Single-sideband modulation
Encyclopedia
Single-sideband modulation (SSB) or Single-sideband suppressed-carrier (SSB-SC) is a refinement of amplitude modulation
that more efficiently uses electrical power
and bandwidth.
Amplitude modulation
produces a modulated output signal that has twice the bandwidth of the original baseband
signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of somewhat increased device complexity.
. The U.S. Navy experimented with SSB over its radio circuits before World War I
. 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. The receivers were in very quiet locations in Houlton, Maine
and Cupar
Scotland.
SSB was also used over long distance telephone line
s, as part of a technique known as frequency-division multiplexing
(FDM). FDM was pioneered by telephone companies in the 1930s. This enabled many voice channels to be sent down a single physical circuit, for example in L-carrier
. SSB allowed channels to be spaced (usually) just 4,000 Hz
apart, while offering a speech bandwidth of nominally 300–3,400 Hz.
Amateur radio operator
s began serious experimentation with SSB after World War II
. The Strategic Air Command
established SSB as the radio standard for its aircraft in 1957. It has become a de facto standard for long-distance voice radio transmissions since then.
.
Let
be the baseband
waveform to be transmitted. Its Fourier transform
,
, is Hermitian symmetrical about the 
axis, because 
is real-valued
. Double sideband modulation of
to a radio transmission frequency, 
, moves the axis of symmetry to 
, and the two sides of each axis are called sidebands.
Let
represent the Hilbert transform
of
. Then
is a useful mathematical concept, called an analytic signal
. The Fourier transform of
equals 
, for 
, but it has no negative-frequency
components. So it can be modulated to a radio frequency and produce just a single sideband.
The analytic representation of
is:
(the equality is Euler's formula
)
whose Fourier transform is
.
When
is modulated (i.e. multiplied) by 
, all frequency components are shifted by 
, so there are still no negative-frequency components. Therefore, the complex product is an analytic representation of the single sideband signal:
where
is the real-valued, single sideband waveform. Therefore:
The presence of two out-of-phase (quadrature) carrier waves is now evident.
represents the baseband signal's upper sideband, 
. It is also possible, and useful, to convey the baseband information using its lower sideband, 
, which is a mirror image about f=0 Hz. By a general property of the Fourier transform, that symmetry means it is the complex conjugate of 
:
Note that:
The gain of 2 is a result of defining the analytic signal (one sideband) to have the same total energy as
(both sidebands).
As before, the signal is modulated by
. The typical 
is large enough that the translated lower sideband (LSB) has no negative-frequency components. Then the result is another analytic signal, whose real part is the actual transmission.
Note that the sum of the two sideband signals is
which is the classic model of suppressed-carrier double sideband AM.
amplitude-modulated onto a carrier wave at 
can be expressed as simple multiplication of two cosine waves: 
, where 
. Applying a simple trigonometric identity, we can change the above expression to be 
. Each cosine term in the equation is known as a sideband
.
One method of producing an SSB signal is to remove one of the sidebands via filter
ing, leaving only either the upper sideband (USB), the sideband with the higher frequency, or less commonly the lower sideband (LSB), the sideband with the lower frequency. Most often, the carrier is reduced or removed entirely (suppressed), being referred to in full as single sideband suppressed carrier (SSBSC). Assuming both sidebands are symmetric, which is the case for a normal AM
signal, no information is lost in the process. Since the final RF amplification is now concentrated in a single sideband, the effective power output is greater than in normal AM (the carrier and redundant sideband account for well over half of the power output of an AM transmitter). Though SSB uses substantially less bandwidth and power, it cannot be demodulated by a simple envelope detector
like standard AM.
, uses phasing
to suppress the unwanted sideband. To generate an SSB signal with this method, two versions of the original signal are generated, mutually 90° out of phase for any single frequency within the operating bandwidth. Each one of these signals then modulates carrier waves (of one frequency) that are also 90° out of phase with each other. By either adding or subtracting the resulting signals, a lower or upper sideband signal results. A benefit of this approach is to allow an analytical expression for SSB signals, which can be used to understand effects such as synchronous detection of SSB.
