Microphone signal
A microphone signal is an audio signal with electrical voltages in the millivolt range obtained with a microphone . As sensors, microphones use the membrane to convert the air movement through sound waves into an electrical signal voltage . The electrical signal voltage is also referred to as modulation voltage in connection with sound engineering , but has nothing to do with the technical modulation process.
The low level creates a high susceptibility to electromagnetic interference. Therefore microphone signals are often transmitted symmetrically in order to reduce the susceptibility to interference. For processing or recording, the microphone signal must be voltage- amplified with a microphone preamplifier .
The quality of a microphone signal is assessed based on interfering influences such as noise, frequency response, distortion factor and electromagnetic interference.
sensitivity
Microphones convert sound pressure into alternating voltage . The field operation transfer factor is measured in millivolts per Pascal (mV / Pa), which increases roughly proportionally with the membrane size. For example, small 1/4-inch capsules have 5 to 10 mV / Pa, 1/2-inch capsules up to 30 to 50 mV / Pa, and 1-inch capsules up to 100 mV / Pa.
A capsule with 50 mV / Pa emits exactly 1 volt at the pain threshold of 120 dB (that's six powers of ten more). From 140 dB (10 volts effective ~ 28 volts peak-to-peak) the measurement of sound pressure becomes problematic. Here you need microphone preamps with a large signal swing.
Noise
The smaller a capsule, the more susceptible it is to noise due to the lower transfer factor. The cause of the noise is not the microphone membrane, but the internal resistance of the capsule. In dynamic microphones, for example, this is the resistance of the moving coil, and in electret microphones, it is the load resistance. The higher the internal resistance, the more noisy the microphone, but generally the higher the output voltage. Compared to moving coil microphones, electret capsules have a terminating resistance that is at least ten times higher and therefore at least √10 times (√10 ≈ 3) higher noise - but they also deliver significantly higher signal voltages.
In order to achieve the level of good microphone amplifiers with an input noise of less than 1.8 nV / √Hz, the internal resistance of the (moving coil) microphone must fall below 200 Ω.
An electret capsule with a resistance greater than 5 kΩ due to the junction field effect transistor (JFET) generates around 9 nV / √Hz, but a signal voltage that is already so high that it only needs to be amplified slightly - a particularly low-noise microphone amplifier is therefore not required for these microphones.
Impedance
While dynamic microphones in the home mostly have impedances around 600 ohms, electret foils as capsules have extremely high impedances; therefore, a field effect transistor (FET) is generally integrated in electret capsules as an impedance converter. The FET is operated on load resistors in the range between 1 and 5 kOhm and requires an electrical voltage. The higher the impedance of the microphone output, the more noticeable the cable capacitance of the connection line: high frequencies are attenuated by long cables.
Frequency response
The frequency response of a microphone results from its acoustic design, the microphone tuning and the transducer principle. The smaller and the lighter the membrane (and possibly the moving coil), the fewer natural resonances it has in the audible frequency band (20 Hz to 20 kHz). The less it resonates , the more undistorted it reproduces the sound. With electret capsules up to 1/2 ", for example, it is possible to position the resonance frequency of the membrane outside of the useful band. With larger membranes and moving coil microphones, this is usually not possible.
Moving coil microphones have a relatively good signal-to-noise ratio, but their frequency response is limited by their design. The weight of the coil causes inertia and resonances, which inevitably lie in the useful band.
Piezo microphones also have extremely strong natural resonances.
Condenser microphones, especially ribbon and small electret microphones, are also suitable for ultrasound .
Distortion factor
The distortion factor indicates the percentage of non-linear signal distortions in the useful signal.
With dynamic microphones, the distortion factor is low, non-linear distortion usually only occurs at very high, irrelevant sound levels . However, parasitic vibrations of the membrane can play a role, the rigidity of which is therefore increased by a special shape.
In electret and condenser microphones, the non-linear relationship between the deflection of the diaphragm and the voltage output distorts the signal in a non-linear manner above certain levels, which creates harmonics. This is particularly true of electret microphones.
Electromagnetic susceptibility, hum
Electromagnetic interference often makes itself noticeable as hum. The reason for this is the type of connection and the cable routing.
A microphone cable should allow little interference in the signal. The susceptibility to interference increases with the length of the cable. The signals coming from different sources of interference can be divided into electrical (capacitive) and magnetic (inductive) components.
