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What is DVI? - A Guide to the Digital World

by Mindi Penrod

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Introduction
DVI's History
The Technology Behind DVI
How Does DVI Work?
Basic Configurations
DVI Cabling Standards
Conclusion

Introduction

The PC and media industries are pushing to go "digital," to do that we need a standardized digital interface, or in other words DVI (Digital Video Interface). We've all heard chatter about DVI, but what is it really? What does that little white port on your video card really do? How will it benefit you and your system? Largely it does something with your video; this article will explain exactly what it does.

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DVI's History

DVI is a specification created by the Digital Display Working Group to accommodate analog and digital interfaces with a single connector. It was created because of a lack of standardization in digital interfacing. High prices of completely digital displays and the lack of standardization created a window for DDWG to stimulate the adoption of this standard for the market.

Analog graphics technology is slowly going the way of the Dodo, making it legacy technology. DVI's ability to improve image quality and transfer rates makes it the best predecessor to analog. The flood gates are open, but not everyone has adopted the technology, therefore, it's an ongoing process to move the market solely to digital. The good news is prices are dropping at time goes by, so DVI is becoming lower in cost to use for display makers. One day we will become completely digital.

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The Technology Behind DVI

DVI uses an interface known as Transition Minimized Differential Signaling (TMDS) (Fig. 1). Simply put, this is what transmits data over a DVI connection. DVI specifications call for at least one TMDS "link" to transmit data, though there are DVI specifications that have what's called dual links or two TMDS channels. A single link has three data channels (RGB) and one clock control channel, you get the picture below if you have a dual link.

A 10-bit TMDS link operates at up to 165 MHz and offers 1.65Gbps of bandwidth. This is enough to operate a digital flat panel display at 1920 x 1080 resolution refreshed at 60 Hz. This is virtually doubled with a dual link TMDS. Dual offers 2Gbps of bandwidth but must be operated at 100 MHz to match the second link with the primary link. Its possible to get a resolution of 2048 x 1536 with a dual link TMDS. This ability to achieve high bandwidth and larger resolutions has pushed DVI into the forefront of graphics technology.

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How does DVI work?

To understand how DVI works we must first understand how a PC creates and transmits a video signal. The PC/VGA/CRT set up (Fig. 2) is most common among computer users. PC's create digital signals (0's and 1's). Your typical CRT monitor can only display analog signals. It is your PC's video card (VGA connection) that does the work of converting those digital signals to analog for the monitor to display. DVI has no place here though, really, but to understand how DVI can improve and preserve image quality understanding how video signals work needs to be explained. This setup would not improve even if you could use a DVI connection on it.

Many people are now moving up from old style CRT technology to small and nice looking LCD monitors. One thing they may not know is that although all LCDs are digital, not all LCD monitors have a DVI connection. Without a DVI connection image quality is diminished because the video card on your PC has to turn the signals into analog and the LCD has to convert them back to digital when it receives them. (An LCD monitor without a direct DVI connection still has a graphics interpreter in the monitor itself to convert those analog signals back into digital.) In basic terms that 1 could end up looking like either a 0.935 or 1.062 to an LCD monitor, no difference in image quality yet you spent all that money on a good flat panel LCD. What a waste, huh?

This is where DVI comes in. Most video cards now-a-days have DVI ports built into them as well as the normal VGA connection. This is where I explain the usefulness of them. It makes no sense to convert digital signals into analog just to be forced to convert it back to digital form once more at the display. The loss of image quality between non-DVI LCD's and DVI LCD's is minimal at lower resolutions, but at higher ones, you'll start to notice quickly. DVI allows this loss to be virtually non-existent. There are many different ways to implement the DVI specification too, all the way from just buying an LCD with DVI or implementing the cables into your system set up.

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Basic DVI Configurations

Before listing the types of DVI standards out there, it would be nice to know the typical types of configurations you will see using this standard. There are so many possibilities when thinking about using DVI. Whether you want to use your existing set up and add DVI to it or you want to build a new set up using DVI, there are many ways you could go.

DVI output to DVI input (Digital to Digital): Always the best situation to have, there are no adapters to mess with and no headaches in the set up. Basically, this is a DVI connection off the video card to a DVI connection on an LCD monitor (Fig. 3). CRT's will rarely have a DVI connection since they are inherently analog devices. Usually there is no loss in image quality here.
Analog output to Analog input LCD This is the worst situation. Your PC creates its digital signal, the video card turns it into an analog signal and then the signal goes back to digital at the LCD via an integrated graphics converter in the LCD. There is image quality loss here due to the fact that the signal is changed to analog and then back again to digital. Any time there is a conversion to analog, you'll notice a quality loss.
Analog output to Analog input CRT: This is the typical set up for most people who have CRT monitors. With this setup there is a minor loss of quality but it's better then analog to analog input LCD. The quality loss will be completely dependent on the image quality of the Video card, and the CRT.

Note: Many video cards these days have 1 DVI port and one standard VGA port. With an adapter you can often make the DVI port function as a second VGA port for dual display use. With this setup (DVI-> VGA CRT via adapter) there may be a very slight quality loss vs VGA -> VGA but it's unlikely to be noticeable with a good adapter.
Analog output to DVI input LCD: This is a bit better as far as image quality, but there is still a loss because the signal is converted to analog once again. This works virtually the same as the connection above, except the LCD is the one who has a DVI connection instead of the PC. An adapter cable is attached to the VGA (analog) output on the video card; it carries the analog signal to the LCD where it's turned into digital at the DVI connection. Again, you'll see a nice quality loss in the image, especially at higher resolutions.

