MEMORY OSCILLOSCOPE

Typical Number in Hospital: 60

Cost Bands: 4

References: 3

To display a graph, or a signal which varies with time (e.g. ECG or arterial pressure) it is now usual to employ a short- persistence CRT in conjunction with a simple computer memory. The memory is usually a 'first in first out' (FIFO) set of shift registers operating rather like a tape loop. The contents of the memory are read repeatedly on to the screen giving a frozen display, or if new information is being added to the memory this is added at one end of the line so the whole line appears to be moving slowly backwards or forwards. The older information is being discarded from the other end of the line.

Memory 'scopes have almost totally replaced long-persistence CRTs ('bouncing ball' or 'comet's tail' types) for ECG and blood pressure display.

The same principle can be used to form a whole image (as opposed to a single line) on the screen for ultrasonic B-scan display, and for images formed during isotope scanning. However, the memory requirements are much larger (e.g. 512 x 512 picture elements) and in the case of ultrasonic scanning the required speed of memory access is near to the practical limit. In these applications the memory is also used to convert the scanning method to a (TV) video format.