A good shielding of the cable can eliminate the electrical interference; coaxial cables are insensitive to magnetic interference anyway.
Dynamic microphones and the adapter transformers in ribbon microphones are, however, potential, sensitive receivers for alternating magnetic fields. This is countered with magnetic shields.
However, the most common hum interference is caused by ground loops (also known as hum loops). These can be eliminated by using differential (symmetrical) cable routing or ground cables that are routed separately for shielding.
Microphone cable partially have a Mikrofonieeffekt , they are sensitive to impact sound and motion when their braid or shielding generated upon movement changing contact resistances. This is particularly noticeable as noise with phantom power or ground loops. Changes in the distance between the screen and the core and the contact with the screen foils may also play a role. Microphony poverty is a quality criterion for microphone cables.
Connection standards
- Symmetrical signal routing : mono signal, three wires: ground, positive signal polarity "Hot", negative signal polarity "Cold"
- Asymmetrical signal routing: mono signal, two wires: ground, signal
- Asymmetrical signal routing: stereo signal, three wires: ground, left signal, right signal
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| standard |
XLR -Cannon connector, 3 -pin + housing ground also: 5-pin |
NAB 6.35 mm jack plug , 3-pin |
NAB 3.5 mm jack plug , 3-pin |
Large / small cloth plug , 3-pin + housing ground; also: 5-pol |
5-pin DIN plug , 3-pin + housing ground; also: 5-pol |
|---|---|---|---|---|---|
| application | Analog mono microphone, AES42 digital microphone signal , studio and stage |
Mono microphone, stereo microphone, home recording |
Stereo microphone, home recording |
Mono microphone, old standard clip-on microphones |
Mono microphone, 3-pin + chassis ground ; also: 5-pol |
| Occupancy | Pin1 = ground Pin2 = hot Pin3 = cold housing = shielding |
Tip = Hot / Left Ring = Cold / Right Ground = Ground, shielding |
Tip = left ring = right ground = ground |
Pin1 = Hot Pin2 = Ground Pin3 = Cold |
Pin1 = ground Pin2 = hot Pin3 = cold housing = shielding |
| electric wire | three-wire, shielded | three-wire, possibly shielded | three-wire, unshielded | three-wire, shielded | three-wire or five-wire, shielded |
| other applications |
Stereo signals Line signals digital audio ( AES / EBU ) Loudspeaker signals DMX (lighting technology) |
Stereo signals Line signals loudspeaker signals insert signals (amplification) |
Headphone signals Line signals Remote Control |
Microphone signals Loudspeaker signals Stereo signals Line IN / OUT |
Microphone signals Line signals Stereo signals Line IN / OUT |
These connection standards are the most common today. Older microphones may have a DIN or Tuchel plug . You can re-solder them or build an adapter. Occasionally there is also the "Klein-Tuchel" - especially for compact clip-on microphones with a separate radio transmitter.
The following applies to all microphone plugs: The “male” connector emits the signal, the “female” connector accepts the signal.
Digital microphone interface
The AES42 standard defines a digital interface for microphones that directly generate a digital audio stream. The processing chain impedance converter - microphone preamplifier - A / D converter is integrated in the microphone housing. The connection is made via an XLR plug, the power supply of the electronics via phantom power (Digital Phantom Power (DPP), 10 V, max. 250 mA). By modulating the phantom voltage, such microphones can be operated remotely, for example to set attenuation / directional characteristics.
literature
- Fritz Kühne: Mono, stereo and transistor microphones (= Radiopraktikerbücherei. No. 11/12, ZDB -ID 1108351-7 ). 7th, revised and expanded edition. Franzis-Verlag, Munich 1969.
- Norbert Pawera: Microphone Practice . Tips and tricks for stage and studio. Technology, acoustics and recording practice for instruments and vocals. 4th, completely revised and expanded edition. PPV-Medien, Bergkirchen 2003, ISBN 3-932275-54-3 .
- Thomas Görne: Microphones in theory and practice. 8th, new, revised and expanded edition. Elektor-Verlag, Aachen 2007, ISBN 978-3-89576-189-8 .
- Michael Dickreiter, Volker Dittel, Wolfgang Hoeg, Martin Wöhr (eds.): Manual of the recording studio technology . 8th, revised and expanded edition. 2 volumes. Walter de Gruyter, Berlin / Boston 2014, ISBN 978-3-11-028978-7 or e-book ISBN 978-3-11-031650-6
