Double-sideband
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radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
communications, a sideband is a
band Band or BAND may refer to: Places *Bánd, a village in Hungary * Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran * Band, Mureș, a commune in Romania * Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, ...
of
frequencies Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
higher than or lower than the
carrier frequency In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...
, that are the result of the
modulation Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The process encodes information in form of the modulation or message ...
process. The sidebands carry the information transmitted by the radio signal. The sidebands comprise all the
spectral component In psychoacoustics, a pure tone is a sound with a sinusoidal waveform; that is, a sine wave of constant frequency, phase-shift, and amplitude. By extension, in signal processing a single-frequency tone or pure tone is a purely sinusoidal signal ...
s of the modulated signal except the carrier. The signal components above the carrier frequency constitute the upper sideband (USB), and those below the carrier frequency constitute the lower sideband (LSB). All forms of modulation produce sidebands.


Sideband creation

We can illustrate the creation of sidebands with one trigonometric identity: :\cos(A)\cdot \cos(B) \equiv \tfrac\cos(A+B) + \tfrac\cos(A-B) Adding \cos(A) to both sides: :\cos(A)\cdot +\cos(B)= \tfrac\cos(A+B) + \cos(A) + \tfrac\cos(A-B) Substituting (for instance)  A \triangleq 1000\cdot t  and  B \triangleq 100\cdot t,  where t represents time: :\underbrace_\cdot \underbrace_ = \underbrace_ + \underbrace_ + \underbrace_. Adding more complexity and time-variation to the amplitude modulation also adds it to the sidebands, causing them to widen in bandwidth and change with time. In effect, the sidebands "carry" the information content of the signal.Tony Dorbuck (ed.), ''The Radio Amateur's Handbook, Fifty-Fifth Edition'', American Radio Relay League, 1977, p. 368


Sideband Characterization

In the example above, a
cross-correlation In signal processing, cross-correlation is a measure of similarity of two series as a function of the displacement of one relative to the other. This is also known as a ''sliding dot product'' or ''sliding inner-product''. It is commonly used f ...
of the modulated signal with a pure sinusoid, \cos(\omega t), is zero at all values of \omega except 1100, 1000, and 900. And the non-zero values reflect the relative strengths of the three components. A graph of that concept, called a
Fourier transform In mathematics, the Fourier transform (FT) is an integral transform that takes a function as input then outputs another function that describes the extent to which various frequencies are present in the original function. The output of the tr ...
(or ''spectrum''), is the customary way of visualizing sidebands and defining their parameters.


