TAPE RECORDER

Typical Number in Hospital: 20

Cost Bands: 1,2

References: 3

These store information by magnetizing small particles on the surface of a plastic tape on open reels, on reels in cassettes or cartridges, or the same principle may be applied on discs, drums or strips (e.g. magnetic credit cards). There are several types of tape recorder:

1. Direct recorders (DR). The tape is drawn past a metal or ferrite ring which has a small gap in it at its point of contact with the tape. The ring is subjected to a varying magnetic flux (the signal) by a coil wound round it so that the tape is magnetized to an extent corresponding to the signal amplitude. On replay the tape passes the same head (usually) and induces current in the coil corresponding to the extent of tape magnetization. Unfortunately the current induced depends on the rate of change of magnetism passing the head and so slow changes are lost, and also very rapid changes are lost due to different effects connected with the width of the head gap and the tape speed. Thus direct recording machines are only suitable for a limited range (e.g. the audible frequencies). This frequency range can be extended by rotating the tape head at high speed using the method employed in video recorders.

2. FM (frequency modulated) recorders. The recording of low frequencies and steady signals can be achieved by representing the voltage to be recorded as the frequency of a tone which is recorded in the normal way on a direct recording machine. On playback the tone is converted back into voltage, the higher the frequency the higher the voltage. Such recorders are useful for logging relatively slow-moving signals such as physiological pressures. The frequency response of FM recorders depends on the tape speed and width of the head gap. A steady tape speed is essential since this affects the frequency of the tone when replayed.

3. Digital recorders. Signals may be recorded by representing the signal as a series of numbers in binary code on the tape. Such recording is relatively independent of variations in tape speed and the principle is used extensively in computers.

4. Video recorders. These are direct recording machines with very high bandwidth sufficient to register television signals (approximately 5 MHz). There are several methods in use (e.g. Umatic, VHS, Betamax) which are mutually incompatible.