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Balanced line
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Balanced line
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A signal transmitted over a balanced line. The signal is kept intact while the noise (which appears as a common-mode signal at the receiving end) is rejected perfectly.

In telecommunications and professional audio, a balanced line or balanced signal pair is an electrical circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths, to ground, and to other circuits.[1] The primary advantage of the balanced line format is good rejection of common-mode noise and interference when fed to a differential device such as a transformer or differential amplifier.[2]

As prevalent in sound recording and reproduction, balanced lines are referred to as balanced audio.

A common form of balanced line is twin-lead, used for radio frequency communications. Also common is twisted pair, used for traditional telephone, professional audio, or for data communications. They are to be contrasted to unbalanced lines, such as coaxial cable, which is designed to have its return conductor connected to ground, or circuits whose return conductor actually is ground (see earth-return telegraph). Balanced and unbalanced circuits can be interfaced using a device called a balun.

Circuits driving balanced lines must themselves be balanced to maintain the benefits of balance. This may be achieved by transformer coupling (repeating coils) or by merely balancing the impedance in each conductor.

Lines carrying symmetric signals (those with equal amplitudes but opposite polarities on each leg) are often incorrectly referred to as "balanced", but this is actually differential signalling. Balanced lines and differential signalling are often used together, but they are not the same thing. Differential signalling does not make a line balanced, nor does noise rejection in balanced cables require differential signalling.

Explanation

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Fig. 1. Balanced line in twisted pair format. This line is intended for use with 2-wire circuits.
Fig. 2. Balanced line in star quad format. This line is intended for use with 4-wire circuits or two 2-wire circuits. It is also used with microphone signals in professional audio.
Fig. 3. Balanced line in DM quad format. This line is intended for use with 4-wire circuits or two 2-wire circuits.
Fig. 4. Balanced line in twin lead format. This line is intended for use with RF circuits, particularly aerials.

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 induce only a common-mode signal on the line, and the balanced impedances 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 electrostatic shielding to reduce the amount of noise introduced. The cable is often wrapped in foil, copper wire, or a copper braid. This shield provides immunity to RF interference but does not provide immunity to magnetic fields.

Some balanced lines use 4-conductor star quad cable to provide immunity to magnetic fields. The geometry of the cable ensures that magnetic fields will cause equal interference of both legs of the balanced circuit. This balanced interference is a common-mode signal that can easily be removed by a transformer or balanced differential receiver.[3][4][5][6][7]

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 a balanced line is used in an unbalanced circuit, with different impedances from each conductor to ground, currents induced in the separate conductors will cause different voltage drops to ground, thus creating a voltage differential, making the line more susceptible to noise. Examples of twisted pairs include category 5 cable.

Compared to unbalanced lines, 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

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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 pair; 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.[8]: 323  Twisted pair balanced lines are still widely used for local loops, the lines that connect each subscriber's premises to their respective exchange.[8]: 314–316 

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 (Dieselhorst-Martin) quad which consists of two twisted pairs with the twisting at different pitches.[8]: 320 

Audio systems

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Main article: Balanced audio
Fig. 5. Microphones connected to star quad cable join together diametrically opposite conductors to maintain balance. This is different from the usage on 4-wire circuits. The colours in this diagram correspond with the colouring in figure 2.

An example of balanced lines is the connection of microphones to a mixer in professional systems. Classically, both dynamic and condenser microphones used transformers to provide a differential-mode signal[citation needed]. 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 connectors. One connects to the shield or chassis ground, while the other two are for the signal conductors. The signal wires can carry two copies of the same signal with opposite polarity (differential signalling) but need not do so. 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

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Main article: Balanced circuit

