Browse Prior Art Database

Scanning System with Hierarchical Interlacing

IP.com Disclosure Number: IPCOM000082621D
Original Publication Date: 1975-Jan-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 4 page(s) / 54K

Publishing Venue

IBM

Related People

Touw, TR: AUTHOR

Abstract

In raster scanning systems such as those used in TV displays or scanning electron microscopes, it is common practice to use interlaced scanning, which allows slower frame rates without flicker than would be possible without interlacing. Commercial TV uses 2:1 interlacing (2 fields per frame); some scanning electron microscopes use interlace ratios of 3:1 or 4:1.

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Scanning System with Hierarchical Interlacing

In raster scanning systems such as those used in TV displays or scanning electron microscopes, it is common practice to use interlaced scanning, which allows slower frame rates without flicker than would be possible without interlacing. Commercial TV uses 2:1 interlacing (2 fields per frame); some scanning electron microscopes use interlace ratios of 3:1 or 4:1.

It is desirable to increase the interlace ratio, because this extends to lower frame rates the scanning rates at which flicker is not observed. This reduction in frame frequency, in turn, decreases the bandwidth required for a given number of lines per frame. With analog scanning circuitry, it becomes complicated and prohibitively expensive to use higher interlace ratios; with digital scanning circuitry it becomes practical to increase interlace ratios.

The extension to higher interlace ratios can become limited by the persistence of the display or the persistence of the observer's visual perception. At very high-interlace ratios (comparable with the number of lines per frame), a limiting factor is the fact that the individual fields become visible to the observer. The optimum interlace ratio must be determined by experiment for a given display size, brightness, number of lines, and observing conditions. For a display consisting of N lines, the optimum interlace ratio lies somewhere between 1:1 and (N/2):1.

Given a total number of lines N in the display and a chosen line interlace ratio R, an algorithm can be defined which will generate a corresponding sequence of line numbers to produce the desired line interlace ratio. The sequence of line numbers thus generated can be the input to a digital-to-analog converter whose output drives a frame-scan amplifier, which in turn drives the display. (In a scanning electron microscope, the same scan generator and scan amplifier that drive the display also drive the scanning electron beam in the frame direction, synchronously with the display). Fig. 1 illustrates the system.

In interlacing a display of N lines with interlace ratio R, the N lines are divided into R subsets (fields). Consider this the first level of hierarchy. In the second level of the hierarchy these R fields themselves may be interlaced at some interlace ratio (which need not be the same ratio R). This determines a time sequence of the fields. In a third level of the hierarchy the individual lines within each field may be interlaced at some interlace ratio, determining the time sequence of the lines within each field. Thus, the three levels of the hierarchy correspond to three steps which, taken together, define a complete line sequence for the frame: 1) dividing the frame of N lines into R fields of N/R (+/-1) lines each; 2) choosing a sequence of the R fields (the field sequence); and, 3) choosing a sequence of lines within each field.

A particular advantageous scanning pattern that can be produced by hierarchical scann...