Sunday, February 26, 2017
PC monitor technology and interfaces
PC monitor technology and interfaces
Monitors general information
PC monitor system is complicated system, but luckily for us its one thats easy to understand. The following description is centered on traditional analogue CRT PC monitors.
The video adapter in PC sends the signals from its image memory at fixed rate (usually configurable) through the DAC (digital to analog converter) circuit to the monitor connector on the graphics card. The DAC converts numeric pixel color values to voltage levels for red, green, and blue which are sent to monitor through the monitor cable. Most monitors today use the traditional CRT, which works on the same scientific principle as a television set. This vacuum tube produces an image when an electron beam strikes the phosphorescent surface inside the monitor. Normal PC VGA monitors nowadays are so called "non-interlaced" monitors. The computer requires a "video Interface" sometimes referred to as a video card to communicate with your monitor. Your monitor is the single most important component of your computer system if you want to get good picture quality (also the graphics card can contribute to this).
The visual quality, depends on the quality of your monitor. Consumers have now become more concerned about the visual quality. The flat screens, high resolution, high refresh rates, and recently the USB and solid state screens top the list of desirable features. The multimedia monitor includes loudspeakers of some sort, maybe a microphone and in some cases a camera for video conferencing all in the same box as the monitor.All analog monitors can produce thousands of colors, it is inherent in the design. The limitation on color registration is directly related to what is available in the interface card and the mode selected. There are practically infinite number of colors possible with the analog monitors (although they can not properly display all natural colors correctly).
Resolution is the number of pixels the graphics card is describing the desktop with, expressed as a horizontal by vertical figure. Standard VGA resolution is 640 x 480 pixels. The commonest SVGA resolutions are 800 x 600 and 1024 x 768 pixels. A typical PC monitor is designed to accept signals at wide resolution and frequency range. When you change the resolution on refresh rate on monitor, just the scan frequencies that are changing to accommodate the new timing/pixel format. The focus (which is sort of the electron beam width, at least as it is seen at the screen) MAY be altered slightly as well, if the monitor has the capability of storing adjustments for that and other parameters (geometry, convergence, etc.) for specific timings, although it is VERY unusual for focus to be included in this. Please not that the monitor physical dot pitch cant change - thats a fixed physical parameter of the CRT itself - but the physical dots on the screen (or the holes in the shadow mask) really have nothing at all to do with the logical pixels of the image, other than being one of the things which ultimately limits the resolution. The scan frequencies do not necessarily change at all when you change the resolution. What happens is that the signal as seen on the VGA plug has (for example) 1024 discrete values between 2 consecutive line syncs as opposed to 800 discrete values and 768 line syncs between frame syncs as opposed to 600 (assuming non interlaced). Typical PC CRT monitor an display all resolutions from the lowest up to the highest supported resolution well. If you have a modern flat panel display, things can be different. On TFT monitors they specify a "recommended" resolution that the TFT works best at and when not run at this resolution they get seriously blocky and in some cases unreadable text.
Refresh rate, or vertical frequency, is measured in Hertz (Hz) and represents the number of frames displayed on the screen per second. Too few, and the eye will notice the intervals in between and perceive a flickering display. The world-wide accepted refresh rate for a flicker-free display is 70Hz and above (preferably 75 Hz or more). The flicker is strongly dependent upon visual angle, because eye peripheral vision response is faster than the higher resolution center of field vision. The bigger the monitor, or the closer you are to it, the worse the flicker will be, so you will need higher refresh rate to get "flicker free" picture. CFF (Critical Flicker Fusion) also depends on illumination levels. The CFF frequency is lower at lower illumination levels. As the height of the picture increases, it is necessary to increase the number of horizontal lines to create a smooth line-free display image. To do this, the monitor and the interface card increase the frequency of the repetitive horizontal scan rate.
In order to consistently reproduce the video information at a high resolution, the monitor must have a wide video bandwidth. In order for the term to be meaningful for comparison purposes, the bandwidth expressed in mhz. must be within +- 3dB You might see a term "sync signal" sometimes.All computer monitors require a "sync" signal which determines the resolution of the display. Some monitors require the sync signal to be a separate electrical connection, some monitors require the sync signal to be mixed in with the green video signal (sync on green). Some monitors support both separate sync and sync on green. PC VGA card uses separate sync signals and PC monitors are designed to accept at least this sync format.