Shifting the baseband signal 90° out of phase cannot be done simply by delaying it, as it contains a large range of frequencies. In analog circuits, a wideband 90-degree phase-difference network is used. The method was popular in the days of vacuum-tube radios, but later gained a bad reputation due to poorly adjusted commercial implementations. Modulation using this method is again gaining popularity in the homebrew
and DSP
fields. This method, utilizing the Hilbert transform
to phase shift the baseband audio, can be done at low cost with digital circuitry.
In Weaver's method, the band of interest is first translated to be centered at zero, conceptually by modulating a complex exponential
with frequency in the middle of the voiceband, but implemented by a quadrature pair of sine and cosine modulators at that frequency (e.g. 2 kHz). This complex signal or pair of real signals is then lowpass filtered to remove the undesired sideband that is not centered at zero. Then, the single-sideband complex signal centered at zero is upconverted to a real signal, by another pair of quadrature mixers, to the desired center frequency.
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.
To recover the original signal from the IF SSB signal, the single sideband must be frequency-shifted down to its original range of baseband
frequencies, by using a product detector
which mixes it with the output of a beat frequency oscillator (BFO). In other words, it is just another stage of heterodyning.(mixing down to base band). For this to work, the BFO frequency must be exactly adjusted.
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.
For audio communications, there is a common agreement about the BFO oscillator shift of 1.7kHz. A voice signal is sensitive to about 50Hz shift, with up to 100Hz still bearable. Some receivers use a carrier recovery
system, which attempts to automatically lock on to the exact IF frequency. The carrier recovery doesn't solve the frequency shift. It gives better S/N ratio on the detector output.
As an example, consider an IF SSB signal centered at frequency
= 45000 Hz. The baseband frequency it needs to be shifted to is 
= 2000 Hz. The BFO output waveform is 
. When the signal is multiplied by (aka 'heterodyne
d with') the BFO waveform, it shifts the signal to
and to 
, which is known as the beat frequency or image frequency. The objective is to choose an 
that results in 
= 2000 Hz. (The unwanted components at 
can be removed by a lowpass filter (for which an output transducer or the human ear
may serve)).
Note that there are two choices for
: 43000 Hz and 47000 Hz, a.k.a. low-side and high-side injection. With high-side injection, the spectral components that were distributed around 45000 Hz will be distributed around 2000 Hz in the reverse order, also known as an inverted spectrum
. That is in fact desirable when the IF spectrum is also inverted, because the BFO inversion restores the proper relationships. One reason for that is when the IF spectrum is the output of an inverting stage in the receiver. Another reason is when the SSB signal is actually a lower sideband, instead of an upper sideband. But if both reasons are true, then the IF spectrum in not inverted, and the non-inverting BFO (43000 Hz) should be used.
If
is off by a small amount, then the beat frequency is not exactly 
, which can lead to the speech distortion mentioned earlier.
s.
These effects were used, in conjunction with other filtering techniques, during World War II
as a simple method for speech encryption
. Radiotelephone
conversation
s between the US and Britain
were intercepted and "decrypted" by the Germans; they included some early conversations between Franklin D. Roosevelt
and Churchill
. In fact, the signals could be understood directly by trained operators. Largely to allow secure communications between Roosevelt and Churchill, the SIGSALY
system of digital encryption was devised.
Today, such simple inversion-based speech encryption
techniques are easily decrypted using simple techniques and are no longer regarded as secure.
communication) is a sideband
that has been only partly cut off or suppressed. Television broadcasts (in analog video formats) use this method if the video
is transmitted
in AM
, due to the large bandwidth used. It may also be used in digital transmission, such as the ATSC standardized 8-VSB. The Milgo 4400/48 modem
(circa 1967) used vestigial sideband and phase-shift keying
to provide 4800-bit/s transmission over a 1600 Hz channel.
