Rotational Stability
This is an absolute goal of audiophile turntables, many of which are belt-driven. Problems with belt instability and deterioration in the past have largely been solved by use of modern elastic polymers.
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. It is difficult to design multiple-speed synchronous motors whose speed can be controlled electronically. Consequently, turntables with electronics providing speed control are nowadays most frequently used with DC servomotors. 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. On some designs, the manufacturer prefers to ensure pitch stability by generating its own sinusoidal waveform to power the motor. Others attempt to achieve this by using optical sensors on the platter which feed back to the electronics to ensure the speed of the platter remains stable.
Read more about this topic: Belt-drive Turntable
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