Balanced line
Encyclopedia
In telecommunication
s and professional audio
, a balanced line or balanced signal pair is a transmission line
consisting of two conductors of the same type, each of which have equal impedances
along their lengths and equal impedances to ground
and to other circuits. The chief advantage of the balanced line format is good rejection of external noise. Common forms of balanced line are twin-lead
, used for radio frequency signals and twisted pair
, used for lower frequencies. They are to be contrasted to unbalanced line
s, such as coaxial cable
, which is designed to have its return conductor connected to ground
, or circuits whose return conductor actually is ground. Balanced and unbalanced circuits can be interconnected using a transformer
called a balun
.
Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by differential signaling
, transformer
coupling or by merely balancing the impedance in each conductor.
Lines carrying symmetrical signals (those with equal but opposite voltages to ground on each leg) are often referred to as balanced, but this is an entirely different meaning. The two conditions are not related.
only a common mode signal
on the line and the balanced impedance
s to ground minimizes differential pickup due to stray electric fields. The conductors are sometimes twisted together
to ensure that each conductor is equally exposed to any external magnetic fields that could induce unwanted noise.
Some balanced lines also have electromagnetic shielding
to reduce the amount of noise introduced.
A balanced line allows a differential receiver to reduce the noise
on a connection by rejecting common-mode interference
. The lines have the same impedance
to ground, so the interfering fields or currents induce the same voltage in both wires. Since the receiver responds only to the difference between the wires, it is not influenced by the induced noise voltage. If twisted pair becomes unbalanced, for example due to insulation failure, noise will be induced. Examples of twisted pairs include Cat-3
Ethernet
cables or telephone wires.
Compared to unbalanced circuits, balanced lines reduce the amount of noise per distance, allowing a longer cable run to be practical. This is because electromagnetic interference will affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other.
, however, this did not become widespread until repeater amplifiers became available. For an unamplified telephone line, a twisted pair cable could only manage a maximum distance of 30 km. Open wires, on the other hand, with their lower capacitance had been used for enormous distances - the longest was the 1500 km from New York to Chicago built in 1893. Loading coils were used to improve the distance achievable with cable but the problem was not finally overcome until amplifiers started to be installed in 1912. Twisted pair balanced lines are still widely used for the telephone subscribers local end.
Telephone trunk lines, and especially frequency division multiplexing carrier systems, are usually 4-wire circuits rather than 2-wire circuits (or at least they were before fibre-optic became widespread) and require a different kind of cable. This format requires the conductors to be arranged in two pairs, one pair for the sending (go) signal and the other for the return signal. The greatest source of interference on this kind of transmission is usually the crosstalk between the go and return circuits themselves. The most common cable format is star quad, where the diagonally opposite conductors form the pairs. This geometry gives maximum common mode rejection between the two pairs. An alternative format is DM quad which consists of two twisted pairs with the twisting at different pitches.
s to a mixer in professional systems. Classically, both dynamic and condenser microphones used transformer
s to provide a differential-mode signal. While transformers are still used in the large majority of modern dynamic microphones, more recent condenser microphones are more likely to use electronic drive circuitry. Each leg, irrespective of any signal, should have an identical impedance to ground. Pair cable (or a pair-derivative such as star quad) is used to maintain the balanced impedances and close twisting of the cores ensures that any interference is common to both conductors. Providing that the receiving end (usually a mixing console
) does not disturb the line balance, and is able to ignore common-mode (noise) signals, and can extract differential ones, then the system will have excellent immunity to induced interference.
Typical professional audio sources, such as microphones, have three-pin XLR connector
s. One is the shield or chassis ground, while the other two are signal connections. These signal wires carry two copies of the same signal, but with opposite polarity. (They are often termed "hot" and "cold," and the AES14-1992(r2004) Standard [and EIA Standard RS-297-A] suggest that the pin that carries the positive signal that results from a positive air pressure on a transducer will be deemed 'hot'. Pin 2 has been designated as the 'hot' pin, and that designation serves useful for keeping a consistent polarity in the rest of the system.) Since these conductors travel the same path from source to destination, the assumption is that any interference is induced upon both conductors equally. The appliance receiving the signals compares the difference between the two signals (often with disregard to electrical ground) allowing the appliance to ignore any induced electrical noise. Any induced noise would be present in equal amounts and in identical polarity on each of the balanced signal conductors, so the two signals’ difference from each other would be unchanged. The successful rejection of induced noise from the desired signal depends in part on the balanced signal conductors receiving the same amount and type of interference. This typically leads to twisted, braided, or co-jacketed cables for use in balanced signal transmission.
between "legs" but do not provide symmetrical signals. Symmetrical differential signals exist to prevent interference with other circuits - the electromagnetic fields are canceled out by the equal and opposite currents. But they are not necessary for interference rejection from other circuits.
