| How can I hook more than one monitor to my video card? |
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| [ From: Sam Goldwasser (sam@stdavids.picker.com) with a bit from Michael Scott (scott@bme.ri.ccf.org) and Bill Nott (BNott@bangate.compaq.com)] The following discussion assumes that you want to display the same video signal on a number of monitors. If instead you want use 2 or more monitors to increase your screen real estate, refer to the section "Can I use two video cards in the same system?". The best way to do this is to purchase a commercial VGA signal splitter or video distribution amplifier.. These are not cheap, but they will provide the best results. A video splitter designed for VGA or SVGA will include the proper high bandwidth video amplifiers as well as the proper cable termination and shielding. Someone may suggest that you just cut and splice a couple of VGA cables together, but this won't provide good results. Major problems relate to cable termination and interference. In order for the video to be sharp and clear without ghosting or ringing, the video cable must be treated as a transmission line. What this means from a practical point of view is that it must use high quality coaxial cable, multiple monitors must be daisychained and not star connected, and the proper terminating resistors must be put only at the very end. Another problem is that video signals operate at high frequencies, and as a result they can cause interference with neighbouring electronic devices, and even the monitor itself. In fact, the video cable can, when designed improperly, act like a nice big antenna. To minimize the interference emanating from the cable, considerations like conductor material, length, shielding, connectors and chokes are taken into account. Chokes are those (usually cylindrical) objects that are located at the ends of many video cables. The result of a good cable design is an impedance matched circuit, which causes a minimum amount of interference, and provides a clean crisp signal to the monitor. If you know enough about electronics, and the monitors and video card in question, then go ahead and design and build a splitter. If you don't, you may cause additional problems. Basic rules for a cable-only solution: 1. Use high quality 75 ohm coax - RG59 is a generic part number but many variations are available. 2. Multiple monitors must be daisychained and not split in a star configuration. 3. Only the last monitor should have its 75 ohm terminating resistors in place. They should be removed from all other monitors or if they have switches, set for HiZ. 4. Pay attention to the grounds - signal returns. Keep the stubs - the connections to intermediate monitors - as short as possible. This will work quite well for workstation monitors - those with BNC coax connectors. Most PC monitors with the 15 pin VGA connectors do not have any means of disconnecting the terminating resistors without actually doing some desoldering - which you really should not attempt unless you are familiar with the safety issues involved in working inside a monitor. If you decide to build an active video splitter which uses video amplifiers, be aware that the video and sync voltage levels are different in a PC: The video is typically 0.7 V p-p, and the sync's are typically TTL level (5 V p-p), so the splitter or amplifier must be able to handle both levels. Finally, pay attention to the video bandwidth capability of the splitter/amp if you care about preserving image detail information. As noted a better solution is to buy an active video splitter. This will include the proper high bandwidth video amplifiers and termination. |
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