Amplitude modulation

Amplitude modulation Amplitude modulation (AM) is a signal modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the instantaneous amplitude of the wave is varied in proportion t ...
of a
carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...
normally results in two mirror-image sidebands. The signal components above the carrier frequency constitute the upper sideband (USB), and those below the carrier frequency constitute the lower sideband (LSB). For example, if a 900kHz carrier is amplitude modulated by a 1kHz audio signal, there will be components at 899kHz and 901kHz as well as 900kHz in the generated
radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...
spectrum; so an
audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum *Digital audio, representation of sound ...
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
of (say) 7kHz will require a
radio spectrum The radio spectrum is the part of the electromagnetic spectrum with frequencies from 3  Hz to 3,000 GHz (3  THz). Electromagnetic waves in this frequency range, called radio waves, are widely used in modern technology, particula ...
bandwidth of 14kHz. In conventional AM
transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...
, as used by ''broadcast band'' AM stations, the original audio signal can be recovered ("detected") by either
synchronous detector In electronics, a synchronous detector is a device that recovers information from a modulated signal by mixing the signal with a replica of the unmodulated carrier. This can be locally generated at the receiver using a phase-locked loop or othe ...
circuits or by simple
envelope detector An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively) high-frequency signal as input and outputs the '' envelope'' of the original signal. Diode detector A simple form of envelope detect ...
s because the carrier and both sidebands are present. This is sometimes called double sideband amplitude modulation (DSB-AM), but not all variants of DSB are compatible with envelope detectors. In some forms of AM, the carrier may be reduced, to save power. The term ''DSB reduced-carrier'' normally implies enough carrier remains in the transmission to enable a receiver circuit to regenerate a strong carrier or at least synchronise a
phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
but there are forms where the carrier is removed completely, producing double sideband with ''suppressed'' carrier (DSB-SC). Suppressed carrier systems require more sophisticated circuits in the receiver and some other method of deducing the original carrier frequency. An example is the
stereophonic Stereophonic sound, commonly shortened to stereo, is a method of sound reproduction that recreates a multi-directional, 3-dimensional audible perspective. This is usually achieved by using two independent audio channels through a configurat ...
difference (L-R) information transmitted in stereo
FM broadcasting FM broadcasting is a method of radio broadcasting that uses frequency modulation (FM) of the radio broadcast carrier wave. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to transmit high fidelity, high-f ...
on a 38 kHz
subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio bro ...
where a low-power signal at half the 38-kHz carrier frequency is inserted between the monaural signal frequencies (up to 15kHz) and the bottom of the stereo information sub-carrier (down to 38–15kHz, i.e. 23kHz). The receiver locally regenerates the subcarrier by doubling a special 19 kHz
pilot tone In telecommunications, a pilot signal is a signal, usually a single frequency, transmitted over a communications system for supervisory, control, equalization, continuity, synchronization, or reference purposes. Uses in different communication ...
. In another example, the
quadrature modulation Quadrature may refer to: Mathematics * Quadrature (geometry), drawing a square with the same area as a given plane figure (''squaring'') or computing that area ** Quadrature of the circle ** ''Quadrature of the Parabola'' ** Quadrature of the ...
used historically for chroma information in
PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
television broadcasts, the synchronising signal is a short burst of a few cycles of carrier during the "back porch" part of each scan line when no image is transmitted. But in other DSB-SC systems, the carrier may be regenerated directly from the sidebands by a
Costas loop A Costas loop is a phase-locked loop (PLL) based circuit which is used for carrier signal, carrier frequency Carrier recovery, recovery from suppressed-carrier modulation signals (e.g. double-sideband suppressed carrier signals) and phase modulatio ...
or squaring loop. This is common in digital transmission systems such as
BPSK Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency carrier wave. The modulation is accomplished by varying the sine and cosine inputs at a precise time. It is w ...
where the signal is continually present. If part of one sideband and all of the other remain, it is called vestigial sideband, used mostly with
television Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
broadcasting Broadcasting is the data distribution, distribution of sound, audio audiovisual content to dispersed audiences via a electronic medium (communication), mass communications medium, typically one using the electromagnetic spectrum (radio waves), ...
, which would otherwise take up an unacceptable amount of
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
. Transmission in which only one sideband is transmitted is called
single-sideband modulation In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of signal modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitu ...
or SSB. SSB is the predominant voice mode on
shortwave radio Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the High frequency, high frequency band (HF), which extends from 3 to 30& ...
other than
shortwave broadcasting Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...
. Since the sidebands are mirror images, which sideband is used is a matter of convention. In SSB, the carrier is suppressed, significantly reducing the
electrical power Electric power is the rate of transfer of electrical energy within a electric circuit, circuit. Its SI unit is the watt, the general unit of power (physics), power, defined as one joule per second. Standard prefixes apply to watts as with oth ...
(by up to 12dB) without affecting the information in the sideband. This makes for more efficient use of transmitter power and RF bandwidth, but a
beat frequency oscillator In a radio receiver, a beat frequency oscillator or BFO is a dedicated electronic oscillator, oscillator used to create an audio frequency signal from Morse code radiotelegraphy (Continuous wave, CW) transmissions to make them audible. The signal ...
must be used at the receiver to reconstitute the carrier. If the reconstituted carrier frequency is wrong then the output of the receiver will have the wrong frequencies, but for speech small frequency errors are no problem for intelligibility. Another way to look at an SSB receiver is as an RF-to-audio frequency
transposer In broadcasting, a transposer or translator is a device in or beyond the service area of a radio or television station transmitter that rebroadcasts signals to receivers which can’t properly receive the signals of the transmitter because of a p ...
: in USB mode, the dial frequency is subtracted from each radio frequency component to produce a corresponding audio component, while in LSB mode each incoming radio frequency component is subtracted from the dial frequency.


