In analogue audio recording, a tachometer is a tool that measures the speed of audiotape as it passes across the head. On most audio tape recorders the tachometer (or basically "tach") is a comparatively huge spindle near the ERP head stack, isolated from the feed & take-up spindles by tension idlers.
On plenty of recorders the tachometer spindle is connected by an axle to a rotating magnet that induces a changing magnetic field on a Hall effect transistor. Other systems connect the spindle to a stroboscope, which alternates light & dark on a photodiode.
Having perfectly regulated tape speed is important because the human ear is sensitive to changes in pitch, sudden ones, & without a self-regulating method to control the speed of tape across the head the pitch could drift several percent. This effect is called a wow-and-flutter, & a contemporary, tachometer-regulated cassette deck has a wow-and-flutter of 0.07%.
The tape recorder's drive electronics use signals from the tachometer to be definite that the tape is played at the proper speed. The signal is compared to a reference signal (either a quartz crystal or alternating current from the mains). The comparison of the frequencies drives the speed of the tape transport. When the tach signal & the reference signal match, the tape transport is said to be "at speed." (To this day on film sets, the director calls "Roll sound!" & the sound man replies "Sound speed!" This is a vestige of the days when recording devices necessary several seconds to reach a regulated speed.)
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.
Tachometers are acceptable for high-fidelity sound playback, but not for recording in synchronization with a film camera. For such purposes, special recorders that record pilottone must be used..
On plenty of recorders the tachometer spindle is connected by an axle to a rotating magnet that induces a changing magnetic field on a Hall effect transistor. Other systems connect the spindle to a stroboscope, which alternates light & dark on a photodiode.
Having perfectly regulated tape speed is important because the human ear is sensitive to changes in pitch, sudden ones, & without a self-regulating method to control the speed of tape across the head the pitch could drift several percent. This effect is called a wow-and-flutter, & a contemporary, tachometer-regulated cassette deck has a wow-and-flutter of 0.07%.
The tape recorder's drive electronics use signals from the tachometer to be definite that the tape is played at the proper speed. The signal is compared to a reference signal (either a quartz crystal or alternating current from the mains). The comparison of the frequencies drives the speed of the tape transport. When the tach signal & the reference signal match, the tape transport is said to be "at speed." (To this day on film sets, the director calls "Roll sound!" & the sound man replies "Sound speed!" This is a vestige of the days when recording devices necessary several seconds to reach a regulated speed.)
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.
Tachometers are acceptable for high-fidelity sound playback, but not for recording in synchronization with a film camera. For such purposes, special recorders that record pilottone must be used..
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