Pickups
A pickup is an electroacoustic transducer ( sensor ) that converts mechanical vibrations in solids ( structure-borne sound ) into electrical voltage (the sound or audio signal ). This distinguishes the pickup from air-borne sound converters ( microphone ) and liquid- borne sound converters ( hydrophone ).
The audio signal is processed in terms of sound and / or made audible, for example, via an audio amplifier with a loudspeaker .
Typical examples are pickups of record players , which absorb the vibrations of the needle sliding through the record groove, as well as pickups for stringed instruments such as. B. Electric guitars .
Mechanical predecessors
As with the mechanical needle movements in the phonograph or gramophone , in the straw violin the mechanical vibration of the string is transmitted to a membrane, the air vibrations are amplified by a horn and thus audible. The stylus is a sharpened steel needle (more rarely wood or hard rubber).
Pickups for records
The pickup is an assembly of the record player that is mounted on the tonearm and consists of a stylus, stylus holder, transducer and housing. A cut diamond is attached to the end of the needle carrier , rarely a sapphire or ruby . On the shaft of the elastically mounted needle carrier, the vibrations are transmitted to the transducer via a fork construction. This converts the movement into an electrical voltage . Sometimes a so-called traveling broom is attached to the housing for the needle carrier, which is primarily intended to remove dust and dirt in front of the scanning needle.
With the crystal pickup - with a piezo crystal as a transducer - and the magnetic pickup - with a magnet coil system - the long-playing record was made possible, which was only operated at 33⅓ min −1 and thus allowed significantly longer playing times compared to the shellac record (78 min −1 ). Compared to gramophone needles, these pickups required significantly lower tracking forces, which increased the service life of the record and needle accordingly.
In order to be able to scan two audio channels independently for stereophony , stereo pickups consist of two transducer systems on the still single needle, which are inclined at 45 ° to the vertical and transversely to the record groove so that they are offset from each other by 90 °. This means that they are orthogonal and achieve the desired decoupling.
With the help of a pickup scale , the correct tracking force of the pickup can be set.
Electromagnetic converter (MM)
Electromagnetic pickups are the most widely used pickups for vinyl records. They are relatively cheap to manufacture and the stylus can be replaced on most systems. In the electromagnetic transducer of the type Moving Iron (moving iron) is moved a small iron part in the vicinity of coils with an iron core. The magnetic circuit is excited by a permanent magnet. By changing the distance between the iron parts, a change in the magnetic flux is generated in the coils, which leads to an induction voltage . A variant is the principle of the variable magnetic short circuit (Variable Magnetic Shunt), where an iron part at the end of the needle carrier in the magnet reduces the flux through the coils by deflecting the magnetic flux.
The needle carrier usually has a small magnet at its other end, which moves between the pole pieces of the coils of the two stereo channels. Pickups with moving magnets are called Moving Magnets or MM. The change in flux in the magnetic circuit is linked to the inhomogeneity of the magnetic field - the greater it is, the greater the signal. Therefore, in principle, they cause non-linear distortion and intermodulation when the deflection is large.
Electromagnetic pickups generate higher signal voltages than electrodynamic systems because the coil can have a high number of turns due to its less restricted mass.
Electromagnetic converters require an equalization preamplifier for playback, which corrects the frequency response of the record ( de-emphasis , cutting characteristic ). The pickup is connected to the equalization preamplifier, which is usually integrated in a hi-fi amplifier, via the phono input . Up until the late 1990s, hi-fi amplifiers were equipped as standard with such a phono input, the input resistance of which is usually 47 kOhm.
Electrodynamic Converter (MC)
Electrodynamic moving coil pickup with are called moving coil (dt., Moving coil ') referred to or abbreviated as MC. In the dynamic scanning system, coils are moved in a constant and as homogeneous as possible field of a permanent magnet. Due to induction, a voltage arises in the coils when they move. In order to keep the moving masses, more precisely the moments of inertia (scanning needle, needle carrier, coil carrier and coils) as low as possible, the coils usually only have a small number of turns. Too fine a light wire is difficult to work with and unreliable. As a result, the signal voltage in electrodynamic scanners is very low. In practice a distinction is made here between low output (below 0.3 mV), medium output (around 1 mV) and high output (3 mV and more). An indication based on a speed of 5 cm / s is common, the indication is an effective voltage value based on a speed peak value. MC systems require a particularly sensitive, very low-noise preamplifier (prepre) or a matching transformer ( transformer ) in front of the equalizer preamplifier, which increases the voltage by a factor of 30 or 10 (30 or 20 dB). Line level systems are sometimes used without a special amplifier. In terms of measurement technology, the systems can be simulated with 3.2 ohms, 32 ohms or 150 or 316 ohms. (For MM systems, 1 kOhm is usual.) The associated input impedances of the amplifiers are around 20 ohms to 1000 ohms, with transformers usually somewhat lower.
