, special forms
In the broadcasting sector, the introduction of SSB as a replacement for AM has been discussed for decades, ended after the development of digital transmission methods (DRM and DAB). In principle the transmission of Stereofoniesignalen under AM stereo is possible with the single-sideband modulation on a radio frequency, but is never carried out. The main reason is that the achievable quality of music because of the low bandwidth of the AM channels is modest beyond the pale of only 9 kHz.
Programs that are broadcast in single sideband, can not be played in radio receivers with normal AM demodulation. But there is also a special form of single-sideband modulation, which can be accurately reproduced by normal radios with AM demodulators (AM -compatible single-sideband modulation, in this case also, the carrier is also sent ). This modulation was used 1953-1962 in a long-wave transmitter of the Germany radio.
A widely used special case of single-sideband modulation is used in the analog transmission of television signals so-called vestigial sideband modulation with respect to the double-sideband AM also achieved a significant increase in energy and spectrum efficiency. In contrast to the pure single-sideband, however, a reduced carrier and a portion of the second side of the tape is transferred here. Characterized (for example, a TV set ), the demodulation of the signal is substantially simplified and cheapened at the receiving side.