Frequency modulation

Frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...
also generates sidebands, the bandwidth consumed depending on the
modulation index The modulation index (or modulation depth) of a modulation Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information. The proces ...
- often requiring significantly more bandwidth than DSB.
Bessel functions Bessel functions, named after Friedrich Bessel who was the first to systematically study them in 1824, are canonical solutions of Bessel's differential equation x^2 \frac + x \frac + \left(x^2 - \alpha^2 \right)y = 0 for an arbitrary complex ...
can be used to calculate the bandwidth requirements of FM transmissions. Carson's rule is a useful approximation of bandwidth in several applications.


Effects

Sidebands can interfere with
adjacent channel Adjacent or adjacency may refer to: *Adjacent (graph theory) in a graph, two vertices that are both endpoints of the same edge, or two distinct edges that share an end vertex * Adjacent (music), a conjunct step to a note which is next in the scale ...
s. The part of the sideband that would overlap the neighboring channel must be suppressed by filters, before or after modulation (often both). In
broadcast band A broadcast band is a segment of the radio spectrum used for broadcasting. See also * North American broadcast television frequencies * AM broadcasting * FM broadcasting * Dead air * Internet radio * Radio network * Music radio * Old-time radio ...
frequency modulation Frequency modulation (FM) is a signal modulation technique used in electronic communication, originally for transmitting messages with a radio wave. In frequency modulation a carrier wave is varied in its instantaneous frequency in proporti ...
(FM),
subcarrier A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio bro ...
s above 75
kHz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose formal expression in terms of SI base uni ...
are limited to a small
percent In mathematics, a percentage () is a number or ratio expressed as a fraction of 100. It is often denoted using the ''percent sign'' (%), although the abbreviations ''pct.'', ''pct'', and sometimes ''pc'' are also used. A percentage is a dime ...
age of modulation and are prohibited above 99 kHz altogether to protect the ±75 kHz normal deviation and ±100 kHz channel boundaries.
Amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency radio spectrum, spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emer ...
and public service FM transmitters generally utilize ±5 kHz deviation. To accurately reproduce the modulating waveform, the entire signal processing path of the system of transmitter, propagation path, and receiver must have enough bandwidth so that enough of the sidebands can be used to recreate the modulated signal to the desired degree of accuracy. In a non-linear system such as an amplifier, sidebands of the original signal frequency components may be generated due to distortion. This is generally minimized but may be intentionally done for the
fuzzbox Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly ...
musical effect.


See also

*
Independent sideband Independent sideband (ISB) is an AM single sideband mode which is used with some AM radio transmissions. Normally each sideband carries identical information, but ISB modulates two different input signals — one on the upper sideband, th ...
*
Out-of-band In telecommunications, out-of-band activity is activity outside a defined frequency band, or, metaphorically, outside of any primary communication channel. Protection from falsing is among its purposes. Examples General usage * Out-of-band agr ...
communications involve a channel other than the main communication channel. *
Side lobe In antenna engineering, sidelobes are the lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the ''main lobe''. The radiation pattern of most antennas shows a pattern of "''lobes'' ...
* Sideband computing is a distributed computing method using a channel separate from the main communication channel. *
TV transmitter A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized a ...


References

* *
Department of The Army The United States Department of the Army (DA) is one of the three military departments within the United States Department of Defense. The DA is the federal government agency within which the United States Army (U.S.) is organized. It is led ...
Technical Manual TM 11-685 "Fundamentals of Single Sideband Communications" {{Authority control Amateur radio