Replacing a worn or damaged stylus with MC cartridges is only possible with specially designed types, as the coils must be firmly connected to the vibrating needle carrier. Some manufacturers offer replacement programs for systems with a worn needle. Specialized companies can install a new needle on the needle carrier (retipping).
Piezoelectric converter
In the case of the piezoelectric pickup (crystal pickup), piezoelectric ceramic strips generate the signal voltage when they are bent by the scanning needle. Seignette salt was previously used as a piezo material , today barium titanate / zirconate. Piezo scanners first deliver an electrical charge, which is converted into a voltage proportional to it by a charge amplifier for good depth reproduction . Due to the required bending of the ceramic strips, they are quite stiff and require high bearing forces. The reproduction is influenced by natural resonance. Because of these properties, this type of converter is now only used in very low-priced devices.
Piezoelectric converters usually do not use an equalizer preamplifier, as the output voltage is not determined by the speed - as with electromagnetic pickups - but by the deflection. This corrects the cutting characteristic (RIAA) in a (very) rough approximation . The voltage is so high that amplifiers can be controlled directly. The input impedance should, however, be high, which today is almost only found in 'instrument' amplifiers.
Differences and other procedures
While the crystal pickup generates a signal voltage proportional to the deflection of the record groove , with the magnetic pickup this is proportional to the speed of the needle. At low frequencies, magnetic pickups therefore generate signal levels that are too low - one would have had to produce different records for both pickups, whereby a larger track width would have had to be provided for the magnetic pickup due to the greater deflection, with a corresponding reduction in the playing time. With the adjustment of the frequency response by the RIAA - characteristic that in the equalizer pre- compensates for magnetic systems is adapted to the recorded signal so that the records can be scanned with both crystal and magnetic pickups.
The development of the pickups reached its peak in the 1980s after the establishment of HiFi standards, when quadrophone (four-channel) records could also be scanned with low wear and tear with tracking forces of less than 10 mN and frequency ranges from 20 Hz to 30 kHz .
In order to improve the sound quality by reducing the noise, in addition to the usual dry scanning methods, wet methods were also tested in which the scanning needle moves in a scanning liquid. Here, the lower friction and improved cooling cause less wear on the plate and needle. The sound quality is significantly increased. However, this method has the disadvantage that dissolved particles stick together in the groove after drying and the records can practically only be played while wet or have to be cleaned at great expense. However, if demineralized water and not isopropanol is used, it dries almost without leaving any residue and no longer has the disadvantage (once wet, always wet). The proven composition of the market leader was ethanol 40% and a surfactant additive . Critics of the process attribute the screeching treble when played back dry to wear in the treble range, which is due to the damping of the fastest needle movement in the liquid. This contradicts the claim that the records are spared during scanning. Experience has shown that the scanning diamonds last more than five times as long as they are dry when they are liquid, until they show signs of wear.
Alternatively, there were also feasibility studies for non-contact scanning using laser or infrared diodes . The scanning is wear-free, but these devices reacted much more sensitively than needle scanners to dust and small scratches. In the 1950s and 1960s, electrostatic pickups were on the market that required special operating devices or preamplifiers ; the optical implementation of the needle movement was similarly complex (around 1975).
With certain non-contact optical processes, destroyed (cracked or broken) plates can be regenerated. Because these methods are very expensive, they are only used for historically valuable recordings.
Pickups for musical instruments
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In musical instruments, pickups are used to convert the vibration of the tone generator ( string , tongue ) of the instrument into electrical alternating voltage, which can be fed to a mixer or amplifier .
This mainly applies to stringed instruments, especially guitars and electric basses , but also keyboard instruments ( electric pianos , Hammond organs ). In the case of wind instruments, on the other hand, it is common to record the airborne sound generated anyway using microphones .
Depending on how they work, a distinction is made between electromagnetic and piezoelectric pickups.
Electromagnetic pickups
By means of an electromagnetic pickup (engl. Pickup ), the string vibration at an electric guitar , wherein a bass or electromechanical electric pianos into electrical signals ( AC voltage converted). In the simplest case it consists of a permanent magnet around which a coil is wound. The movement of the strings (they must be made of a ferromagnetic material) in the magnetic field changes its field strength. An alternating voltage with the frequency of the oscillation of the string is thus generated in the coil by electromagnetic induction . This voltage is around 0.1 V , but this also depends on the thickness of the string and its direction and amplitude of vibration: the thicker a string, the higher the voltage it induces. Vibrations in the direction of the magnetic pole lead to higher voltage induction than vibrations to the side of the pole. The voltage generated in this way is then fed to an audio amplifier (possibly processed by effects devices) . In order to reduce the sensitivity of the pickup for sound waves ( microphony ) and thus the tendency to feedback, the coils are often fixed in paraffin with a 20% addition of beeswax.
