Connecting DVI to HDTV

Originally Posted by rbinck

I get PMs and questions about connecting a computer video card output to an HDTV quite often. It came to me that a series of threads that deal with this proceedure and the issues involved would be helpful. This thread will deal with the connecting of a computer video card with DVI outputs to an HDTV with a DVI input.

Also for you strictly technical guys out there, I do take a little poetic license to try and simplify the subject somewhat.

Introduction
The reason many TV manufacturers include a DVI connection in addition to component video was not to allow the connection of a computer. The main reason was to allow the exchange of video information in a digital format rather than analog. The DVI interface came into being due to the LCD and plasma flat panel computer monitors and projectors, and later DLP and LCOS (D-ILA) displays and projectors. Since these are fixed pixel devices an interface that would present a digital value for each pixel rather than an analog signal level made sense.

The DVI interface answered the question of, "Why start out with a digital array of pixel values then convert them to an analog signal to send to a monitor only to have the analog signal reconverted back to digital?" With the DVI interface the possibility of the signal remaining all digital is possible.

The DVI interface was also given interaction capabilities between the video card and the monitor. This allowed the computer to inquire of the monitor what the resolution and timing that it supported and then via software in the computer set the video output accordingly. This capability is actually the essence of the DVI interface, the ability to tell the source what signals the monitor will display. The earliest DVI-I interfaces were actually just VGA analog signals with the digital link to allow the information to be exchanged between the computer and the monitor. There is a cable available to convert a DVI-I output from a video card to a VGA input on a monitor.

The DVI connection that is most desirable for fixed pixel devices is the DVI-D, dual link connection. This is the connection that most HDTV manufacturers use primarily for compatibility reasons.

Adoption of DVI for HDTV Signals
Since the computer was able to ask the monitor to supply information concerning its signal capabilities, as well as other information, the consumer electronics industry saw this interface (and later the HDMI) as a possible answer to the copy protection issue that had prevented HD movies from being made available for broadcast in HDTV. The entertainment industries were concerned with copy protection and insisted on a method of preventing cloning of their movies. The HDCP protocol was developed and the DVI became the first interface to have HDPC implemented. This allowed the broadcasters to "raise the HDCP flag" that would prevent a recording device connected to the DVI output from recording by shutting off the output. As long as a monitor only device was connected the output was enabled.

Computer Output Compatibility
Many people think that because there is a DVI connection on their HDTV that it should be a matter of connecting a DVI cable between the two and presto! We have computer video on our HDTV screen. Well I'm sorry to say this is not the always the case, in fact rarely.

There are exceptions to this, primarily plasma, LCD displays and projectors. These are generally higher cost display units that are built with the connection of a computer in mind, but most HDTVs are not built with the connection of a computer. Computer monitors today will accept a vast range of video signals. Refresh rates from 50Hz to 120Hz are typical for most computer monitors, whereas most HDTVs will only handle 59 to 61Hz refresh rates. Horizontal scan ranges are also reduced compared to a computer monitor. Here is my HDTV information returned by MonInfo, a utility that can be downloaded and run to get information from your HDTV via the DVI connection:

Note that the 30Hz indicates these are interlaced timings.

The reason HDTVs have such narrow response is a cost issue. Most people who buy an HDTV do not want to pay the extra $2k or so required to expand the response to allow the computer timings.

What Is Possible
But there are many reasons to connect your computer to your HDTV, even though it will not be a big screen duplicate of your computer monitor. Slide shows of digital photos, DVD viewing and group web surfing are just some of the possibilities, but I find it will not replace my computer monitor.

To find out what is possible for your HDTV the first step, after the connection of the appropriate DVI cable, would be to download and run MonInfo. This utility will tell you what resolutions your TV will display. I have found that this is not the only resolutions, but the recommended by the manufacturer. For example my LG will also display 1920x540 at 60Hz which is the progressive scan counterpart to the 1920x1080 at 30Hz. It is probably not recommended by LG due to the aspect ratio issues.

It is important to know what is possible for your particular HDTV because serious damage can be done to your HDTV if non viewable signals are sent to your TV. If while trying out different resolutions and timings you hear a high pitched noise from your TV, remove the signal or turn off the TV immediately.

Recommended Steps To Get Started
First of all you will need to have a computer monitor connected as well as your HDTV. Since the majority of resolutions that can be displayed on your monitor will not be displayable on your HDTV, you would be operating blind and that just will not work. Some video cards have both a VGA output and a DVI output and if yours is this type, then you can hook up your monitor to the VGA output. If your computer monitor is a DVI unit then you will need a DVI distribution box, like the DTronics DD-12P, for example.

Start by setting your computer to 640x480 and get a stable picture. This should be no problem. Most likely the 16:9 resolutions of 720x480 and 1920x1080 will not be listed with the standard video drivers. In order to be able to set custom resolutions it will be necessary to download a program called powerstrip, which is available here: PowerStrip - English and the home page to read up on the software is here: http://entechtaiwan.net/util/ps.shtm

Most likely without the powerstrip software you will go no further. Some video cards have some HDTV drivers for their latest and greatest cards, but if you have an older card - 6 months old even - you will need powerstrip. In the end, you may need to get a newer card anyway. Even if you get a newer card that supports HDTV resolutions, you may still need powerstrip to tweak the display to reduce the overscan. See Overscan thread for more information.

This is a fairly complex issue to get your computer connected to your HDTV successfully, but with some persistence it can be done.