. For example, baluns can be used to send line level
audio or E-carrier level 1 signals over coaxial cable (which is unbalanced) through 300 feet (91.4 m) of Category 5 cable
by using a pair of baluns at each end of the CAT5 run. The balun takes the unbalanced signal, and creates an inverted copy of that signal. It then sends these 2 signals across the CAT5 cable as a balanced signal. Upon reception at the other end, the balun takes the difference of the two signals, thus removing any noise picked up along the way and recreating the unbalanced signal.
A once common application of a radio frequency
balun was found at the antenna terminals of a television
receiver. Typically a 300-ohm balanced twin lead antenna input could only be connected to a coaxial cable from a cable TV system through a balun.
of a transmission line is an important parameter at higher frequencies of operation. For a parallel 2-wire transmission line,
where
is half the distance between the wire centres, 
is the wire radius and 
, 
are respectively the permeability
and permittivity
of the surrounding medium. A commonly used approximation that is valid when the wire separation is much larger than the wire radius and in the absence of magnetic materials is
where
is the relative permittivity of the surrounding medium.
, the three conductors used for three-phase power
transmission are referred to as a balanced line since the instantaneous sum of the three line voltages is nominally zero. However, balance in this field is referring to the symmetry of the source and load: it has nothing to do with the impedance balance of the line itself, the sense of the meaning in telecommunications.
For the transmission of single-phase electric power
as used for railway electrification system
s, two conductors are used to carry in-phase and out-of-phase voltages such that the line is balanced.
Bipolar HVDC lines at which each pole is operated with the same voltage toward ground are also balanced lines.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s and professional audio
Professional audio
Professional audio, also 'pro audio', refers to both an activity and a type of audio equipment. Typically it encompasses the production or reproduction of sound for an audience, by individuals who do such work as an occupation like live event support, using sound reinforcement systems designed for...
, a balanced line or balanced signal pair is a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
consisting of two conductors of the same type, each of which have equal impedances
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
along their lengths and equal impedances to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
and to other circuits. The chief advantage of the balanced line format is good rejection of external noise. Common forms of balanced line are twin-lead
Twin-lead
Twin-lead cable is a two-conductor ribbon cable used as a transmission line to carry radio frequency signals.- Characteristics and uses:...
, used for radio frequency signals and twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
, used for lower frequencies. They are to be contrasted to unbalanced line
Unbalanced line
In Electrical engineering, an unbalanced line is a transmission line, usually coaxial cable, whose conductors have unequal impedances with respect to ground; as opposed to a balanced line.Microstrip and single-wire lines are also unbalanced lines....
s, such as coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, which is designed to have its return conductor connected to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
, or circuits whose return conductor actually is ground. Balanced and unbalanced circuits can be interconnected using a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
called a balun
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
.
Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by differential signaling
Differential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
, transformer
Repeating coil
In telecommunications, a repeating coil is a voice-frequency transformer characterized by a closed magnetic core, a pair of identical balanced primary windings, a pair of identical but not necessarily balanced secondary windings, and low transmission loss at voice frequencies...
coupling or by merely balancing the impedance in each conductor.
Lines carrying symmetrical signals (those with equal but opposite voltages to ground on each leg) are often referred to as balanced, but this is an entirely different meaning. The two conditions are not related.
Explanation
Transmission of a signal over a balanced line reduces the influence of noise or interference due to external stray electric fields. Any external signal sources tend to induceFaraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
only a common mode signal
Common-mode interference
In telecommunication, the term common-mode interference has the following meanings:#Interference that appears on both signal leads , or the terminals of a measuring circuit, and ground....
on the line and the balanced impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
s to ground minimizes differential pickup due to stray electric fields. The conductors are sometimes twisted together
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
to ensure that each conductor is equally exposed to any external magnetic fields that could induce unwanted noise.
