SCAN CONVERTER

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References: 1

This is a device to change the scanning system of a video (e.g. TV) signal. They were developed for converting television recordings made in one country for showing in another, but are used in medical ultrasound scanners to convert the ultrasound scanning system to a TV video format. There are two main types of scan converter.

The analogue type takes the form of a television tube and television camera tube connected end to end so that an image written on to a screen between the two sections of the tube can be presented at one line and frame rate, and read off by the camera portion of the tube at a different rate. The image is not actually formed as a visible image but is merely an electrostatic image on a silicon plate.

The second, and now more common type, is the digital scan converter which has a large computer memory having one memory location for every picture element (pixel). The memory is written to, and read from, at the two different scan rates.

A major drawback with early ultrasonic B-scanners lay in the image storage mechanism. Bi-stable storage cathode ray tubes were used which could only produce a line image (i.e. black and white with no intermediate shades). Since part of the diagnostic information is contained in the differing amplitudes of the echoes from different tissues, the inability to display this as variations in brightness meant that there was some loss of diagnostic information. The analogue scan converter solved this problem since it could produce an image of wide dynamic range (low contrast) which could be frozen.

It has since been found that scan converters are also useful with moving picture scanners to provide a 'frame freeze' facility. The moving image can be frozen at will, or according to a synchronizing signal (e.g. ECG) and the resulting still picture can be photographed or measurements taken. The digital scan converters have completely supplanted the analogue types for this work because of their stability and flexibility. Being computer compatible, supplementary programs can be run to produce image improvements, area measurements, and alpha-numeric information on the display. Scan converters usually produce a television-type signal which can be viewed on one or more monitors and can be recorded on videotape.