The video baseband signal used in TV in countries that use NTSC or ATSC has a bandwidth of 6 MHz. To conserve bandwidth, SSB would be desirable, but the video signal has significant low frequency content (average brightness) and has rectangular synchronising pulses. The engineering compromise is vestigial sideband modulation. In vestigial sideband the full upper sideband of bandwidth W2 = 4 MHz is transmitted, but only W1 = 1.25 MHz of the lower sideband is transmitted, along with a carrier. This effectively makes the system AM at low modulation frequencies and SSB at high modulation frequencies. The absence of the lower sideband components at high frequencies must be compensated for, and this is done by the RF and IF filters.
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
that more efficiently uses electrical power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
and bandwidth.
Amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
produces a modulated output signal that has twice the bandwidth of the original baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
signal. Single-sideband modulation avoids this bandwidth doubling, and the power wasted on a carrier, at the cost of somewhat increased device complexity.
History
The first U.S. patent for SSB modulation was applied for on December 1, 1915 by John Renshaw CarsonJohn Renshaw Carson
John Renshaw Carson , who published as J. R. Carson, was a noted transmission theorist for early communications systems...
. The U.S. Navy experimented with SSB over its radio circuits before World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
. SSB first entered commercial service on January 7, 1927 on the longwave
Longwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
transatlantic public radiotelephone circuit between New York and London. The high power SSB transmitters were located at Rocky Point, New York
Rocky Point, New York
Rocky Point is a hamlet in Suffolk County, New York on the North Shore of Long Island. As of the United States 2000 Census, the CDP population was 10,185.Rocky Point is a community in the town of Brookhaven.-Geography:...
and Rugby, England. The receivers were in very quiet locations in Houlton, Maine
Houlton, Maine
Houlton is a town in Aroostook County, Maine, on the United States – Canada border, located at . As of the 2010 census, the town population was 6,123. It is perhaps best known as being at the northern terminus of Interstate 95 and for being the birthplace of Samantha Smith...
and Cupar
Cupar
Cupar is a town and former royal burgh in Fife, Scotland. The town is situated between Dundee and the New Town of Glenrothes.According to a recent population estimate , Cupar had a population around 8,980 making the town the ninth largest settlement in Fife.-History:The town is believed to have...
Scotland.
SSB was also used over long distance telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
s, as part of a technique known as frequency-division multiplexing
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
(FDM). FDM was pioneered by telephone companies in the 1930s. This enabled many voice channels to be sent down a single physical circuit, for example in L-carrier
L-carrier
SystemYearFrequencyCoax per cableDistance between repeatersVoice circuits per coax tubeL-119413 MHz48 miles600L-21942840 kHz416 miles360L-319508 MHz84 miles1,860L-4196717 MHz202 miles3,600L-5197257 MHz221 mile10,800L-5E197566 MHz221 mile13,200...
. SSB allowed channels to be spaced (usually) just 4,000 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
apart, while offering a speech bandwidth of nominally 300–3,400 Hz.
Amateur radio operator
Amateur radio operator
An amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...
s began serious experimentation with SSB after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. The Strategic Air Command
Strategic Air Command
The Strategic Air Command was both a Major Command of the United States Air Force and a "specified command" of the United States Department of Defense. SAC was the operational establishment in charge of America's land-based strategic bomber aircraft and land-based intercontinental ballistic...
established SSB as the radio standard for its aircraft in 1957. It has become a de facto standard for long-distance voice radio transmissions since then.
Mathematical formulation
SSB and vestigial side band (VSB) can also be regarded mathematically as special cases of analog quadrature amplitude modulationQuadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
.
Let
be the basebandBaseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
waveform to be transmitted. Its Fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
,
, is Hermitian symmetrical about the axis, because is real-valuedReal number
In mathematics, a real number is a value that represents a quantity along a continuum, such as -5 , 4/3 , 8.6 , √2 and π...
