Use Of Digital TV As A Computer Monitor
Dear Gary: I’m a manager of a large electronics retailer, and have a question regarding the convergence of the video and computer industries. Specifically, how would you go about using your new Digital TV as a computer monitor, if it has full-time progressive scanning but no SVGA input? For instance, I own the Sony KV36XBR450 and tried converting the SVGA out of my computer to component with the RCA VHDC-300 transcoder (for use with the RCA HD receiver that outputs RGB on TVs with only YPbPr in) and got a scrambled pix.
Daniel O’Connor,
mailto:irishoconnor@hotmail.com
Video Technical Editor Greg Rogers Comments:
The short answer to your question is that the SVGA format is 800 x 600, which is equivalent to a video format of 600p. But the Sony KV36XBR450 is only compatible with 480i, 480p, and 1080i YPbPr signals. There are several more issues involved in your question that I would like to comment on for the general benefit of our readers. But first, are you sure that you want to use this TV as a computer monitor? Unless you plan to use the computer strictly as a DVD player, I don’t recommend it. I would never use a CRT direct-view TV (which this is), or a CRT-based front or rear projector, as a general purpose computer monitor, or to display games. Fixed images from the operating system or other applications will create permanent damage (retained images) on the CRT phosphor. Even if you use the computer strictly as a DVD player, you must be sure to completely fill the screen with the DVD picture, and not leave any fixed borders or dialogue boxes on the display. The same warning applies to fixed images in games. The brighter the image the quicker permanent image retention will occur. I don’t believe any TV manufacturer will replace CRTs with retained images under warranty. That is one reason that some manufacturers have been reluctant to include PC-compatible inputs on their HDTVs. Plasma Display Panels (PDPs) also use phosphors and are susceptible to phosphor damage. The reason computer monitors are less susceptible to phosphor damage is because they produce much lower light levels (they just look bright because you sit closer to them). But screen savers are still recommended for computer monitors to reduce phosphor damage over time. If you want to use a large screen TV for a computer monitor, then I suggest a DLP rear projector. They do not use phosphors, and image retention is not a concern. Now that everyone has been warned, let’s look at some of the reasons that a computer output might not work with an HDTV. A) The computer output is RGB with H (horizontal) and V (vertical) sync and the display only accepts YPbPr video signals. As you already know it’s not enough to use a breakout cable to convert the DB-15 (VGA style) RGBHV connector to RCA or BNC connectors. You must have an RGBHV-to-YPbPr transcoder. I know of three relatively inexpensive products, the RCA VHDC-300 (www.rca.com), the Key Digital KD-VTCA1 (www.keydigital.com), and the Audio Authority 9A60 (www.audioauthority.com). There are several compatibility issues to consider between the computer video card and the transcoder, and between the transcoder and the display. I’ll discuss these briefly, but I suggest that you contact each manufacturer for more information and provide them with the type of computer video card you have and the type of display you plan to use. The RCA transcoder was designed specifically to work with the RCA DTC-100 DirecTV Satellite Receiver, so I don’t have much information about its compatibility with other products. The DTC-100 produces positive H and V sync signals, while some computers output negative H and V sync. I don’t know if the RCA transcoder will work with negative HV sync signals. Computer monitors usually accept both, and often have a status screen that will tell you which HV sync polarity your video card outputs. The Key Digital and Audio Authority transcoders will accept either positive or negative HV sync. A transcoder will output either bi-level or tri-level (the HD standard) sync to the display. Most new HDTVs will work with tri-level sync, but some older HDTVs and front projectors will only work with bi-level sync. Some displays will work with either tri-Level or bi-level sync. Your display manual will usually indicate what type of sync signals it works with. The RCA transcoder outputs tri-level sync. The Audio Authority transcoder produces bi-level sync. The Key Digital transcoder also outputs bi-level sync, and is advertised to work with Sony HD monitors. The Key Digital product also has a special sync output mode for compatibility between the RCA DTC-100 and Panasonic HD monitors. B) The transcoder is not a scaler. The computer must output a video format that is compatible with the display. The transcoder must also be compatible with that format. Most rear projection and direct-view HDTVs only accept 480i, 480p, 540p, and 1080i formats. A few rear-projection and direct-view HDTVs, particularly new DLP rear-projection HDTVs, will also accept 720p. Most front projectors accept a wide range of computer compatible RGB signals and don't need a transcoder. Most computer video cards only produce progressive formats and will not be compatible with 480i or 1080i. There are some exceptions. A few will output 720p, but most will not output 540p or 720p. That leaves 480p as the one format that just about all computers and HDTVs are certain to work with. When in doubt, try to get 480p working first, before trying another format. You can match computer formats to TV formats by looking at the last number in the computer format, which is the number of active (visible) scan (horizontal) lines on the video display. For instance, the VGA format is 640 x 480. That is equivalent to a video format of 480p. SVGA is 800 x 600. That is equivalent to 600p. But you won’t find a TV that is compatible with 600p YPbPr because there is no video standard for it. The transcoder must also be compatible with whatever video format you use. The RCA transcoder is compatible with at least 1080i and 540p. The Key Digital transcoder is compatible with 480i, 480p, 540p, 576i, 576p, 720p, and 1080i. The Audio Authority transcoder is compatible with 480p, 540p, 720p, and 1080i. C) The display’s vertical scan frequency must be compatible with the vertical scan frequency selected on the computer video card. Most rear projection and direct-view HDTVs are only compatible with 60 Hz (59.94 Hz) vertical scan rates in North America, and 50 Hz in Europe. Computers are usually operated with vertical scan rates set to 72 Hz or higher. That will not work with YPbPr inputs on HDTV displays. Therefore, you must change the video card setting to 60 Hz. Many front projectors have RGB inputs that are compatible with the higher vertical frequencies normally used with computers. However, when playing DVDs on a computer, you will usually get the best picture using 60 Hz, unless the DVD software has made special provisions to optimize playback at 72 Hz. Consult you DVD software for more information on this point. The transcoder must also be compatible with the vertical scan rate chosen for the computer's video card. The RCA and Audio Authority transcoders are compatible with 60 Hz, and the Key Digital is compatible with 50 and 60 Hz.
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