Tape recorder
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In general, a tape recorder, tape deck, cassette deck or tape machine is any device that records a fluctuating signal by moving a strip of magnetic tape across a tape head, which is a strong electromagnet. Current flowing in the coils of the electromagnet cause the magnetic material on the tape to align in a manner proportional to the original signal. The signal can be reproduced by running the tape back across the tape head, where the reverse process occurs - the magnetic imprint on the tape induces a small current in the read head which approximates the original signal. This is then amplified for playback. Many tape recorders are capable of recording and playing back at once by means of separate record and playback heads in line or combined in one unit.
The storage of an analogue signal on tape works well, but is not perfect. In particular, the granular nature of the magnetic material adds noise to the signal, which is usually heard as tape hiss. Also, the magnetic characteristics of tape are not linear, they exhibit a characteristic hysteresis curve. The curvature causes unwanted distortion of the signal. Some of this distortion is overcome by using an inaudible high-frequency AC bias signal when recording, though the amount of bias needs careful adjustment for best results. Different tape material requires differing amounts of bias, which is why most recorders have a switch to select this (or switch automatically). Additionally, systems such as Dolby B and Dolby HX-Pro have been devised to ameliorate some of the noise and distortion problems. Variations in tape speed cause flutter, which can be reduced by using dual capstan.
There are a wide variety of tape recorders in existence, from small hand held devices to large multitrack machines.
An important use of tape recorders is the recording of video. Video cassette recorders differ subtantially from audio recorders because of the use of a rotating magnetic head that uses a helical scan of the tape medium. Helical scan is used to allow for faster movement of the tape surface over the head.
While they are primarily used for sound recording, tape machines were also important for data storage before the advent of floppy disks and CDs, and are still used today, although primarily to provide an offline backup to hard disk drives.