. Double sideband modulation of
to a radio transmission frequency, , moves the axis of symmetry to , and the two sides of each axis are called sidebands.Let
represent the Hilbert transformHilbert transform
In mathematics and in signal processing, the Hilbert transform is a linear operator which takes a function, u, and produces a function, H, with the same domain. The Hilbert transform is named after David Hilbert, who first introduced the operator in order to solve a special case of the...
of
. Thenis a useful mathematical concept, called an analytic signal
Analytic signal
In mathematics and signal processing, the analytic representation of a real-valued function or signal facilitates many mathematical manipulations of the signal. The basic idea is that the negative frequency components of the Fourier transform of a real-valued function are superfluous, due to the...
. The Fourier transform of
equals , for , but it has no negative-frequencyNegative frequency
The concept of negative and positive frequency can be as simple as a wheel rotating one way or the other way. A signed value of frequency indicates both the rate and direction of rotation...
components. So it can be modulated to a radio frequency and produce just a single sideband.
The analytic representation of
is: (the equality is Euler's formulaEuler's formula
Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the deep relationship between the trigonometric functions and the complex exponential function...
)
whose Fourier transform is
.When
is modulated (i.e. multiplied) by , all frequency components are shifted by , so there are still no negative-frequency components. Therefore, the complex product is an analytic representation of the single sideband signal:where
is the real-valued, single sideband waveform. Therefore: |
 |
 |
|
 |
The presence of two out-of-phase (quadrature) carrier waves is now evident.
Lower sideband
represents the baseband signal's upper sideband, . It is also possible, and useful, to convey the baseband information using its lower sideband, , which is a mirror image about f=0 Hz. By a general property of the Fourier transform, that symmetry means it is the complex conjugate of :Note that:
The gain of 2 is a result of defining the analytic signal (one sideband) to have the same total energy as
(both sidebands).As before, the signal is modulated by
. The typical is large enough that the translated lower sideband (LSB) has no negative-frequency components. Then the result is another analytic signal, whose real part is the actual transmission. |
 |
 |
Note that the sum of the two sideband signals is
which is the classic model of suppressed-carrier double sideband AM.
Bandpass filtering
A signal at frequency amplitude-modulated onto a carrier wave at can be expressed as simple multiplication of two cosine waves: , where . Applying a simple trigonometric identity, we can change the above expression to be . Each cosine term in the equation is known as a sidebandSideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
.
One method of producing an SSB signal is to remove one of the sidebands via filter
Filter (signal processing)
In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal...
ing, leaving only either the upper sideband (USB), the sideband with the higher frequency, or less commonly the lower sideband (LSB), the sideband with the lower frequency. Most often, the carrier is reduced or removed entirely (suppressed), being referred to in full as single sideband suppressed carrier (SSBSC). Assuming both sidebands are symmetric, which is the case for a normal AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
signal, no information is lost in the process. Since the final RF amplification is now concentrated in a single sideband, the effective power output is greater than in normal AM (the carrier and redundant sideband account for well over half of the power output of an AM transmitter). Though SSB uses substantially less bandwidth and power, it cannot be demodulated by a simple envelope detector
Envelope detector
An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls...
like standard AM.
Hartley modulator
An alternate method of generation known as a Hartley modulator, named after R. V. L. HartleyRalph Hartley
Ralph Vinton Lyon Hartley was an electronics researcher. He invented the Hartley oscillator and the Hartley transform, and contributed to the foundations of information theory.-Biography:...
, uses phasing
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
to suppress the unwanted sideband. To generate an SSB signal with this method, two versions of the original signal are generated, mutually 90° out of phase for any single frequency within the operating bandwidth. Each one of these signals then modulates carrier waves (of one frequency) that are also 90° out of phase with each other. By either adding or subtracting the resulting signals, a lower or upper sideband signal results. A benefit of this approach is to allow an analytical expression for SSB signals, which can be used to understand effects such as synchronous detection of SSB.