In the case of pickups for electric guitars and basses, a distinction is made between the single coil (single coil) and humbucker (double coil) designs , which are characterized by two very different sound characteristics. Single-coil pickups often (but not always) have a clearer sound that is richer in overtones, while humbuckers tend to sound soft and centered. In addition, thanks to the special interconnection of their two coils, humbuckers offer better protection against interfering noises caused by electromagnetic radiation from power supply units. a. be generated. The position of the pickup is also important for the sound. A hard, high-pitched sound is created near the bridge , while a pickup near the guitar neck delivers a softer, deeper-heavy tone. Electric guitars and basses are usually equipped with two or three separate pickups that can be combined to create the desired sound.
Guitar pickups are differentiated between
- passive ones , which make the unprocessed audio signal available for further processing, and
- Active pickups: these have an electronic preamplifier integrated in the instrument, which is usually powered by a small battery. It often also has tone and volume controls.
Piezoelectric pickups
Piezoelectric pickups are made of piezoelectric ceramics: Mechanical pressure or structure-borne sound from the sound body creates an electrical voltage.
Since there is no underlying magnetic effect here, piezo pickups work on stringed instruments with all types of strings, including nylon or gut strings.
Piezo pickups are suitable for use on all acoustic instruments with which the body vibrates. They are used on acoustic guitars (western guitars, concert guitars), double basses and other plucked instruments such as mandolins . Violins and electric violins also predominantly use these customers. Some electric guitars use piezo transducers to pick up a sound similar to an acoustic guitar instead of the magnetic pickup. They are useful for pieces of music in which acoustic and other passages alternate - the musician does not need to change the guitar, but only has to operate a switch to change the pickup.
Usually the pickup is built into the bridge or clamped between the bridge and the body. He then receives the pressure differences directly. If it is glued to the body of the instrument, it works like a structure-borne sound microphone: the vibration of the string, which is transmitted via the bridge to the top, is converted from a pressure fluctuation into an alternating voltage by the pickup due to its inertia. It is not only important to have a position from which as many vibrations as possible are transmitted to the consumer, but also the desired sound character, which can be very different at different points.
Since piezo pickups have a high output impedance , the system almost always includes a battery-operated preamplifier that is built into the instrument and usually has a volume control and a simple tone control .
Since a sensitive pickups each recorded from the body -borne sound converts (knocking, scraping, background noise) into signals interfering can feedback (also feedback given) occurring when such a pickup sound signals of the speaker receives. To combat this effect, many preamplifiers are equipped with notch filters or phase switches, which allow the annoying background noise to be minimized.
Piezo pickups are also used in tuners . Sounds of a musical instrument are recorded at a vibrating point and measured with the device. The pickup is usually designed as a clamp. This clamp can be firmly attached to the tuner, but is also available as a separate part.
MIDI pickup
MIDI pickups pick up the string vibrations of the individual strings of a guitar or another instrument. Either the piezoelectric or the electromagnetic pick-up of the string oscillation is used here. The vibrations converted into electrical signals are digitized by an additional electronic unit ( A / D converter ) and converted into MIDI signals by a microprocessor (see guitar synthesizer ).
Early systems analyzed the time interval between the zero crossings of the string oscillation. However, this led to significant latencies in the MIDI signal, especially at low frequencies. Modern systems use the characteristic attack (attack) of a string for frequency analysis. With this method, the latencies are in the range of 1 to 5 ms, a barely perceptible delay.
Other uses
The pickups also include the laser microphone , which is popular with secret services, as well as the larynx microphone and the bone conduction microphone used for communication in noisy environments . All of these converters use structure-borne sound for voice transmission.
The geophone , an instrument for detecting mechanical vibrations in the earth, represents a special case of the pickup .
literature
- Helmuth Lemme: Pickups, Potis & Co. - The inner workings of the electric guitar and bass. PPVMEDIEN, Bergkirchen 2018, ISBN 978-3-95512-121-1 .
- Helmuth Lemme: Electric guitars - technology and sound. Elektor-Verlag, Aachen 2003, ISBN 3-89576-111-7
- Bernhard Walter Panek: Pickup systems for stringed instruments: Overviews of acoustic and electric guitars, electric bass guitars, electromagnetic, electrodynamic, hexaphonic and piezo pickups, microphone pickups. Wiener Universitätsverlag Facultas ISBN 978-3-7089-0323-1
- Cathy van Eck: Between Air and Electricity. Microphones and Loudspeakers as Musical Instruments. Bloomsbury Academic, New York 2017. ISBN 978-1-5013-2760-5
