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Turntable drive systems

Introduction

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, wear and decomposition of the wheel, as well as the direct mechanical coupling to a vibrating motor, introduced low-frequency noise ("rumble") and speed variations ("wow and flutter") into the sound. These systems generally used a synchronous motor which ran at a speed synchronized to the frequency of the AC power supply. Portable record players typically used an inexpensive shaded-pole motor. At the end of the motor shaft there was a stepped driving capstan; to obtain different speeds, the rubber idler wheel was moved to contact different steps of this capstan. The idler was pinched against the bottom or inside edge of the platter to drive it. The idler-wheel drive was the most common on turntables, except for higher-end audiophile models.

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. Many belt-drive turntables having multiple speeds used a simple mechanical system to change speeds, using a mechanism to move the belt between different-sized pulleys on the motor shaft. For electronic speed control, it is difficult to design multiple-speed synchronous motors; consequently, DC servomotors with electronics providing speed control have gained favor. 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 or printed around their edge to provide optical pulses to these speed-control systems. Viewing these markings in artificial light at mains frequency produces a stroboscopic effect, which can be used 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. Problems with belt instability and deterioration have largely been solved by use of modern elastic polymers.

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 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.