Understanding the spectrum of configurations with DVI connections and monitor choices can be confusing, what can be even more confusing is the abundance in different types of DVI cables. Which one do you buy? Which type of DVI does my video card have? How do I make use of my DVI connection even if I have a standard VGA connection? All will be explained here, plus a little more.

There are three basic rules of thumb when thinking about DVI and what you want to do with it:

Above all DVI to DVI is always best. Here you will lose virtually no image quality because video signals are not even converted to analog, so a 0 or 1 will look just as is to a monitor. As far as monitors, DVI will only be found in your upper price range LCD's. A few rare CRT's are coming out with digital, but DVI?, I'm not for sure. Some rare CRTs will use the DVI-A standard (Analog DVI). The main point, if at all possible, just make sure to buy a video card that supports DVI as well as a DVI monitor. Your best shot at flawless image quality lies here.
Check your connectors. It's important to know what you have before you know what you can do. Check your video card and monitor to see if they even support DVI and what kind of DVI standard they use. Remember adapters too. If you need one, know what kind of connection from one end to the other is needed. This is important when thinking about buying a monitor or video card to add DVI support to your set up.
Know what you want and your limitations. Knowing the connections you have is the first step, but knowing what you want and what you want to do is the next. DVI standards will be explained later, but if you have one type of standard as your video connection and you want an LCD monitor with DVI, make sure the standards match. You can't mix standards. And if you have a DVI monitor but a video card that doesn't support it, you're better off getting a new video card with DVI support. Just know what you're dealing with before you go shopping. This article should help you with that.

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DVI Cabling Standards

There are three types of standards you will see in DVI cables as well as many types of cross over adapter cables. I will explain several here. You should get to know what each type looks like before you go out and buy your equipment to upgrade to DVI. It's important to know how each works and which ones are needed for what you want.

DVI-D - DVI Digital (True Digital)

DVI-D is a direct digital connection between your video card and your monitor. This type of connection provides a fast transfer rate, high quality image and no quality loss due to the fact that all signals from PC's are purely digital; no digital-analog-digital conversion is needed. Unfortunately, due to the lack in conformity by the monitor and video card industry, this standard is not usually seen in either piece of hardware. (Of the two it's slightly more common on monitors)
DVI-A - DVI Analog (High-Resolution Analog)

This format is used to carry a DVI signal to an analog display, and we all know what that is ?CRT. This format is also used to carry a DVI signal to some HDTV's. Some image quality is lost of course due to the conversion process, but the format transmits a picture of higher quality than standard VGA formats. Also, this format is virtually obsolete. I have seen cables for this standard, but no equipment that actually supports it.
DVI-I - DVI Integrated (Digital and Analog in One Format)

DVI-I is the format de jour. Any video card out there that does support DVI will more often than not have a DVI-I connection on it. It supports both analog signals and digital signals in one cable. This means that the cable can transmit either a digital-to-digital signal or an analog-to-analog one, but it will not transmit crossovers of either one (digital-to-analog or analog-to-digital). It also means that if you have a DVI-I port on your video card your fine hooking up most DVI-D or DVI-A devices without needing a seperate adapter.

Note: None of these cables can be interchanged with other cables. A DVI-D cable cannot be placed on an analog system nor can a DVI-A cable be placed on a digital system. The standards can not be mixed. A DVI-I port however can accept another DVI-I, DVI-D or DVI-A cable.
DVI to VGA connectors

These adapter cables or plugs are DVI at one end and VGA at the other. These are typically used to transmit a DVI signal from the video card to VGA at the monitor or vice versa. Each standard (DVI-D, A and I) has a respective adapter cable to accommodate the type of standard your system uses.

DVI-A to VGA adapter plug

Another note to add in this section, all cables and adapter plugs have male and female ends depending on what you need. Know which ones you need to attach your PC to your monitor. Usually, the video card has the female end of the DVI and VGA connectors, so the adapter cable should be a male end for the PC and a male end for the monitor. If you need an adapter plug to change the DVI or VGA port to the opposite standard, the plug will usually be male to female so the cable can attach to the plug properly.

DVI-I Female & Male connectors DVI-D Female & Male connectors

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Conclusion

Going truly digital means you need an understanding of DVI and how it works. This article has fully explained what DVI is, its technology and how to determine what you need to make it work with your ideal system. DVI is a vastly confusing world, so knowing what you're getting into is important. Simply knowing what you have, what you need and what is best is your best defense against moving yourself into the true digital age. Though DVI hasn't spread like wildfire yet, you are now armed with the knowledge you need to tackle that obstacle between analog and digital video imaging.

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Last Updated: 11/17/03

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Very Informative - Oct 10, 2005
Reviewer: Paul
For a newbie like me, very helpful.

DVI Clarified - Oct 7, 2005
Reviewer: Jim Pratt
Great, straight-forward article on something that was a mystery to me. Thanks for shining a light on DVI.

Excellent guide - Sep 29, 2005
Reviewer: Jarod
Great job reviewing not only the technology from the ground up, but also in reviewing the types of connections out there. Recommended for anyone looking to get a grasp of this emerging technology.

thanks - Sep 21, 2005
Reviewer: Debauc ;)

WOOT!! - Aug 10, 2005
Reviewer: Pyro
Full ov info easy to understand with diagrams thanx for tha hlp