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COMPUTER BITS By JENNIFER BONNITCHA* A look at video monitors Every personal computer has a video monitor but very little is written on the many different types of monitor. All monitors work in much the same way as television screens but they are driven by digital instead of analog signals. IBM's entry into personal computing in the autumn of 1981 caused more than a little excitement. Prior to the PC's development, IBM's policy was that anything connected to its machines had to be an IBM product. Keeping the inner workings of their computers secret created for IBM a seemingly unassailable monopoly. In a landmark decision, the company decided to make available all the necessary technical details on the PC. Since Big Blue's logo means business to most people, the general perception of the personal computer changed almost overnight from that of a hobby machine to that of a serious business tool. Suddenly there was an army of small entrepreneurs eager to make peripheral products for the PC. Literally hundreds, probably thousands, of companies now exist because the PC does. However, it is also true to say that the PC and its compatibles sell because these businesses exist. Perhaps the most important peripheral for the PC is the monitor, through which the PC communicates with you. The continuing development of both monitor and adaptor boards means that the picture you see is becoming more and more detailed. The TV-like screen through which your computer communicates with you is called many names: display, CRT (Cathode Ray Tube), video monitor, screen, VDT [Visual Display Terminal) or simply terminal. It forms a necessary part of your computer since it provides visual feedback as you run your programs. Typically you make a choice from a menu or enter data at appropriate locations via the keyboard. The display you see on the screen is the result of some activity on the part of the program. So, now you've decided to put your carefully saved computer dollars down on a You Beaut Turbo XL computer. The next question is: What monitor do you put on top of it? Answer: that depends. It depends on what you need your * Jennifer Bonnitcha is an independent consultant and has a wide experience of computers ranging from mainframes to personal computers and peripheral devices. She is also experienced in the use of integrated and spreadsheet software, word processing and desktop publishing. Jennifer is the founder of Business Action, a company specialising in on-site training in the use of personal computers for business applications. 70 SILICON CHIP computer to do, your personal tastes, and of course the bottom line - how much you have to spend. At one end, you could get a standard low-priced monochrome monitor suitable for your network server. At the other end, you can get 50cm or so of high-powered resolution and colour, suitable for graphics or CAD. How monitors work Monitor jargon is as daunting as any in the field, with plenty of acronyms, like MDA, CGA, EGA and VGA, and technical specifications like resolution, scan frequency and dot pitch. All video monitors function like a television set in that an electron beam is swept across and down the screen by the deflection circuitry. However, instead of having a continuous picture made up of interlaced lines, the image produced by a monitor comprises a large number of small dots in fixed position. The dots are arranged in horizontal rows (corresponding to the horizontal scan lines) and the term resolution refers to the number of points across the screen and the number of rows (or lines) up and down the screen. What it all ultimately comes down to is dots on the screen, or picture elements (pixels for short). Therefore a high-resolution monitor implies that the number of pixels is larger than in medium or low resolution monitors. Higher resolution means a more detailed image on the screen. All the specifications ultimately say are how many dots the screen has and the acronyms represent standard ways to create the dots. High resolution monitors are not confined to IBM machines. These two monitors are designed to suit the Apple Macintosh. The larger of the two is a nominal 21-inch unit with 1152 x 870 dots of resolution. Its refresh or vertical scan rate is 75Hz. Initially, video monitors used the American NTSC TV standard as the basis of their operation. This means a field frequency of 60Hz and a line (or horizontal scan) frequency of 15,750Hz. Higher resolution monitors use higher scanning frequencies so they can put more dots on the screen. The image is generated by an electron beam sweeping over the screen one line at a time while the