Star quad cable

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Star-quad cable intended for use with a single two-wire circuit or two two-wire circuits. It is often used with microphone signals in professional audio. Star quad.svg
Star-quad cable intended for use with a single two-wire circuit or two two-wire circuits. It is often used with microphone signals in professional audio.
Star-quad cable cross-section Star-Quad Cable Cross Section.jpg
Star-quad cable cross-section
Star-quad exploded view showing the geometric centers of the dual-conductors used for each leg of the balanced line. Star-Quad exploded.jpg
Star-quad exploded view showing the geometric centers of the dual-conductors used for each leg of the balanced line.
To achieve magnetic immunity, geometric centers of both dual-conductors must be aligned to a common point. Star-Quad Simple.jpg
To achieve magnetic immunity, geometric centers of both dual-conductors must be aligned to a common point.

Star-quad cable is a four conductor cable that has a special quadrupole geometry that provides magnetic immunity when used in a balanced line. Four conductors are used to carry the two legs of the balanced line. All four conductors must be an equal distance from a common point (usually the center of a cable). The four conductors are arranged in a four-pointed star (forming a square). Opposite points of the star are connected together at each end of the cable to form each leg of the balanced circuit.

A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.

In telecommunications 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 when fed to a differential amplifier. 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 lines, 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.

Contents

Star quad cables often use filler elements to hold the conductor centers in a symmetric four-point arrangement about the cable axis. All points of the star must lie at equal distances from the center of the star. When opposite points are connected together they act as if they are one conductor located at the center of the star. This configuration places the geometric center of each of the two legs of the balanced circuit in the center of the star. To a magnetic field, both legs of the balanced circuit appear to be in the exact center of the star. This means that both legs of the balanced circuit will receive exactly the same interference from the magnetic field and a common-mode interference signal will be produced. This common-mode interference signal will be rejected by the balanced receiver.

In telecommunication, the term common-mode interference has the following meanings:

  1. Interference that appears on both signal leads, or the terminals of a measuring circuit, and ground.
  2. A form of coherent interference that affects two or more elements of a network in a similar manner as distinct from locally generated noise or interference that is statistically independent between pairs of network elements.

The magnetic immunity of star quad cable is a function of the accuracy of the star-quad geometry, the accuracy of the impedance balancing, and the common-mode rejection ratio of the balanced receiver. [1] Star-quad cable typically provides a 10 dB to 30 dB reduction [2] in magnetically-induced interference. [3] [4] [5]

Advantages

When star-quad cable is used for a single balanced line, such as professional audio applications and two-wire telephony, two non-adjacent conductors are terminated together at both ends of the cable, and the other two conductors are also terminated together. Interference picked up by the cable arrives as a virtually perfect common mode signal, which is easily removed by a coupling transformer or differential amplifier. The combined benefits of twisting, differential signalling, and quadrupole pattern give outstanding noise immunity, especially advantageous for low signal level applications such as long microphone cables, even when installed very close to a power cable. It is particularly beneficial compared to twisted pair when AC magnetic field sources are in close proximity, for example a stage cable that can lie against an inline power transformer. [6]

Professional audio

Professional audio, abbreviated as pro audio, refers to both an activity and a category of high quality, studio-grade audio equipment. Typically it encompasses sound recording, sound reinforcement system setup and audio mixing, and studio music production by trained sound engineers, audio engineers, record producers, and audio technicians who work in live event support and recording using audio mixers, recording equipment and sound reinforcement systems. In contrast, consumer audio equipment is a lower grade of gear which is used by regular people for the reproduction of sound in a private home on a home stereo or home cinema system.

In telecommunication, a two-wire circuit is characterized by supporting transmission in two directions simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and receive. In either case they are twisted pairs. Telephone lines are almost all two wire, while trunks and switching are almost entirely four wire. To communicate in both directions in the same wire pair, conversion between four-wire and two-wire is necessary, both at the telephone and at the central office. A hybrid coil accomplishes the conversion for both. At the central office, it is part of a four-wire terminating set, more often as part of a line card.

