What is DVI ? - Digital Visual Interface

Contents

Introduction
What is DVI ?
DVI's History
The Technology Behind DVI
How Does DVI Work ?
Basic DVI Configurations
What are single and dual links ?
DVI Cabling Standards
What are the DVI Formats ?
How do I know which cable to use ?
How to recognize a DVI Cable
DVI with HDCP
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 Visual 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.

What is DVI ?

DVI stands for (D)igital (V)isual (I)nterface. DVI is a new form of video interface technology made to maximize the quality of flat panel LCD monitors and high- end video graphics cards. It is a replacement for the P&D Plug & Display standard, and a step up from the digital-only DFP format for older flat panels. DVI is becoming increasingly popular with video card manufacturers, and most cards purchased include both a VGA and a DVI output port. In addition to being used as the new computer interface, DVI is also coming out as the digital transfer method of choice for HDTV, EDTV, Plasma Display, and other ultra-high-end video displays for TV, movies, and DVDs. Likewise, even a few of the top-end DVD players are now featuring DVI outputs in addition to the high-quality analog Component Video. Don't expect to throw away all your old video cables just yet, but keep an eye out for DVI availability in the future.

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.

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.

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.

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.

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.

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 noticable with a good adapter.

What are Single and Dual Links ?

The Digital formats are available in DVI-D Single-Link and Dual-Link as well as DVI-I Single-Link and Dual-Link format connectors. These cables send information using a digital information format called TMDS (transition minimized differential signaling). Single link cables use one TMDS 165Mhz transmitter, while dual links use two. The dual link effectively doubles the power of transmission and provides an increase of speed and signal quality; i.e. a single link 60-Hz LCD can display a resolution of 1920 x 1080, while a dual link can display a resolution of 2048 x 1536.

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 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)

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 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.

 

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

What are the DVI Formats ?

There are three types of DVI connections: 1. DVI-D (Digital) 2. DVI-A (Analog) 3. DVI-I (Integrated Digital/Analog)

DVI-D format is used for direct digital connections between source video (namely, video cards) and digital LCD (or rare CRT) monitors. This provides a faster, higher-quality image than with analog, due to the nature of the digital format. All video cards initially produce a digital video signal, which is converted into analog at the VGA output. The analog signal travels to the monitor and is re-converted back into a digital signal. DVI-D eliminates the analog conversion process and improves the connection between source and display.

DVI-A format is used to carry a DVI signal to an analog display, such as a CRT monitor or an HDTV. Although some signal quality is lost from the digital to analog conversion, it still transmits a higher quality picture than standard VGA.

DVI-I format is an integrated cable which is capable of transmitting either a digital-to-digital signal or an analog-to-analog signal, but it will not work transmitting a digital-to-analog or analog-to-digital signal. Like any other format, DVI digital and analog formats are non-interchangeable. This means that a DVI-D cable will not work on an analog system, nor a DVI-A on a digital system. Make sure that you know what format each part of your equipment is before you purchase any DVI cables. Only equipment with a DVI port labeled 'DVI-I' will accept both a DVI-D and DVI-A source signal.

How do I know which cable to use ?

Determining which type of cable to use for your DVI products is critical in getting the right product the first time. Check both of the female DVI plugs to determine what signals they are compatible with.

Note: To prevent pins being broken off of mistmatched cables, most manufacturers will make their female plugs with all available pins. This means that most every female DVI plug will look like a DVI-I, but this is not necessarily true. Be sure to look for a label, or check the product documentation to make sure you know what type it is. If you have plugs that are DVI-D, they will accept a DVI-D or DVI-I cable. If you have plugs that are DVI-A, they will accept a DVI-A or DVI-I cable. If you have plugs that are DVI-I, they will accept any type of DVI cable. If you have mistmatched plugs, such as DVI-D and DVI-I or DVI-A and DVI-I, you may use either a DVI-I cable or the cable that matches the other plug. For example, you may use a DVI-D cable on a DVI-I to DVI-D connection, but not a DVI-A cable.

Note: You may not mismatch a DVI-D and a DVI-A connection. It is possible to extend existing DVI cables by the use of DVI-I extension and DVI-D Extension cables. The maximum signal length, however, is 5 meters, and going beyond that may result in quality loss.

How to recognize a DVI Cable

There are two variables in every DVI connector cable, and each represents one characteristic. The flat pin on one side denotes whether the cable is digital or analog: A flat pin with four surrounding pins is either DVI-I or DVI-A A flat pin alone denotes DVI-D The pinsets vary depending on whether or not the cable is single- or dual-link: A solid 24-pin set (rows of 8) for a dual- link cable Two separated 9-pin sets (rows of 6) for a single-link cable NOTE: To distinguish from DVI-I and DVI-A, check the pinset. A solid 24-pin set is for a DVI-l; a separated 8-pin and 4-pin set is for DVI-A.

 

DVI with HDCP

High bandwidth Digital Content Protection (HDCP) is a standard that the Motion Picture Industry is endorsing to provide a protected interface which will ensure the availability of high value content, such as premium high-definition video from HD broadcast satellite receivers and many new digital cable boxes, while preventing recording of the signal.

Click Here for more Information

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.