Shifting the baseband signal 90° out of phase cannot be done simply by delaying it, as it contains a large range of frequencies. In analog circuits, a wideband 90-degree phase-difference network is used. The method was popular in the days of vacuum-tube radios, but later gained a bad reputation due to poorly adjusted commercial implementations. Modulation using this method is again gaining popularity in the homebrew
Amateur radio homebrew
-History:In the early years of amateur radio, long before factory-built gear was easily available, most hams built their own transmitting and receiving equipment, a process that came to be known as "homebrewing." In the 1930s, 40s, and 50s, hams handcrafted reasonable-quality vacuum tube-based...
and DSP
Digital signal processor
A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
fields. This method, utilizing the Hilbert transform
Hilbert transform
In mathematics and in signal processing, the Hilbert transform is a linear operator which takes a function, u, and produces a function, H, with the same domain. The Hilbert transform is named after David Hilbert, who first introduced the operator in order to solve a special case of the...
to phase shift the baseband audio, can be done at low cost with digital circuitry.
Weaver modulator
Another variation, the Weaver modulator, 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
with frequency in the middle of the voiceband, but implemented by a quadrature pair of sine and cosine modulators at that frequency (e.g. 2 kHz). This complex signal or pair of real signals is then lowpass filtered to remove the undesired sideband that is not centered at zero. Then, the single-sideband complex signal centered at zero is upconverted to a real signal, by another pair of quadrature mixers, to the desired center frequency.Demodulation
The front end of an SSB receiver is similar to that of an AMAmplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
or FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
receiver, consisting of a superheterodyne RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
front end that produces a frequency-shifted version of the radio frequency (RF) signal within a standard intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF) band.
To recover the original signal from the IF SSB signal, the single sideband must be frequency-shifted down to its original range of baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
frequencies, by using a product detector
Product detector
A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name...
which mixes it with the output of a beat frequency oscillator (BFO). In other words, it is just another stage of heterodyning.(mixing down to base band). For this to work, the BFO frequency must be exactly adjusted.
If the BFO frequency is off, the output signal will be frequency-shifted (up or down), making speech sound strange and "Donald Duck
Donald Duck
Donald Fauntleroy Duck is a cartoon character created in 1934 at Walt Disney Productions and licensed by The Walt Disney Company. Donald is an anthropomorphic white duck with a yellow-orange bill, legs, and feet. He typically wears a sailor suit with a cap and a black or red bow tie. Donald is most...
"-like, or unintelligible.
For audio communications, there is a common agreement about the BFO oscillator shift of 1.7kHz. A voice signal is sensitive to about 50Hz shift, with up to 100Hz still bearable. Some receivers use a carrier recovery
Carrier recovery
A carrier recovery system is a circuit used to estimate and compensate for frequency and phase differences between a received signal's carrier wave and the receiver's local oscillator for the purpose of coherent demodulation....
system, which attempts to automatically lock on to the exact IF frequency. The carrier recovery doesn't solve the frequency shift. It gives better S/N ratio on the detector output.
As an example, consider an IF SSB signal centered at frequency
= 45000 Hz. The baseband frequency it needs to be shifted to is = 2000 Hz. The BFO output waveform is . When the signal is multiplied by (aka 'heterodyneHeterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
d with') the BFO waveform, it shifts the signal to
and to , which is known as the beat frequency or image frequency. The objective is to choose an that results in = 2000 Hz. (The unwanted components at can be removed by a lowpass filter (for which an output transducer or the human earEar
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
may serve)).
Note that there are two choices for
: 43000 Hz and 47000 Hz, a.k.a. low-side and high-side injection. With high-side injection, the spectral components that were distributed around 45000 Hz will be distributed around 2000 Hz in the reverse order, also known as an inverted spectrumInverted spectrum
Inverted spectrum is the apparent possibility of two people sharing their color vocabulary and discriminations, although the colours one sees — their qualia — are systematically different from the colours the other person sees....