Some balanced lines also have electromagnetic shielding
Electromagnetic shielding
Electromagnetic shielding is the process of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive and/or magnetic materials. Shielding is typically applied to enclosures to isolate electrical devices from the 'outside world' and to cables to isolate...
to reduce the amount of noise introduced.
A balanced line allows a differential receiver to reduce the noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
on a connection by rejecting common-mode interference
Common-mode interference
In telecommunication, the term common-mode interference has the following meanings:#Interference that appears on both signal leads , or the terminals of a measuring circuit, and ground....
. The lines have the same impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
to ground, so the interfering fields or currents induce the same voltage in both wires. Since the receiver responds only to the difference between the wires, it is not influenced by the induced noise voltage. If twisted pair becomes unbalanced, for example due to insulation failure, noise will be induced. Examples of twisted pairs include Cat-3
Category 3 cable
Category 3 cable, commonly known as Cat 3 or station wire, is an unshielded twisted pair cable designed to reliably carry data up to 10 Mbit/s, with a possible bandwidth of 16 MHz...
Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
cables or telephone wires.
Compared to unbalanced circuits, balanced lines reduce the amount of noise per distance, allowing a longer cable run to be practical. This is because electromagnetic interference will affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other.
Telephone systems
The first application for balanced lines was for telephone lines. Interference that was of little consequence on a telegraph system (which is in essence digital) could be very disturbing for a telephone user. The initial format was to take two single-wire unbalanced telegraph lines and use them as a pair. This proved insufficient, however, with the growth of electric power transmission which tended to use the same routes. A telephone line running alongside a power line for many miles will inevitably have more interference induced in one leg than the other since one of them will be nearer to the power line. This issue was addressed by swapping the positions of the two legs every few hundred yards with a cross-over, thus ensuring that both legs had equal interference induced and allowing common-mode rejection to do its work. As the telephone system grew, it became preferable to use cable rather than open wires to save space, and also to avoid poor performance during bad weather. The cable construction used for balanced telephone cables was twisted pairTwisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
, however, this did not become widespread until repeater amplifiers became available. For an unamplified telephone line, a twisted pair cable could only manage a maximum distance of 30 km. Open wires, on the other hand, with their lower capacitance had been used for enormous distances - the longest was the 1500 km from New York to Chicago built in 1893. Loading coils were used to improve the distance achievable with cable but the problem was not finally overcome until amplifiers started to be installed in 1912. Twisted pair balanced lines are still widely used for the telephone subscribers local end.
Telephone trunk lines, and especially frequency division multiplexing carrier systems, are usually 4-wire circuits rather than 2-wire circuits (or at least they were before fibre-optic became widespread) and require a different kind of cable. This format requires the conductors to be arranged in two pairs, one pair for the sending (go) signal and the other for the return signal. The greatest source of interference on this kind of transmission is usually the crosstalk between the go and return circuits themselves. The most common cable format is star quad, where the diagonally opposite conductors form the pairs. This geometry gives maximum common mode rejection between the two pairs. An alternative format is DM quad which consists of two twisted pairs with the twisting at different pitches.
Audio systems
An example of balanced lines is the connection of microphoneMicrophone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s to a mixer in professional systems. Classically, both dynamic and condenser microphones used transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s to provide a differential-mode signal. While transformers are still used in the large majority of modern dynamic microphones, more recent condenser microphones are more likely to use electronic drive circuitry. Each leg, irrespective of any signal, should have an identical impedance to ground. Pair cable (or a pair-derivative such as star quad) is used to maintain the balanced impedances and close twisting of the cores ensures that any interference is common to both conductors. Providing that the receiving end (usually a mixing console
Mixing console
In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...
) does not disturb the line balance, and is able to ignore common-mode (noise) signals, and can extract differential ones, then the system will have excellent immunity to induced interference.
