 from point to point (Fig. 4-9) on the screen. The storage-tube or DVST type display is a relatively complex structure, but the cost of the tube is balanced by savings in the sup- port electronics (Fig. 4-10). There is no constraint in the amount of time required to trace an image. The amount of data that must be stored for quick-access refresh is reduced to a minimum, yet ex- tremely complex images can be created. Raster type graphic displays draw lines as a series of pixels, which X2 Y2 X3 Y3 X4 Y4 Fig. 4-9. A stroke writing sequence for displaying graphics. TERMINAL TECHNOLOGY 63 Terminal controller Keyboard Digital to analog Direct viewing storage tube converters Fig. 4-10. A typical DVST graphics system. makes diagonal lines appear as stairsteps and consequently limits res- olution. Raster terminals are generally less expensive than stroke- writing types and they provide full-color capabilities. The informa- tion on the raster screen is typically refreshed from a "bit map" where each bit in the map, which may be several bits deep to provide color, corresponds to a picture element (pixel) on one of the scan lines. See Table 4-1 for a comparative summary of the various types of graph terminal tradeoffs. Table 4-1. Comparison of Graphic Display Technologies GRAPHICS SCREEN DISPLAY COLOR DEVICE QUALITY DYNAMICS CAPACITY QUALITY Stroke writer Excellent Excellent Storage Excellent Excellent Raster Excellent Excellent Excellent 4.4 FACSIMILE TECHNOLOGY In many business applications, facsimile is used to replace long- winded and costly phone calls. In this technology, the thought that goes into communicating occurs before transmission. Messages and 64 TELECOMMUNICATIONS sketches are composed before rather than during telephone "con- versation." Facsimile may also be employed in place of teletype mes- sage transmission or as a higher speed alternative to the mails. Such use can replace typing operations (either for teletype or the mails), where hand written memos and sketches serve as input documents. One of the key 