intensity is varied. For text or graphics displays which are made up of dots all having the same brightness, the electron beam intensity is switched on and off - on for a dot, off at all other times. Each line is scanned left to right with the intensity of the beam determined by the data stored in the video RAM of the display board. In all but the simplest displays, the intensity can be varied rather than simply being on or off. At the completion of each line, the beam rapidly returns or "retraces" to the beginning of the next line. During this retrace period, the beam is turned off so that it is invisible. When the screen sweep is complete, the deflection coils around the neck of the picture tube move the electron beam back to the top left-hand corner of the screen ready to scan the next screen. So far we have described the operation of a monochrome monitor which has one single electron beam. The actual colour of the screen display is determined by the internal phosphor coating of the picture tube, giving green, amber or white. Colour monitors A colour monitor has three beams sweeping across the screen in unison. By means of a shadowmask inside the tube face, each beam is arranged to strike only its appropriate small dots of red, green and blue phosphor on the surface of the screen. So the red electron beam only excites the red dots, the blue beam excites the blue dots and the green beam the green dots. Thus you get colour and, depending on the intensity of the beam, different degrees of colour brightness. Monochrome Display Adaptor The IBM monochrome display uses a 9 [wide) x 14 [high) dot matrix to form the full range of ASCII characters which comprise 256 different letters, numbers and special characters. Alas, graphics are not supported. Upper and lower case letters are formed from 7 x 9 dots in the 9 x 14 matrix; a further 2 vertical dots are used by the character descenders [the hanging parts of letters such as g's and p's etc). The monitor requires the installation of a Monochrome Display Adapter [MDA) card to display 25 lines of 80 characters in alphanumeric mode [or text) mode. When you type a character on the keyboard, two bytes carry the information to the adaptor. The first byte is an ASCII code [between 0 and 255) defining the character shape. The second byte is called an attribute code and it determines JULY 1989 71 white or green, red and brown). In graphics mode at high resolution (640 by 200 pixel mode) only black and white are available, since it requires all of the adapter card's memory to define the on or off state of each pixel. Enhanced Graphics Adaptor In 1986 IBM released the Enhanced Graphics Adaptor (EGA) monitor, giving 640 x 350 pixels and more colours at the higher resolution. EGA has become the dominant graphics standard although plenty of software is still written for CGA (particularly games). The EGA display adapter supports the foregoing display standards while in monochrome and colour mode it supports a 640 by 350 pixel resolution and up to 64 colours (from the varying intensity of the signal). The monitor has excellent clarity for both text and graphics. MultiSync & VGA For any serious application involving graphic displays you need an EGA (enhanced graphics adapter) monitor. Naturally, for CAD or desktop publishing work, the bigger the monitor, the better. how the character appears (green on black background or reversed out of green etc). The resulting display is very sharp and easy to read. Since the IBM monochrome display has the standard text mode of 25 lines by 80 characters and each character is formed in a matrix of 9 x 14 dots, this means that its resolution is 720 dots (9 x 80) across by 350 (25 x 14) dots down. Colour Graphics Adaptor The IBM colour monitor enables operation in either black and white (monochrome) or colour. The Colour Graphics Adaptor (CGA) permits two basic modes of operation: alphanumeric (text) and graphics [also known as all points addressable or APA). This graphics mode allows any point on the screen to be directly defined 72 SILICON CHIP however the display is limited in detail by the resolution of the screen. Within each mode, several display character widths or pixel sizes are available for selection. The monitor uses a 5 by 8 dot matrix letter inside an 8 by 8 dot box with only one dot for the descenders. Consequently text isn't displayed quite as clearly as on a monochrome monitor but the graphics are quite acceptable. In text mode the display can operate in 80 character by 25 line mode. Colours are sharp with two graphics resolutions available medium resolution of 320 by 200 pixels, and high resolution of 640 (80 x 8) by 200 (25 x 8) pixels. In the 320 