The term "dot pitch" is the measurement in millimeters of the distance between two adjacent phosphor color elements. There are two color phosphor systems in use today in CRT monitors: triad dot shadow mask (most monitors) and aperture grille (used in the trinitron tube from SONY). NEC has developed a hybrid mask type, called slotted mask, which uses elliptically-shaped phosphors grouped vertically and separated by a slotted mask.
Here are some guidelines for suitable resolutions for different monitors:
- 14 inch monitor is adequate for 800 x 600 resolution.
- 15 inch monitor is adequate for 1024 x 768 resolution.
- 17 inch monitor is adequate for 1024 x 768 resolution.
- 19 inch monitor is adequate for 1280 x 1024 resolution.
- 21 inch monitor is adequate for 1600 x 1280 resolution.
If you use a higher resolution exceeding these guidelines, a very good monitor may deliver adequate pictures but you can also run into a poor quality picture. Keep in mind what frequencies and resolution your monitor can handle. Trying to use frequencies and resolutions that the monitor was NOT designed to support can severely damage your monitor. To make the monitor installation easy, VESA has produced several standards for plug-and-play monitors. Those standard features (like DDC) should in theory allow your system to figure out and select the ideal settings, but in practice this very much depends on the combination of hardware.
Here is an overview of different video display resolution standards and de-facto standards in use (not all of them used in PCs):
| Computer Standard | Resolution |
|---|---|
| VGA | 640 x 480 (4:3) |
| SVGA | 800 x 600 (4:3) |
| XGA | 1024 x 768 (4:3) |
| WXGA | 1280 x 768 (15:9) |
| SXGA | 1280 x 1024 (5:4) |
| SXGA+ | 1400 x 1050 (4:3) |
| WSXGA | 1680 x 1050 (16:10) |
| UXGA | 1600 x 1200 (4:3) |
| UXGAW | 1900 x 1200 (1.58:1) |
| QXGA | 2048 x 1536 (4:3) |
| QVGA (quarter VGA) | 320 x 240 (4:3) |
| Analogue TV Standard | Resolution |
| PAL | 720 x 576 |
| PAL VHS | 320 x 576 (approx.) |
| NTSC | 640 x 482 |
| NTSC VHS | 320 x 482 (approx.) |
| Digital TV Standard | Resolution |
| NTSC (preferred format) | 648 x 486 |
| D-1 NTSC | 720 x 486 |
| D-1 NTSC (square pixels) | 720 x 540 |
| PAL | 720 x 486 |
| D-1 PAL | 720 x 576 |
| D-1 PAL (square pixels) | 768 x 576 |
| HDTV | 1920 x 1080 |
| Digital Film Standard | Resolution |
| Academy standard | 2048 x 1536 |
In the late 1980s concern over possible health issues related to monitor use were raised. In Sweden this resulted a standard MPR1 to be developed. This was amended in 1990 to the internationally adopted MPR2 standard, which called for the reduction of electrostatic emissions with a conductive coating on the monitor screen. In 1992 a further stricter standard, entitled TCO (TCO92), was introduced by the Swedish Confederation of Professional Employees. Other relevant monitor safety standards include: ISO 9241 part 3 (the international standard for monitor ergonomics), EN60950 (the European standard for the electrical safety of IT equipment) and the German TUV/EG mark (monitor has been tested to ISO 9241 part 3, EN60950, MPR2 and German standard for basic ergonomics ZH/618). TCP99 is the latest iteration of the standard TCO99 give regulations on screen refresh rates. To reduce eye fatigue caused by image flicker, the minimum required refresh rate is increased to 85Hz for displays of less than 20in, with 100MHz recommended, and to a minimum of 75Hz for 20in or greater.
Power consumed by the monitor can also be a significant figure. In 1993, VESA initiated its DPMS standard, or Display Power Management Signaling, which allowed a DPMS compliant graphics card to turn the monitor to standby more or suspend modes which consume considerably less power than normal operation. EPA Energy Star is a power saving standard, mandatory in the US and widely adopted in Europe, requiring a mains power saving mode drawing less than 30W. In 1995, TCO was expanded with a range of conditions to cover environmental issues. TCO95 became the first global environmental labeling scheme. Over and above TCO92, the product may not contain cadmium or lead, the plastic housing must be of biodegradable material and free of brominated flame retardants and the production process must avoid use of CFCs (freons) and chlorinated solvents.
