Editing Tachometer (section)

==In analogue audio recording==

In [[sound recording|analogue audio recording]], a tachometer is a device that measures the speed of [[audiotape]] as it passes across the head.  On most audio tape recorders the tachometer (or simply "tach") is a relatively large spindle near the [[ERP head stack]], isolated from the feed and take-up spindles by tension idlers.

On many recorders the tachometer spindle is connected by an axle to a rotating magnet that induces a changing [[magnetic field]] upon a [[Hall effect]] [[transistor]].  Other systems connect the spindle to a [[stroboscope]], which alternates light and dark upon a [[photodiode]].

The tape recorder's drive electronics use signals from the tachometer to ensure that the tape is played at the proper speed.  The signal is compared to a reference signal (either a [[quartz clock|quartz crystal]] or [[alternating current]] from the [[Mains electricity|mains]]).  The comparison of the two [[frequency|frequencies]] drives the speed of the tape transport.  When the tach signal and the reference signal match, the tape transport is said to be "at speed."  (To this day on film sets, the [[film director|director]] calls "Roll sound!" and the sound man replies "Sound speed!"  This is a vestige of the days when recording devices required several seconds to reach a regulated speed.)

Having perfectly regulated tape speed is important because the human ear is very sensitive to changes in pitch, particularly sudden ones, and without a self-regulating system to control the speed of tape across the head, the pitch could drift several percent.  This effect is called a [[wow (recording)|wow]]-and-[[Flutter (electronics and communication)|flutter]], and a modern, tachometer-regulated [[cassette deck]] has a wow-and-flutter of 0.07%.

Tachometers are acceptable for [[high-fidelity]] sound playback, but not for recording in synchronization with a [[movie camera]].  For such purposes, special recorders that record [[pilottone]] must be used.

Tachometer signals can be used to synchronize several tape machines together, but only if in addition to the tach signal, a directional signal is transmitted, to tell slave machines in which direction the master is moving.