Phonograph

The phonograph, or gramophone, was the most common device for playing recorded sound from the 1870s through the 1980s.

Turntable technology

Turntable drive systems

Most turntables employ an idler-wheel drive, belt drive or direct drive system to rotate the turntable platter:

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• Idler-wheel drive system

:Earlier designs used a rubberized idler-wheel drive system. However, non-linear wear and decomposition of the wheel introduced noise and speed variations into the desired audio. These systems generally used a synchronous motor which ran at a speed synchronized to the AC frequency of the power supply. Different speeds were obtained by bringing differing diameter wheels into position against the bottom or inside edge of the platter.

Related Topics:
Synchronous motor - AC - Frequency

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• Belt drive system

:Belt drives brought improved motor and platter isolation compared to idler-wheel designs. Motor noise heard as low frequency rumble was much reduced. It is difficult to design multiple speed synchronous motors, consequently DC servomotors with electronics providing speed control, have gained favour. On the most sophisticated designs, optical sensors on the platter are used to ensure the speed of the platter remains stable. Many platters have a continuous series of strobe markings machined around their edge to provide these pulses. A strobe effect can be observed by the operator to verify rotational speed. DC servomotors rotate in steps rather than continuously. This is referred to as 'cogging', and can add noise during playback. Helical armature motors can be used to overcome this. Modern high fidelity applications favor the use of belt-driven systems, as these isolate the revolving platter from motor-induced vibrations. Problems with material instability and deterioration have largely been solved by use of modern elastic polymers.

Related Topics:
Rumble - DC servomotor - Strobe - Helical armature

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• Direct drive system

:Direct drive turntables drive the platter directly without utilizing intermediate wheels, belts, or gears as part of a drive train. The platter functions as a motor armature. This requires good engineering, with advanced electronics for acceleration and speed control. Matsushita's Technics division introduced the first commercially successful direct drive platter, model SP10, in 1969. The Technics SL-1200 turntable, introduced in 1972, was one of the most successful direct drive turntables ever produced. Its rapid acceleration up to speed, quartz locked speed control, electric braking system and its reliability made it a favourite with radio stations and disc jockeys right across the world. It was particularly popular with the disc jockeys who used it for beatmixing because it had a variable pitch control (first a knob and then a slider on the Mk 2), allowing variations of the rotational speed above and below the usual 33 and 45 rpm settings. The SL-1200 Mk2 turntable was still in production in the 1990s - a remarkable achievement in an increasingly digital world.

Related Topics:
Matsushita - Technics - 1969 - 1972 - 1990s

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Pickup systems

Another major component is the pickup or cartridge. Early electronic phonographs used a piezo-electric quartz crystal for pickup, where the mechanical movement of the stylus in the groove generates a proportional electrical voltage. Crystal pickups are relatively robust, and yield a good level of signal which requires only a modest amount of amplification. A crystal's output tends not to be very linear, that is, it introduces unwanted distortion. It is difficult to make a crystal pickup suitable for stereo reproduction, as the stiff coupling between the crystal and the stylus prevents close tracking of the needle to the groove modulations. This tends to increase wear on the record, and introduces distortion.

Related Topics:
Piezo-electric - Quartz - Crystal - Stylus - Voltage - Distortion - Stereo

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In all high-fidelity systems, the crystal pickup has been replaced by the magnetic cartridge using either a moving magnet or moving coil. In the moving magnet system, the stylus carries a tiny permanent magnet, which is positioned between a series of fixed coils. As the magnet vibrates in response to the stylus following the record groove, it induces a tiny current in the coils. This current, now a weak alternating current representing the original sound wave from the recording session, is fed to an amplifier which boosts the signal, and then to a loudspeaker where it is converted to sound waves. Because the magnet is so light, and is not coupled mechanically to the coils, the stylus follows the groove far more gently and faithfully. Moving coil systems, are generally more expensive and are preferred by some audiophiles. Here a tiny coil is attached to the stylus, and moves within the field of a permanent magnet. Magnetic cartridges provide a much lower output than a crystal pickup, in the range of a few millivolts, thus requiring a preamplifier stage. Moving-coil cartridges generate an even smaller signal, of a few hundred microvolts, and require additionally a transformer or pre-preamplifier stage. Electrical noise induced by power lines or other EMI are attenuated by various methods, including electromagnetic shielding in the signal cables connecting the pickup to the amplifier.

