Balanced audio is a method of interconnecting audio equipment using impedance-balanced lines. This type of connection is very important in sound recording and production because it allows for the use of long cables while reducing susceptibility to external noise.
Balanced connections use three-conductor connectors, usually the XLR or TRS jack plug. The XLR connector is an Electrical connector design XLR Plugs and sockets are used mostly in professional audio and Video A TRS connector, also called an audio jack, phone plug, jack plug, stereo plug, mini-jack, or mini-stereo, is a common XLR connectors, for instance, are usually used with microphones because of their durable construction, while TRS jack plugs are usually used for mixer inputs and outputs because of their smaller profile. In professional audio, a mixing console, or audio mixer, also called a sound board or soundboard, is an electronic device for combining
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Applications
Many microphones operate at low voltage levels and some with high output impedance (hi-Z), which makes long microphone cables especially susceptible to electromagnetic interference. Any linear electronic circuit or device which supplies a current may be modelled as an Ideal voltage source in series with an impedance. Microphone interconnections are therefore a perfect application for a balanced interconnection, which cancels out most of this induced outside noise.
If the power amplifiers of a public address system are located at any distance from the mixing console, it is also normal to use balanced lines for the signal paths from the mixer to these amplifiers. Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. A public address or " PA " system is an electronic amplification system with a mixer, Amplifier and Loudspeakers used to In professional audio, a mixing console, or audio mixer, also called a sound board or soundboard, is an electronic device for combining Many other components, such as graphic equalizers and effects units, have balanced inputs and outputs to allow this. In recording and for short cable runs in general, a compromise is necessary between the noise reduction given by balanced lines and the cost introduced by the extra circuitry they require. An electronic circuit is a closed path formed by the interconnection of Electronic components through which an Electric current can flow
Interference reduction
Balanced audio connections use a number of techniques to reduce noise.
A typical balanced cable contains two identical wires, which are twisted together and then wrapped with a third conductor (foil or braid) that acts as a shield. A shielded cable is an electrical Cable of one or more insulated conductors enclosed by a common conductive layer
The term "balanced" comes from the method of connecting each wire to identical impedances at source and load. Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal Alternating current (AC Any linear electronic circuit or device which supplies a current may be modelled as an Ideal voltage source in series with an impedance. The Input impedance, Load impedance, or external impedance of a circuit or electronic device is the Thévenin This means that much of the electromagnetic interference will induce an equal noise voltage in each wire. Since the amplifier at the far end measures the difference in voltage between the two signal lines, noise that is identical on both wires is rejected. Subtraction is one of the four basic Arithmetic operations it is the inverse of Addition, meaning that if we start with any number and add any number and then subtract The noise received in the second, inverted line is applied against the first, upright signal, and cancels it out when the two signals are subtracted.
This differential signal recombination can be implemented with a differential amplifier. A differential amplifier is a type of Electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential Gain) A balun may also be used instead of an active differential amplifier device. A balun, ("bal-un" is a passive electronic device that converts between '''bal'''anced and '''un'''balanced Electrical A differential amplifier is a type of Electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential Gain)
The wires are also twisted together, to reduce interference from electromagnetic induction. Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of Twisting makes the loop area between the conductors as small as possible, and ensures that a magnetic field that passes equally through adjacent loops will induce equal but opposite currents, which cancel out.
The separate shield of a balanced audio connection also yields a noise rejection advantage over an unbalanced two-conductor arrangement (such as used in typical home stereos) where the shield must also act as the signal return wire. Noise reduction is the process of removing Noise from a signal. High fidelity or hi-fi reproduction is a term used by home stereo listeners and home audio enthusiasts ( Audiophiles to refer to high-quality reproduction Any noise currents induced into a balanced audio shield will not therefore be directly modulated onto the signal, whereas in a two-conductor system they will be. This also prevents ground loop problems, by separating the shield/chassis from signal ground. In an Electrical System, a ground loop usually refers to a current
Differential signalling
Signals are often transmitted over balanced connections using the differential mode, meaning the wires carry signals of opposite polarity to each other (for instance, in an XLR connector, pin 2 carries the signal with normal polarity, and pin 3 carries an inverted version of the same signal). Differential signaling is a method of transmitting Information electrically by means of two complementary Signals sent on two separate wires The XLR connector is an Electrical connector design XLR Plugs and sockets are used mostly in professional audio and Video
Despite popular belief, this is not necessary for noise rejection. As long as the impedances are balanced, noise will couple equally into the two wires (and be rejected by a differential amplifier), regardless of the signal that is present on them. [1][2] A simple method of driving a balanced line is to inject the signal into the "hot" wire through a known source impedance, and connect the "cold" wire to ground through an identical impedance. Any linear electronic circuit or device which supplies a current may be modelled as an Ideal voltage source in series with an impedance. Due to common misconceptions about differential signalling, this is often referred to as a quasi-balanced or impedance-balanced output, though it is, in fact, fully balanced and will reject common-mode interference.
There are some benefits to driving the line with a fully differential output, though:
- The electromagnetic field around a differential line is ideally zero, which reduces crosstalk into adjacent cables. In Electronics, the term crosstalk ( XT) refers to any phenomenon by which a signal transmitted on one circuit or channel of a Transmission system
- Though the signal level would not be changed due to nominal level standardization, the maximum output from the differential drivers is twice as much, giving 6 dB extra headroom. Nominal level is the operating level at which an electronic Signal processing device is designed to operate In a digital and analog audio systems headroom is the amount by which linear signal capabilities exceed actual signal level [1] (If the amplifiers are identical, though, their output noise sums to 3 dB more than a single amplifier, decreasing dynamic range. )
- Noise that is correlated between the two amps (from poor power supply rejection, for instance), would be cancelled out.
