Several approaches to digital conversion of the video signal may be made. First, a very high speed A/D may be employed doing a line by line conversion and dumping into a digital memory. Such A/Ds are not too expensive when operating in a reduced resolution, reduced grayscale, format such as 128x128x4 and, in fact, you can get a complete package including a buffer memory in the Robot Research Model 400 Ham Slow Scan TV Converter. A note of caution, however, 4 bits of grayscale, or 16 levels, will normally not give completely satisfactory quality due to a phenomenon known as "contouring" which gives a distinctly artificial appearance to the reproduced image and frequently causes a loss of important detail. A 5 bit system is preferable for naturalness.
A second form of A/D conversion uses relatively slow speed sampling, essentially taking one sample per TV line which allows approximately 60 microseconds to make an A/D conversion. This is a very effective and economical approach to achieve both high resolution and extended grayscale.
With an actual minicomputer based system, a wide range of programming variations may be used to alter the end picture product. For example, pictures may be individually titled and dated, with perhaps a few pertinent comments added. One system uses a computer generated border, and other variations include multiple images, strip pictures, giant pictures (made by gluing two or more strips together), and deliberate tone reduction or "posterization" which reproduces an image having from two to four shades of gray.
Most systems in the field appear to employ conventional line printers such as manufactured by Centronics and Printronix, frequently with slight modifications to achieve closer character spacing. A variety of papers and ribbons are available for use with this type of printer, including sublimating inks for heat transfer processing. More expensive units such as the electrostatic printer-plotters can produce appreciably higher resolution images for a given paper area. It is also feasible to use direct CRT/hard copy output devices but, unfortunately, the dry silver paper used in this process has some disadvantages, namely limited size, high expense, and a fugitive image quality when exposed to intense illumination such as direct sunlight.
As in any computer based system, a large number of variations may be anticipated. These include not only ingenious programs but the use of accessories such as floppy disc memories for reference picture storage or customer reorders. Two or more TV cameras may be used with combined signals and special effects equipment to expand graphic input capabilities. For the serious operator, picture output may be produced photographically in
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