Related Topics:
Magnetic cartridge - Magnet - Induces - Current - Amplifier - Loudspeaker - EMI - Attenuated - Electromagnetic shielding

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The stylus is typically a conical diamond tip on an aluminum tubular cantilever for a monophonic sound or rugged use, and an elliptical diamond tip for a stereo or binaural signal. Some very expensive styli have ruby, boron, or carbon fiber cantilevers chosen for their exceptional stiffness. DJs use the more rugged conical (sometimes inaccurately called spherical) styli due to the frequent reversals of direction involved in scratching.

Related Topics:
Diamond - Cantilever - Monophonic - Binaural - Ruby - Boron - Carbon fiber - DJ - Scratching

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Phonograph recordings are made with high frequencies boosted. Then during playback the high frequencies are rescaled to the initial level, which reduces groove background noise including clicks or pops. This is accomplished in the amplifier with a 'PHONO' input that uses a standardized RIAA equalization curve.

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Arm systems

Basic arm design has changed relatively little. S-Type tone-arms can be found on even the 1925 Victor Orthophonic phonograph. Originally, even though the tone-arm was light for earlier electric pickup, the full weight rested on the record. Right through to the crystal pickup, this was required to create sufficient tracking force to follow the grooves adequately with the relatively stiff styli. Naturally, record wear was not given much consideration. With the advent of the better technologies, including more powerful rare-earth magnetic cartridges, far lighter tracking forces became possible, and a balanced arm came into use. Many use a counterweight to offset the weight of the arm. The addition of a calibrated dial on the weight, provides for quick change of stylus pressure. Stylus pressure of 1 to 2 grams are currently the standard.

Related Topics:
Rare-earth - Counterweight - Gram

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Two types of tracking error incident to a standard arm can affect the sound. As the tone arm tracks the groove, the stylus drags tangent to the disc surface and resistance along the arm combines to create a horizontal skating force towards the center of the disc. Modern arms provide a spring-loaded or hanging weight bias which offsets this force, so as to leave the net horizontal force near zero. The second error occurs as the arm sweeps in an arc across a disc recording, causing the angle between the cartridge head and groove direction to change slightly. A change in angle, albeit small, may have an audible effect by creating a differential force on the groove walls. Making the arm longer so as to reduce this angle is one solution. Some arms have been manufactured with an auxillary arm which pivots the cartridge head on the main arm to maintain a constant angle.

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If the arm is not pivoted at a fixed point, but travels horizontally along a radius of the disc, there will be no skating force and no cartridge angle error. The arm is driven along a linear track using a servomechanism to position it properly. Matsushita's Technics division developed one such practical system with its model SL-10 turntable in 1979, ten years after the introduction of the first direct-drive turntable. Early Edison phonographs had utilized similar but spring powered drives to carry the stylus across the record at a pre-determined rate. In practice the linear tracking system is not widely used today due to its complexity and attendant expense. However some of the most sophisticated systems do employ this technique.

Related Topics:
Servomechanism - Matsushita - Technics - 1979 - Direct-drive turntable

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Front-loading systems

A brief mention could be made of one attempt to make the use of records more convenient, in the dawning age of the compact disc. In the early 1980s, one manufacturer designed an upright (front loading) record playing music centre, in which the record was placed in a door which hinged downwards to accept it. The door retracted automatically and the record was spun in the vertical plane. A pair of linear-tracking arms traversed the disk, one on each side, meaning that the whole record could be played without stopping and turning it over. The whole system was mechanically and electronically exceedingly complex, and while it worked, the system as a whole was aimed at the mass market and had only mediocre sound quality. The large physical size of the hinged door made it vulnerable to damage, and the retraction motor was barely able to lift its weight, especially after some years of use.

Related Topics:
1980s - Music centre

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