- At higher frequencies, the output impedance of the output amplifier can change, resulting in a small imbalance. When driven in differential mode by two identical amplifiers, this impedance change will be the same for both lines, and thus cancelled out. [1]
- Differential drivers are also more forgiving of incorrectly wired adapters or equipment that unbalance the signal by shorting pin 2. [1]
Internally balanced audio design
Most professional audio products (recording, public address, etc. ) provide differential balanced inputs and outputs, typically via XLR or TRS connectors. The XLR connector is an Electrical connector design XLR Plugs and sockets are used mostly in professional audio and Video A TRS connector, also called an audio jack, phone plug, jack plug, stereo plug, mini-jack, or mini-stereo, is a common However, in most cases, a differential balanced input signal is internally converted to a single-ended signal via transformer or electronic amplifier. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. After internal processing, the single-ended signal is converted back to a differential balanced signal and fed to an output. A small number of professional audio products have been designed as an entirely differential balanced signal path from input to output; the audio signal never unbalances. This design is achieved by providing identical (mirrored) internal signal paths for both pin 2 and pin 3 signals (AKA "hot" and "cold" audio signals). In critical applications, a 100% differential balanced circuit design can offer better signal integrity by avoiding the extra amplifier stages or transformers required for front-end unbalancing and back-end rebalancing. A transformer is a device that transfers Electrical energy from one circuit to another through inductively coupled Electrical conductors Fully balanced internal circuitry has been promoted as yielding 3dB better dynamic range. [3]
Connectors
While 3-pin XLR connectors are the most common balanced connector, quarter-inch (¼" or 6. The XLR connector is an Electrical connector design XLR Plugs and sockets are used mostly in professional audio and Video Inches redirects here To see the Les Savy Fav album see Inches. 35 mm) TRS connectors (tip-ring-sleeve) are also commonly used. A TRS connector, also called an audio jack, phone plug, jack plug, stereo plug, mini-jack, or mini-stereo, is a common Many hybrid jacks are now designed to take either XLR or TRS.
 2. 5, 3. 5 and 6. 35 mm TRS plugs |
 XLR3 cable connectors, female on left and male on right |
 XLR and 1/4" TRS combo jack. |
On TRS plugs, the tip is "hot" (positive), the ring is "cold" (negative), and the sleeve is ground (earthed or chassis). In Electrical engineering, the term ground or earth has several meanings depending on the specific application areas A chassis (plural "chassis" (ˈʃæːsiː ˈtʃæːsiː consists of a framework that supports an inanimate object analogous to an Animal 's If a stereophonic or other binaural signal is plugged into such a jack, one channel (usually the right) will be subtracted from the other (usually the left), leaving an unlistenable L − R (left minus right) signal instead of normal monophonic L + R. Stereophonic sound, commonly called stereo, is the reproduction of Sound, using two or more independent audio channels through a Symmetrical Monaural (often shortened to mono) sound reproduction is single-channel Reversing the polarity at any other point in a balanced audio system will also result in this effect at some point when it is later mixed-down with its other channel.
Telephone lines also carry balanced audio, though this is generally now limited to the local loop. A telephone line or telephone circuit (or just line or circuit within the Industry) is a single-user circuit on a Telephone In Telephony, the local loop (also referred to as a subscriber line) is the physical link or circuit that connects from the Demarcation point of the It is called this because the two wires form a balanced loop through which both sides of the conversation travel. Banter redirects here for the Radio 4 panel show see Banter (radio show A conversation is Communication by two or more people or by one's self
Data lines, including digital audio, are also frequently balanced, normally using AES/EBU (AES3) with XLR connectors for pro audio. Digital audio uses Digital signals for Sound reproduction. This includes analog-to-digital conversion, digital-to-analog conversion, storage The digital audio standard frequently called AES/EBU, officially known as AES3, is used for carrying Digital audio signals between various devices The digital audio standard frequently called AES/EBU, officially known as AES3, is used for carrying Digital audio signals between various devices Eight-channel analog balanced audio connectors like ADAT use DB25 connectors, which can also carry up to 16 digital channels. Alesis Digital Audio Tape or ADAT, first introduced in 1991 was used for simultaneously recording eight tracks of Digital audio at The D-subminiature or D-sub is a common type of Electrical connector used particularly in Computers Calling them "subminiature" was appropriate
Converters
Unbalanced signals can be converted to balanced signals by the use of a balun, often through a DI unit. A balun, ("bal-un" is a passive electronic device that converts between '''bal'''anced and '''un'''balanced Electrical A DI unit, DI box, Direct Box or simply DI is an electronic device that connects a high impedance Line level signal that has an unbalanced
If balanced audio must be fed into an unbalanced connection the electronic design used for the balanced output stage must be known. In most cases the negative output can be tied to ground but in certain cases the negative output should be left disconnected. See Douglas Self's page for specific details.
References
- ^ a b c d Graham Blyth. Audio Balancing Issues. Professional Audio Learning Zone. Soundcraft. Retrieved on 2007-05-24. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1218 - The Fifth Crusade leaves Acre for Egypt. 1276 - Magnus Ladulås is crowned
- ^ (2000) "Part 3: Amplifiers", Sound system equipment, Third edition, Geneva: International Electrotechnical Commission, p. The International Electrotechnical Commission ( IEC) is a not-for-profit, non-governmental international Standards organization that prepares and publishes 111. IEC 602689-3:2001. “Only the common-mode impedance balance of the driver, line, and receiver play a role in noise or interference rejection. This noise or interference rejection property is independent of the presence of a desired differential signal. ”
- ^ Sweetwater. Grace Design m201 (promotional ad copy)
See also
External links
- Uk Sound And Lighting Community — Article On Balanced Lines
- A Simple Implementation of the Impedance Balanced Output
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