Many explanations of balanced lines assume symmetric signals (i.e. signals equal in magnitude but of opposite polarity) but this can lead to confusion of the two concepts—signal symmetry and balanced lines are quite independent of each other.[2] Essential in a balanced line is identical impedances in the two conductors in the driver, line and receiver (impedance balancing). These conditions ensure that external noise affects each leg of the line equally and thus appears as a common-mode signal that is rejected by the receiver.[2] There are balanced drive circuits that have excellent common-mode impedance balancing between the legs but do not provide symmetric signals.[9][10] Symmetric differential signals concern headroom and are not necessary for interference rejection.[11]

Baluns

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Main article: Balun

Interfacing balanced and unbalanced lines requires a balun. 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 m) of balanced category 5 cable by using a pair of baluns at each end of the CAT5 run. As the signal travels through the balanced line, noise is induced and added to the signal. As the CAT5 line is carefully impedance balanced, the noise induces equal (common-mode) voltages in both conductors. At the receiving end, the balun responds only to the difference in voltage between the two conductors, thus rejecting the noise picked up along the way and leaving the original signal intact.

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.

Characteristic impedance

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The characteristic impedance 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.

Electric power lines

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In electric power transmission, 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, 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

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Balanced transmission standards

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Balanced line

View on Grokipedia
from Grokipedia
A balanced line is a transmission line in electrical engineering consisting of two conductors of the same type that carry signals of equal magnitude but opposite polarity relative to a common reference, such as ground, with equal impedances to ground and each other along their lengths. This configuration enables differential signaling, where the receiver measures the voltage difference between the two conductors to extract the intended signal while rejecting that affects both lines equally as common-mode interference. The principle of balanced lines originated in for long-distance transmission, allowing signals to travel over extended distances without significant degradation from . In practice, balanced lines typically employ twisted-pair cabling with a separate shield that is connected to chassis ground at both ends, as specified by standards like AES48-2019, to further enhance and prevent ground loops. Various implementations exist, including transformer-balanced outputs for electrical isolation, electronically balanced outputs using operational amplifiers to maintain equal impedances, and impedance-balanced designs that provide robustness against cable faults. Balanced lines are widely used in professional audio systems, data communications like , and to achieve high immunity, support longer cable runs (up to hundreds of feet), and ensure in noisy environments. Compared to unbalanced lines, which use a single signal conductor relative to ground and are more prone to interference, balanced lines offer superior common-mode rejection ratios, often exceeding 60 dB, making them essential for applications requiring reliable, low- performance.

Core Concepts

Definition and Principles

A balanced line is a two-conductor electrical in which the two conductors are electrically symmetric with respect to ground and each other, such that they carry equal and opposite voltages or currents relative to a reference point, ensuring no net current flows to ground. This symmetry distinguishes balanced lines from other configurations and enables efficient of transverse electromagnetic (TEM) waves along the line. The core operating principles of balanced lines revolve around their inherent rejection of common-mode and interference through maintained electrical . In operation, the useful signal is represented by the differential voltage or current between the two conductors, while any external interference—such as electromagnetic fields from nearby sources—couples equally to both conductors, manifesting as a common-mode component that does not affect the differential signal. This rejection occurs because the line's ensures that common-mode signals produce equal and opposite effects at the receiving end when processed differentially, effectively canceling out unwanted disturbances without impacting the intended transmission. Consequently, balanced lines provide robust performance in environments prone to electrical , prioritizing the potential difference between conductors over their individual potentials relative to ground. A basic representation of a balanced line consists of two parallel wires serving as the conductors, with the signal propagating as equal and opposite currents flowing along each wire in the forward direction. External noise couples inductively or capacitively to both wires in the same manner, inducing common-mode currents that flow in the same direction on both conductors but are subsequently rejected by differential sensing at the receiver. This configuration highlights how the line's confines the differential signal to the space between the conductors while isolating it from environmental influences. The origins of balanced lines trace back to the late in the development of , where two-wire metallic circuits were introduced around to replace noisy single-wire ground-return systems, providing the foundational approach for balanced transmission in communication networks.