Differential amplifier

A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages

Disadvantages

The disadvantage is that star quad, in combining two conductors, typically has more capacitance than similar two-conductor twisted and shielded audio cable. High capacitance causes an increasing loss of high frequencies as distance increases. [7] [8] The high-frequency loss is due to the RC filter formed by the output impedance of the cable driver and the capacitance of the cable. In some cases an increase in distortion can occur in the cable driver if it has difficulty driving the higher cable capacitance.

The capacitance of a 4-conductor quad-star cable is roughly equal to the capacitance of a standard 2-conductor cable about 1.5 times as long. The increased capacitance of the star quad cable is not usually a problem with short cable runs, but it can be an issue for long cable runs. For example, an 8 m (25 ft) star-quad cable has a capacitance of 150 pF/m [9] for a total capacitance of 1200 pF for the entire length of cable. With a 150-ohm source impedance and 1200 pF load capacitance, the frequency response of this RC circuit is -0.02 dB at 20 kHz. If the cable was 80 m instead of 8 m, then the frequency response would be -0.2 dB at 20 kHz, and -3 dB at 88 kHz.

A resistor–capacitor circuit, or RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current source. A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit.

Other applications for star quad cable

While the above discussion focuses on preventing noise from getting in (e.g. into a microphone cable) the same star-quad quadrupole configuration is useful for audio speaker cable, [10] for split-phase electric power wiring, and even for open-wire star quad transmission line.

In these cases, the purpose of the star quad configuration is reversed. The star-quad geometry cancels the magnetic fields that are produced by the two pairs of conductors. This cancellation reduces the magnetic emissions of the cable. To work properly, the cable must be wired in the same fashion as the microphone cable example above. Wires on opposite sides of the star must be shorted together at each end of the cable. This means that 4 conductors are required for a two-wire circuit. Furthermore, this scheme only works if the two pairs of conductors carry equal and opposite currents.

If a ground conductor is also needed, it must be added in a way that will not interfere with the star-quad geometry. It should also be added in a geometric configuration that exposes the ground conductor to equal interference from all four star-quad conductors. The most common solution is to wrap the star quad with a cylindrical ground conductor.

Star-quad cable can be used for two circuits, such as four-wire telephony and other telecommunications applications, but it will not provide magnetic immunity in this application. In this configuration each pair uses two non-adjacent conductors. Because the conductors are always the same distance from each other, crosstalk is reduced relative to cables with two separate twisted pairs. Each conductor of one pair sees an equal capacitance to both wires in the other pair. This cancels the capacitive crosstalk between the two pairs. The geometry also cancels the magnetic interference between the two pairs.

Related Research Articles

Electromagnetic compatibility

Electromagnetic compatibility (EMC) is the branch of electrical engineering concerned with the unintentional generation, propagation and reception of electromagnetic energy which may cause unwanted effects such as electromagnetic interference (EMI) or even physical damage in operational equipment. The goal of EMC is the correct operation of different equipment in a common electromagnetic environment.

In telecommunication and electrical engineering, a phantom circuit is an electrical circuit derived from suitably arranged wires with one or more conductive paths being a circuit in itself and at the same time acting as one conductor of another circuit.

Transmission line specialized cable or other structure designed to carry alternating current of radio frequency

In radio-frequency engineering, a transmission line is a specialized cable or other structure designed to conduct alternating current of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses.

Electrical cable two or more wires running side by side and bonded, twisted, or braided together to form a single assembly

An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current.

Coaxial cable A type of electrical cable with an inner conductor surrounded by concentric insulating layer and conducting shield

Coaxial cable, or coax, is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Many coaxial cables also have an insulating outer sheath or jacket. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis. Coaxial cable was invented by English engineer and mathematician Oliver Heaviside, who patented the design in 1880.

Twisted pair wiring in which two conductors of a circuit are twisted together to improve electromagnetic compatibility

Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference. It was invented by Alexander Graham Bell.

Balanced audio is a method of interconnecting audio equipment using balanced lines. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference.

