FM TAPE RECORDER

Typical Number in Hospital: 2

Cost Bands: 4

References: 3

Ordinary direct recording (DR) tape recorders cannot record continuous or slow-moving signals and are therefore unsuitable for recording most biological signals. This problem can be overcome by using a carrier signal. In an FM (frequency modulated) recorder this is a tone which has a frequency which can be recorded on a direct recording machine (e.g. 1 kHz), and the signal to be recorded causes the frequency of the tone to change. For instance an input signal of 1 V may cause the tone to move to 2 kHz and a 2 V signal may move it to 3 kHz. When the recording is played back, a frequency to voltage converter will decode the signal, back into the original voltages, for off-line analysis or for demonstration purposes. Clearly the played back frequencies must be a faithful reproduction of those presented during recording and this requires a very stable tape speed, or a compensating mechanism, so that variations in tape speed are identified and a correction factor applied to the output signals.

Biological variables such as pressures, ECG, EEG, blood flow, blood gas tension, etc. may be recorded on an FM tape recorder and replayed for analysis. FM tape recorders are less used than a few years ago because of the capability of some modern computing equipment to perform high speed analysis of signals in real time. They would be found in the physics and bioengineering departments, in some clinical research units and possibly in the intensive care department.