Balanced vs. Unbalanced Lines

Balanced lines consist of two symmetric conductors that carry equal and opposite signals relative to a common reference, such as a where each wire has identical impedance to ground. In contrast, unbalanced lines employ a single signal conductor paired with a ground or shield as the return path, exemplified by coaxial cable where the inner conductor carries the signal and the outer shield serves as ground. This structural symmetry in balanced lines ensures that external electromagnetic fields induce identical noise voltages on both conductors, which can then be rejected at the receiver. Performance-wise, balanced lines offer superior noise immunity due to their ability to cancel common-mode interference through differential signaling, making them less susceptible to (EMI) and ground loops compared to unbalanced lines. Unbalanced lines, however, are more vulnerable to such issues because noise induced on the signal conductor relative to the ground does not cancel out, potentially leading to higher rates or signal degradation in noisy environments. For instance, unbalanced configurations can generate and pick up more due to their reliance on a shared ground, exacerbating problems like hum in audio or in communications. Balanced lines are preferred in scenarios involving long cable runs or high-noise settings, such as industrial environments, where their noise rejection maintains over distances up to several hundred meters. Unbalanced lines, by comparison, suit shorter distances and low-noise applications, like local video connections, where simplicity and lower cost outweigh the need for robust interference . When interfacing balanced and unbalanced systems, challenges arise from differing characteristic impedances—typically higher in balanced lines (e.g., 100–120 Ω) than unbalanced (e.g., 50–75 Ω)—necessitating converters like baluns to match impedances and prevent reflections or signal loss.

Signaling and Transmission

Differential and Common-Mode Signals

In balanced lines, the differential signal represents the intended information-carrying component, defined as the voltage difference between the two conductors, Vdifferential=V1−V2V_{\text{differential}} = V_1 - V_2, where V1V_1 and V2V_2 are the voltages on the respective conductors relative to a . This formulation arises from the of the balanced line, where the differential mode corresponds to the antisymmetric component of the voltages, ensuring that the signal propagates as equal and opposite currents on the two conductors, thereby maintaining the line's balanced nature and minimizing . Conversely, the consists of voltages that appear equally on both conductors relative to ground, given by Vcommon-mode=V1+V22V_{\text{common-mode}} = \frac{V_1 + V_2}{2}. This derives from the symmetric nature of the balanced line, capturing the in-phase component where both conductors experience the same potential shift, often due to external influences rather than the transmitted . Common-mode signals primarily embody unwanted noise or interference, as they do not contribute to the useful information transfer and can degrade if not suppressed. The noise rejection mechanism in balanced lines exploits the separation of these modes at the receiver, where a subtracts the voltages from the two conductors: the output amplifies the differential signal while ideally canceling the common-mode component. This subtraction process, rooted in the line's , ensures that common-mode noise—induced equally on both lines by , such as from nearby power lines—affects V1V_1 and V2V_2 identically, resulting in zero net contribution to the output. For instance, if an external electromagnetic field induces a 1 V common-mode voltage on both conductors, the receiver's differential operation yields Vout=VdifferentialV_{\text{out}} = V_{\text{differential}}, with the noise fully rejected, provided the circuit's (CMRR) is high.

Characteristic Impedance

The characteristic impedance Z0Z_0 of a balanced line is defined as the ratio of the differential voltage to the differential current associated with a propagating electromagnetic wave along the line, and this value remains independent of the line's physical length for a uniform structure. This property arises because Z0Z_0 is an intrinsic characteristic determined by the line's and materials, ensuring that the wave behaves as if the line were infinitely long when properly terminated. The value of Z0Z_0 is influenced by the distributed electrical parameters per unit length of the line: series resistance RR, series LL, shunt conductance GG, and shunt CC. For lossless lines where R=0R = 0 and G=0G = 0, which is a common approximation at high frequencies, Z0Z_0 simplifies to the real-valued expression Z0≈LCZ_0 \approx \sqrt{\frac{L}{C}}
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