During inspection in which spatial resolution and contrast are critical, analog information alone may be inadequate, and computer enhancement is required. Digital image processing, which is possible through desktop computers with the necessary internal components, is a solution. As indicated, the RS170 signal is an analog signal that must be converted to a digital signal before a computers digital imaging processor can work with it. This is referred to as an A to D converter, or an analog to digital converter. There are also D to A converters, or digital to analog converters. Digital codes are based on the binary number system having 2 as the base number raised to various powers, e.g., 200 = 1 21 = 2, 22 = 4, 23 = 8, 24 = 16, 25 = 32, 26 = 64, 27 = 128, 28 = 256, etc. Digital electronic devices differ from analog devices because analog values change continuously in regard to what is being represented. Digital devices change in discreet increments rather than continuously. The digital signal has two modes: it is either on or off. These are discrete values. Modern computers are digital because the signals have better quality and can transmit and store more information than analog systems can.
By use of an A to D converter, the RS170 signal can be digitized or changed to a digital value. Digital image processors are designed to work with 525 lines, 30 frames-per-second TV images. The computer hardware used to convert the signal is referred to as a frame grabber, which converts the signal as it is creating the frames. This converter also starts with the upper left-hand corner of the image. With an eight-bit converter, A to D conversion will take place and resolve the gray scales into 256 levels (28 = 256), a value of 0 being black and a value of 255 being white. The digitization process takes place line by line until all 525 lines are complete. The number of pixels for 525 lines is in many instances 640 horizontally and 480 vertically. The analog signal consists of gray scales: no color is involved and none produced as a result of digitization. Benefits of this process are that (1) some electronic noise detracting from the quality of the image is eliminated, (2) when the gray scale is resolved by 256 levels, more information can be obtained from the image than otherwise and (3) edge detail is improved, and so is spatial resolution. Pixel size influences resolution because each pixel represents the smallest bit of information available, i.e., an image made up of small pixels will show greater resolution than an image made up of large pixels. More information on A to D conversion can be found in Appendix III