Monitor interfaces
Analogue VGA interface
- 1 Red Video
- 2 Green Video
- 3 Blue Video
- 4
- 5
- 6 Red Return (ground)
- 7 Green Return (ground)
- 8 Blue Return (ground)
- 9
- 10 Sync Return (ground)
- 11
- 12
- 13 Horizontal Sync
- 14 Vertical Sync
- 15
In addition to video signal, the VGA connector has some monitor identification pins (pins 11, 12 and 15) that allow PC video cards to determine what type of monitor is connected to the graphics card. The original plan used to such that the monitors grounded some of those pins to tell that the monitor is there and what type of monitor is there. Modern plug&play monitor systems have changed their use in such way that pins 11 and 15 are used for DCC data communications between computer and monitor (pin 12 = DDC DATA, pin 15 = DCC Clock). The extra control signals are generally carried through separate wires (all inside one cable main shield). Pin 5 is sometimes referred as GND TEST and sometimes just ground.Pins 4 and 9 are not generally used. Not all connector pins are used in VGA cables. Generally pins 9 has been removed because is is used in many devices as key to stop plugging in full 15 pin connectors. In some scales also pin 15 have been removed for compatibility with all VGA computers (also older ones, because pins 9 and 12 pins were removed in early VGA cables and blocked in old VGA cards).Here is one common wiring used:
Pin New VESA DDC Old VGA1 Red Red2 Green Green3 Blue Blue4 No Connect Reserved5 Ground Ground6 Ground Red Ground7 Ground Green Ground8 Ground Blue Ground9 No Connect No connect10 Ground Ground for syncs11 No Connect Monitor ID 0 (ground)12 DDC DAT Monitor ID 1 (no connect)13 Horizontal Sync Horizontal Sync14 Vertical Sync Vertical Sync15 DDC Clock No Connect
The video signals carried in VGA connector are designed to be matched to 75 ohm load and use coaxial cable. At least the RGB signals on the cable used to connect VGA signals must have 75 ohm coaxial construction to guarantee good quality high resolution image. A typical high quality VGA monitor cable or VGA extension cable has three three 75 ohm mini coax cables to carry RGB signals and 9 other wires (typically 24 AWG) to carry other signals like syncs and monitor identification. The whole cable has a good metallic shield around all of those wires. Some very high quality cables use five 75 ohm coaxial cables and for RGBHV signals and just few extra wires. In applications where monitor ID signals are not needed, just five 75 ohm coaxes are used to transfer VGA signals. Typical features of flexible mini coax cables (from http://www.drakausa.com/pdfsHHT/AVprcise.pdf for extra flexible miniature coax):
1 MHz: 0.6 dB/100ft5 MHz: 1.3 dB/100ft10 MHz: 1.8 dB/100ft30 MHz: 10.2 dB/100ft100 MHz: 17.1 dB/100ft
Some attenuation figures of high quality VGA extension cable (for reference of typical features):
10 MHz: 1.6 dB/100ft50 MHz: 4.0 dB/100ft100 MHz: 6.1 dB/100ft200 MHz: 9.8 dB/100ft300 MHz: 13.0 dB/100ft400 MHz: 15.9 dB/100ft
The connector uses in VGA connections is HD 15 connector. This ubiquitous connector is convenient, low cost, and most importantly, adopted by IBM, but technically not the best possible connector. The connector was originally selected to be good enough for the signals existing in the early days of VGA interface and was more than good enough for this use, but VGA connector has its limitations at high resolutions. Does anyone know the impedance of a 15-pin VGA connector? Unfortunately the HD15 connector used does not match to 75 ohm impedance (in reality the impedance of a typical VGA connection is about 100 ohms). Even though the connector impedance is not exactly right, the primary issue centers on the limited length of the connector interface, so it does not significantly hamper performance in systems we most often deal with.This HD15 connector is still used, because this ubiquitous connector is convenient, low cost, and most importantly, adopted by IBM. It is still with the limitations considered "good enough". And in practice one VGA connector on the route from the graphics card to the monitor does not cause too much problems for picture quality. The primary issue for this centers on the limited length of the connector interface. Because of the limited length, it does not significantly hamper performance in systems we most often deal with. Because there is no no significant effects, hence the popularity of the VGA connector as a low-cost, general interface for the PC even nowadays. The problems of connector impedance mismatch becomes visible if you happen to have more than one VGA connector on the route to monitor and you run high frequency video signal (high resolution at high refresh rate). Impedance mismatch degrades the picture quality. You can see the impedance matching problems usually when you use devices like VGA monitor switch boxes, VGA extension
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