. That is in fact desirable when the IF spectrum is also inverted, because the BFO inversion restores the proper relationships. One reason for that is when the IF spectrum is the output of an inverting stage in the receiver. Another reason is when the SSB signal is actually a lower sideband, instead of an upper sideband. But if both reasons are true, then the IF spectrum in not inverted, and the non-inverting BFO (43000 Hz) should be used.
If
is off by a small amount, then the beat frequency is not exactly , which can lead to the speech distortion mentioned earlier.SSB as a speech-scrambling technique
SSB techniques can also be adapted to frequency-shift and frequency-invert baseband waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s.
These effects were used, in conjunction with other filtering techniques, during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
as a simple method for speech encryption
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
. Radiotelephone
Radiotelephone
A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the...
conversation
Conversation
Conversation is a form of interactive, spontaneous communication between two or more people who are following rules of etiquette.Conversation analysis is a branch of sociology which studies the structure and organization of human interaction, with a more specific focus on conversational...
s between the US and Britain
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
were intercepted and "decrypted" by the Germans; they included some early conversations between Franklin D. Roosevelt
Franklin D. Roosevelt
Franklin Delano Roosevelt , also known by his initials, FDR, was the 32nd President of the United States and a central figure in world events during the mid-20th century, leading the United States during a time of worldwide economic crisis and world war...
and Churchill
Winston Churchill
Sir Winston Leonard Spencer-Churchill, was a predominantly Conservative British politician and statesman known for his leadership of the United Kingdom during the Second World War. He is widely regarded as one of the greatest wartime leaders of the century and served as Prime Minister twice...
. In fact, the signals could be understood directly by trained operators. Largely to allow secure communications between Roosevelt and Churchill, the SIGSALY
SIGSALY
In cryptography, SIGSALY was a secure speech system used in World War II for the highest-level Allied communications....
system of digital encryption was devised.
Today, such simple inversion-based speech encryption
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
techniques are easily decrypted using simple techniques and are no longer regarded as secure.
Vestigial sideband (VSB)
A vestigial sideband (in radioRadio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
communication) is a sideband
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
that has been only partly cut off or suppressed. Television broadcasts (in analog video formats) use this method if the video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
is transmitted
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
in AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
, due to the large bandwidth used. It may also be used in digital transmission, such as the ATSC standardized 8-VSB. The Milgo 4400/48 modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
(circa 1967) used vestigial sideband and phase-shift keying
Phase-shift keying
Phase-shift keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal ....
to provide 4800-bit/s transmission over a 1600 Hz channel.
The video baseband signal used in TV in countries that use NTSC or ATSC has a bandwidth of 6 MHz. To conserve bandwidth, SSB would be desirable, but the video signal has significant low frequency content (average brightness) and has rectangular synchronising pulses. The engineering compromise is vestigial sideband modulation. In vestigial sideband the full upper sideband of bandwidth W2 = 4 MHz is transmitted, but only W1 = 1.25 MHz of the lower sideband is transmitted, along with a carrier. This effectively makes the system AM at low modulation frequencies and SSB at high modulation frequencies. The absence of the lower sideband components at high frequencies must be compensated for, and this is done by the RF and IF filters.
See also
- ACSSB, amplitude-companded single sideband
- Independent sidebandIndependent sidebandIndependent sideband 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, the other on the lower sideband...
- ModulationModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
for other examples of modulation techniques - SidebandSidebandIn radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...
for more general information about a sideband
Further reading
- Sgrignoli, G., W. Bretl, R. and Citta. (1995). "VSB modulation used for terrestrial and cable broadcasts." IEEE Transactions on Consumer Electronics. v. 41, issue 3, p. 367 - 382.
- J. Brittain, (1992). "Scanning the past: Ralph V.L. Hartley", Proc. IEEE, vol.80,p. 463.