Balun

A balun is an electrical device that converts between a balanced signal and an unbalanced signal. A balun can take many forms and may include devices that also transform impedances but need not do so. Transformer baluns can also be used to connect lines of differing impedance. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than ignoring, common mode signals.

In an electrical system, a ground loop or earth loop occurs when two points of a circuit both intended to be at ground reference potential have a potential between them. This can be caused, for example, in a signal circuit referenced to ground, if enough current is flowing in the ground to cause two points to be at different potentials.

Ferrite bead passive electric component that suppresses high frequency noise in electronic circuits. It is a specific type of electronic choke

A ferrite bead or ferrite choke is a passive electric component that suppresses high-frequency noise in electronic circuits. It is a specific type of electronic choke. Ferrite beads employ high-frequency current dissipation in a ferrite ceramic to build high-frequency noise suppression devices. Ferrite beads may also be called blocks, cores, rings, EMI filters, or chokes.

Pickup (music technology) transducer that captures or senses mechanical vibrations produced by musical instruments

A pickup is a transducer that captures or senses mechanical vibrations produced by musical instruments, particularly stringed instruments such as the electric guitar, and converts these to an electrical signal that is amplified using an instrument amplifier to produce musical sounds through a loudspeaker in a speaker enclosure. The signal from a pickup can also be recorded directly.

Differential signaling electronics

Differential signaling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conductors can be wires or traces on a circuit board. The receiving circuit responds to the electrical difference between the two signals, rather than the difference between a single wire and ground. The opposite technique is called single-ended signaling. Differential pairs are usually found on printed circuit boards, in twisted-pair and ribbon cables, and in connectors.

Shielded cable electrical conductors enclosed by a conductive layer

A shielded cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper, a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually this shield is covered with a jacket. The shield acts as a Faraday cage to reduce electrical noise from affecting the signals, and to reduce electromagnetic radiation that may interfere with other devices.. The shield minimizes capacitively coupled noise from other electrical sources. The shield must be applied across cable splices.

Feed line

In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from the transmitter to the antenna, where it is radiated as radio waves. In a receiving antenna it transfers the tiny RF voltage induced in the antenna by the radio wave to the receiver. In order to carry RF current efficiently, feed lines are made of specialized types of cable called transmission line. The most widely used types of feed line are coaxial cable, twin-lead, ladder line, and at microwave frequencies, waveguide.

Unbalanced line

In electrical engineering, an unbalanced line is a transmission line, often 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.

Transposition is the periodic swapping of positions of the conductors of a transmission line, in order to reduce crosstalk and otherwise improve transmission. In telecommunications this applies to balanced pairs whilst in power transmission lines three conductors are periodically transposed.

Transformer types

A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.

A balanced circuit is circuitry for use with a balanced line or the balanced line itself. Balanced lines are a common method of transmitting many types of electrical communication signals between two points on two wires. In a balanced line the two signal lines are of a matched impedance to help ensure that interference induced in the line is common-mode and can be removed at the receiving end by circuitry with good common-mode rejection. To maintain the balance, circuit blocks which interface to the line, or are connected in the line, must also be balanced.

Nominal impedance in electrical engineering and audio engineering refers to the approximate designed impedance of an electrical circuit or device. The term is applied in a number of different fields, most often being encountered in respect of:

References

  1. The Importance of Star-Quad Microphone Cable
  2. Evaluating Microphone Cable Performance & Specifications
  3. The Star Quad Story
  4. What's Special About Star-Quad Cable?
  5. How Starquad Works
  6. "SynAudCon EMI Project".
  7. Lampen, Stephen H. (2002). Audio/Video Cable Installer's Pocket Guide. McGraw-Hill. pp. 32, 110, 112. ISBN   0071386211.
  8. Rayburn, Ray (2011). Eargle's The Microphone Book: From Mono to Stereo to Surround – A Guide to Microphone Design and Application (3 ed.). Focal Press. pp. 164–166. ISBN   0240820754.
  9. http://www.canare.com/ProductItemDisplay.aspx?productItemID=54 Canare L-4E6AT Star-Quad Cable Specifications
  10. Canare 4S11 Star Quad Speaker Cable Specifications