Typical professional audio sources, such as microphones, have three-pin XLR connector
XLR connector
The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...
s. One is the shield or chassis ground, while the other two are signal connections. These signal wires carry two copies of the same signal, but with opposite polarity. (They are often termed "hot" and "cold," and the AES14-1992(r2004) Standard [and EIA Standard RS-297-A] suggest that the pin that carries the positive signal that results from a positive air pressure on a transducer will be deemed 'hot'. Pin 2 has been designated as the 'hot' pin, and that designation serves useful for keeping a consistent polarity in the rest of the system.) Since these conductors travel the same path from source to destination, the assumption is that any interference is induced upon both conductors equally. The appliance receiving the signals compares the difference between the two signals (often with disregard to electrical ground) allowing the appliance to ignore any induced electrical noise. Any induced noise would be present in equal amounts and in identical polarity on each of the balanced signal conductors, so the two signals’ difference from each other would be unchanged. The successful rejection of induced noise from the desired signal depends in part on the balanced signal conductors receiving the same amount and type of interference. This typically leads to twisted, braided, or co-jacketed cables for use in balanced signal transmission.
Balanced and differential
Most explanations of balanced lines assume symmetrical (antiphase) signals but this is an unfortunate confusion - signal symmetry and balanced lines are quite independent of each other. Essential in a balanced line is matched impedances in the driver, line and receiver. These conditions assure that external noise affects each leg of the differential line equally and thus appears as a common mode signal that is removed by the receiver. There are balanced drive circuits that have excellent common-mode impedance matchingImpedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
between "legs" but do not provide symmetrical signals. Symmetrical differential signals exist to prevent interference with other circuits - the electromagnetic fields are canceled out by the equal and opposite currents. But they are not necessary for interference rejection from other circuits.
Baluns
To convert a signal from balanced to unbalanced requires a balunBalun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
. For example, baluns can be used to send line level
Line level
Line level is a term used to denote the strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players....
audio or E-carrier level 1 signals over coaxial cable (which is unbalanced) through 300 feet (91.4 m) of Category 5 cable
Category 5 cable
Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
by using a pair of baluns at each end of the CAT5 run. The balun takes the unbalanced signal, and creates an inverted copy of that signal. It then sends these 2 signals across the CAT5 cable as a balanced signal. Upon reception at the other end, the balun takes the difference of the two signals, thus removing any noise picked up along the way and recreating the unbalanced signal.
A once common application of a radio frequency
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...
balun was found at the antenna terminals of a television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
receiver. Typically a 300-ohm balanced twin lead antenna input could only be connected to a coaxial cable from a cable TV system through a balun.
Characteristic Impedance
The characteristic impedanceCharacteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
of a transmission line is an important parameter at higher frequencies of operation. For a parallel 2-wire transmission line,where
is half the distance between the wire centres, is the wire radius and , are respectively the permeabilityPermeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
and permittivity
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
of the surrounding medium. A commonly used approximation that is valid when the wire separation is much larger than the wire radius and in the absence of magnetic materials is
where
is the relative permittivity of the surrounding medium.Electric power lines
In electric power transmissionElectric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
, the three conductors used for three-phase power
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
transmission are referred to as a balanced line since the instantaneous sum of the three line voltages is nominally zero. However, balance in this field is referring to the symmetry of the source and load: it has nothing to do with the impedance balance of the line itself, the sense of the meaning in telecommunications.
For the transmission of single-phase electric power
Single-phase electric power
In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors...
as used for railway electrification system
Railway electrification system
A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...
s, two conductors are used to carry in-phase and out-of-phase voltages such that the line is balanced.
Bipolar HVDC lines at which each pole is operated with the same voltage toward ground are also balanced lines.
See also
- Differential pairDifferential pairA differential pair is a pair of conductors used for differential signaling. Differential pairs are usually found on a printed circuit board, in cables , and in connectors...
- Insert (effects processing)Insert (effects processing)In audio processing and sound reinforcement, an insert is an access point built into the mixing console, allowing the user to add external line level devices into the signal flow between the microphone preamplifier and the mix bus....
- Ladder line (balanced transmission line for radio frequencies)
Balanced transmission standards
- RS-422EIA-422RS-422 is a technical standard that specifies electrical characteristics of a digital signalling circuit. Differential-mode signals can be sent at rates as high as 10 million bits per second, or may be sent on cables as long as 1200 metres. Some systems directly interconnect using RS 422 signals,...
- RS-485
- Low-voltage differential signalling (LVDS)
External links
- Balanced Lines, Phantom Powering, Grounding, and Other Arcane Mysteries — from MackieMackieMackie is a brand of the United States-based company LOUD Technologies. The Mackie brand is used on professional music and recording equipment, such as mixing consoles, loudspeakers, and DAW control surfaces....
;