by 200 pixels graphics mode each pixel can have one of four colours. One of sixteen background colours may be used then three other colours selected from one of two palettes (cyan, magenta and The MDA, CGA and EGA graphics adapters were all designed for connection to TTL monitors and, for this reason, most manufacturers produce monitors which only accept this type of signal. Two of the newer (and consequently more expensive) types of monitors are the MultiSync and Video Graphics Array (VGA) displays. The MultiSync is a high resolution, colour monitor which provides crisp text and vivid colour graphics. The beauty of the MultiSync is its ability to automatically adjust to graphics board scanning frequencies from 15.5kHz to 35kHz with a maximum horizontal resolution of 800 dots and a maximum vertical resolution of 560 lines. The monitor can cope with any of the colour video standards, both digital and analog. While the MultiSync monitor copes well with horizontal frequencies, the vertical frequencies are set to that used by the CGA and EGA monitors - 60Hz. Adjustment of the vertical hold control should, however, adjust the monitor sufficiently to handle the 70Hz rate used for the latest type of monitor, the VGA. VGA is now emerging although TABLE 1: VIDEO MONITOR STANDARDS LOOK AT THESE BARGAIN PRICED KITS ·- Graphics Standard Video Bandwidth Horizontal Scan Vertical Scan OUR LATEST UHF REMOTE CONTROL MDA 720 X 350 16.257MHz 18.43kHz 50Hz X 200 14.318MHz 15.75kHz 60Hz CGA 640 Prices listed apply till August. EGA 640 640 X X 350 200 16.257MHz 14.318MHz 21.85kHz 15.75kHz 60Hz 60Hz 720 X 350 16.257MHz 18.43kHz 50Hz (EA JAN 89 and APRIL 89) • • • COMPLETE KITS TRANSMITTER & RECEIVER ONLY $49.90 Extra Transmitter Kits $17 .OD ea PASSIVE INFRARED MOVEMENT DETECTOR (EA MAY 89) • VGA • 640 720 X 640 X 640 X X 400 400 480 25.175MHz 28 .322MHz 25.175MHz 31.50kHz 31 .50kHz 31 .50kHz 350 25.175MHz 31.50kHz 70Hz 70Hz 60Hz 70Hz High Security, Low Power Consumption. Switch and Indicator Relays. Up to 50 metre range. Uses Special Dual Element Pyroelectric Detector. Kit includes two Lenses. SPECIAL BARGAIN PRICE $34.95 Interface Components $7 .95 extra. 16 CHANNEL UHF REMOTE CONTROL (EA NOV 88 and MARCH 89) • Package including Transmitter, Receiver and Four Channel Relay Driver, plus indicator. SPECIAL TILL END OF AUGUST EGA remains the defacto standard in the IBM compatible world. Most VGA and EGA cards are downward-compatible with the earlier CGA and MDA standards. Colour used to be a luxury which most software did not take advantage of but the demands of many products are making colour monitors increasingly prevalent. Because it is analog rather than TTL, the VGA monitor can provide up to 256,000 colours (although only 256 at any one time). The character size is 8 pixels horizontally by 16 vertically. Taking a step back, the question remains: what should you get? Again, it depends on what your needs are. If all you have in mind is word processing and spreadsheet use, an inexpensive monochrome monitor with reasonable resolution should do fine . But for more complex software, particularly in CAD applications, high resolution colour displays are essential. In fact, to use most CAD packages effectively, you need a large screen of say, 50cm or more. These cost really big dollars. That is probably enough on the subject of video monitors for the moment. Next month, we will have a look at the various graphics adapter "standards" available. ~ $119.00 THE MICROPHONE (EA NOV 86) • • • Professional Quality Up to 1 V RMS Output, Drives any Amplifier , even HI-FI Line inputs. Touch Control. COMPLETE KIT $24.99 MAINS MUZZLER (SC JAN 89) Short Form kit similar to the Mains Muzzler including the Torroid, 3x0.022uF/ 400V metallised polyester capacitors, 1M ohm 1W resistor and a 275V/40joule Varistor THE PRICE? ... ONLY $7 .40 That's less than the price of the Varistor! ULTRASONIC MOVEMENT DETECTOR/ALARM • • • • High Quality Crystal Controlled Kit. Prewired Transducers. PCB & Components plus ABS Case. Relay Output. COMPLETE KIT $42.95 BANKCARD, MASTERCARD and VISA ACCEPTED WITH PHONE ORDERS. P&P FOR ANY OF THE ABOVE $2.50. OATLEY ELECTRONICS MAIL: PO Box 89, Oatley, NSW 2223 SHOP: 5 Lansdowne Pde, Oatley West. PHONE: (02) 579 4985. RCS Radio Pty Ltd is the only company which manufa_ctures and sells every PCB f, front panel published in SILICON CHIP, ETI and EA. You can also buy some of our Kits at the following Distributors. Slightly higher prices may apply. NEWCASTLE MELBOURNE - 651 Forest Road, Bexley, NSW 2207 Phone (02) 587 3491 for instant prices NOVOCASTRIAN (049) 62 1358 ELECTRONICS WORLD (03) 723 3860 WOLLONGONG - ITEC (042) 26 4044 JULY 1989 73