Bandwidth and Sync ratesNow that we know how many dots are on the front of the CRT, it is time to determine whether or not the electron guns can hit all those dots fast enough from the back of the CRT. People generally like to think of computer displays in terms of resolution and refresh rates. A refresh rate of 60Hz (60 complete screen re-paintings per second) is considered the bare minimum. Slower than that, and the viewer percieves that the image is flickering, like an old silent movie. Refresh rates of 66Hz or 72Hz are preferable if you will be spending lots of time looking at the display. For most graphical applications, a display of 1024x768 pixels is also a minimum. The goal is to choose a monitor for a particular resolution and refresh rate. The following tables list the minimum video bandwidth and sync rates a monitor would have to support just to draw the dots on the screen for the given resolutions. As we will soon discover, there is additional bandwidth and time required above this to actually display such an image. 60 Hz Refresh rate res Pixels/screen Bandwidth Hsync Rate In addition to the pixel data signal, a monitor gets two other important signals: the Horizontal Sync Pulse (Hsync), and the Vertical Sync(Vsync). The Hsync pulse tells the monitor that it is time to stop drawing one horizontal line of pixels and get ready to start drawing the next. When the monitor sees the Hsync pulse it turns off the electron guns and repositions them to the begining of the next line. This takes a finite amount of time. The monitor will generally need a little time between the end of the pixel data and the begining of this pulse. This time interval is called the ``Back Porch'' in video terminology. It will also need some time between the end of the Hsync pulse and the begining of the next line pixel data. This interval is called the ``Front Porch.'' There are similar requirements for the Vsync pulse. There is generally a delay between the end of the last displayed line of the monitor and the begining of the Vsync pulse. There is also a delay between the end of the pulse and the begining of the first line to be displayed. The time required by these two pulse signals and the ``blank time'' required around them adds to the bandwidth requirement for a given resolution/refresh-rate. Alas, these times are not standardized. Different monitors have different requirements for the amount of time devoted to each of these intervals. Even for a single monitor the amount of time required for these intervals may vary depending upon the Dot Clock speed. Some manufacturers provide this information in Owner's Manual. It is generally easier and cheaper for a manufacturer to achieve the bandwidth and sync rates necessary for a high resolution display than it is to manufacture a picture tube with a dot pitch fine enough for that same resolution. Usually, if a monitor's dimensions and dot pitch indicate it can display an image of a particular resolution, the electronics driving it can handle the signals to generate such a display. There are exceptions to this rule, so one should not take this for granted. By way of example, consider the Sony CPD-1730. Looking at the Monitor Specifications Table It would appear that this monitor can display a very high-resolution image (1312x968). However, the low 60Mhz maximum bandwidth prevents this monitor from being used for resolutions higher than 1024x768. Even if we were to accept a refresh rate of 60Hz, the tables above show that 62.2Mhz would be required just for displaying the pixels of a 1152x900 image. That does not allow any time for sync pulses or blanking time. Sometimes the best way to get a feel for the inter-relationship of numbers like these is to try variations. The bandwidth-form is provided for this purpose. Using this form, you will be able to fill in timing and resolution values and see the resultant Hsync and Vsync (refresh) rate values. The form starts with the values used for a standard 640x480 display. The following tables are given as an example of the timings that might be required for various display resolutions. These particular timings are from the owner's manual for the Nanao T560i, copied without permission. 640x480
Dotclock Hsync Vsync
25.175Mhz 31.468KHz 59.941Hz
H-pixel Front Hsync Back Pre Vsync Post
time Porch Porch Vsync Vsync
usec 25.422 0.636 3.813 1.907 318 64 1048
If you have read this far perhaps you now know a little bit more about how monitors work and how to choose one for yourself. There are a number of other technical issues worth considering when purchasing a monitor. Ergonomics, radiation levels, divergence, and power consumption just to name a few. If you feel like writing about any of them, send me a copy of what you write and I'll try to include it here so everyone can benefit. butler@hawks.ha.md.us |
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Copyright @ Р. Кожухаров Последно обновена: Ноември 28, 2005 09:52:42 |