May 2000

TECHNOLOGY BRIEF

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DIGITAL VISUAL INTERFACE Luke Chang, Multimedia Technologist Joe Goodart, Displays Engineer The Digital Visual Interface (DVI) is a display interface developed in response to the proliferation of digital flat-panel displays. For the most part, these displays are currently connected to an analog video graphics array (VGA) interface and, thus, require a double conversion. The digital signal from the computer must be converted to an analog signal for the analog VGA interface, then converted back to a digital signal for processing by the flat-panel display. This inherently inefficient process takes a toll on performance and video quality and adds cost. In contrast, when a flat-panel display is connected to a digital interface, no digital-toanalog conversion is required. The DVI interface is becoming more prevalent and is expected to become widely used for digital display devices, including flat-panel displays and emerging digital CRTs. This article reviews the DVI standard, discusses its current status in the market, and its future prospects.

Standards Effort A number of digital interfaces already exist, but none has emerged as an industry standard. Current portable systems use a low voltage differential signaling (LVDS) digital interface to the liquid crystal display (LCD) panel, but this technology does not transition well to external desktop displays. Other digital interfaces—Digital Flat Panel (DFP), VESA® Plug and Display, and OpenLDI— have also failed to achieve widespread acceptance.

TMDS DVI uses Silicon Image’s PanelLink, a high-speed serial interface that uses transition minimized differential signaling (TMDS) to send data to the monitor. The DFP and VESA Plug and Display interfaces also use PanelLink. For this reason, DVI can work with these previous interfaces by using adapter cables (depending on the signal quality of the adapter.) TMDS conveys data by transitioning between “on” and “off” states. An advanced encoding algorithm that uses Boolean exclusive OR (XOR) or exclusive NOR (XNOR) operations is applied to minimize the transitions. Minimizing transitions avoids excessive electromagnetic interference (EMI) levels on the cable. An additional operation is performed to balance the DC signal. Figure 1 shows the flow of display (or pixel) data from the graphics controller through TMDS mechanisms (implemented in a chip on the graphics card or in the graphics chip set) and the display controller. In this process, incoming 8-bit data is encoded into 10-bit transition-minimized, DC-balanced characters. (The first eight bits are the encoded data; the ninth bit identifies whether the data was encoded with XOR or XNOR logic; the tenth bit is used for DC balancing.)

In contrast, DVI is widely expected to become the standard digital interface for flat panels. The specification was developed by the Digital Display Working Group (DDWG), which was formed to develop a universal standard for attaching a flat-panel display to a PC. The resulting specification was published in April 1999 and graphics cards with standards-based DVI connectors are now available. Source: Digital Visual Interface specification, revision 1.0, 4/2/99, Digital Display Working Group

Figure 1. TMDS Logical Links

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As shown in Figure 1, DVI allows for up to two TMDS links. Each link is composed of three data channels for RGB information and has a maximum bandwidth of 165 MHz, which equates to 165 million pixels per second. (See sidebar for further discussion.)

How much bandwidth do you need? Resolutions Supported

The bandwidth required for a given resolution is governed by the refresh rate and blanking interval of the monitor. Bandwidth can be calculated by using the following formula: Resolution x Refresh Rate x (1 + Blanking Interval*) = Pixels/second For example, to display super extended graphics array (SXGA) resolution on a flat-panel display with a refresh rate of 60 Hz and a 5 percent blanking interval:

Table 1 reviews *The blanking interval value is the sum of the vertistandard PC display cal and horizontal blanking intervals. resolutions and Table 2 summarizes the available bandwidth enabled in version 1 of the DVI specification. The bandwidth of a single-link configuration supports flat-panel resolutions in excess of 1920 x 1080 at 60 Hz. The dual-link configuration provides enough bandwidth for resolutions up to 2048 x 1536. A single-link configuration can also support digital CRT resolutions of up to 1600 x 1200, but only at a 60-Hz refresh rate. (For a discussion of digital CRTs, see “The Future of DVI” later in this article.) The practical limit of a single-link configuration for CRTs at current 75- to 85Hz refresh rates is 1280 x 1024 (at 85 Hz). Higher-resolution CRTs require a dual link configuration, which can support up to 1920 x 1080 at 85 Hz. The two links share the same clock so that bandwidth can be divided evenly between them. The system en-

Single-Link DVI

Dual-Link DVI

60-Hz LCD with 5% blanking interval

Up to 1920 x 1080 (HDTV)

Up to 2048 x 1536 (QXGA)

75-Hz CRT with approx. 15% blanking interval

Up to 1280 x 1024 (SXGA)

Up to QXGA

85-Hz CRT with approx. 15% blanking interval

Up to SXGA

Up to HDTV

Table 2. Resolutions Supported by DVI, Version 1

1280 x 1024 x 60 x 1.05 = 82,565,360 (or 83 MHz, rounded) In this case, the bandwidth required to display SXGA—83 MHz—is well within the 165-MHz bandwidth available in a single TMDS channel.

Display

ables one or both links, depending on the capabilities of the monitor.

Plug and Play The DVI specification supports hot plugging of DVI display devices, and the feature is enabled on current DVI displays and display adapters. However, current Microsoft® Windows® operating systems do not support hot plugging of DVI devices. Future Windows operating systems may include this feature. DVI also supports the VESA Display Data Channel (DDC) and the Extended Display Identification Data (EDID) specifications. DDC is a standard communications channel between the display adapter and monitor. EDID is a standard data format containing monitor information such as vendor information, monitor timing, maximum image size, and color characteristics. EDID information is stored in the display and is communicated over the DDC. EDID and DDC enable the system, display, and graphics adapter to communicate so that the system can be configured to support specific features available in the display.

DVI Connectors Resolution Name

Pixel Resolution

Video graphics array (VGA)

640 x 480

Super VGA (SVGA)

800 x 600

Extended Graphics Array (XGA)

1024 x 768

Super XGA (SXGA)

1280 x 1024

Ultra XGA (UXGA)

1600 x 1200

High-Definition TV (HDTV)

1920 x 1080

Quad XGA (QXGA)

2048 x 1536

Table 1. Review of Display Resolutions

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The digital DVI connector has 24 pins that can accommodate up to two TMDS links and the VESA DDC and EDID services. The DVI specification defines two types of connectors: • •

DVI-Digital (DVI-D) supports digital displays only DVI-Integrated (DVI-I) supports digital displays and is backward compatible with analog displays

Figure 2 shows the DVI-D and DVI-I receptacle connectors that would be located on the back of a PC. The DVI-D interface is designed for a 12- or 24-pin DVI plug connector from a digital flat panel. (Single-link DVI plug connectors implement only 12 of the 24 pins. Dual-link connectors implement all 24 pins.) The DVI-I interface accommodates a 12- or 24-pin DVI plug connector or a new type of analog plug connector that uses four additional pins, plus a ground plane plug to maintain a constant impedance for the analog RGB signals. The DVI-D plug connector interfaces with either of the DVI receptacles. In contrast, the new DVI-I plug connector can only be inserted in the DVI-I interface. Neither of these DVI connectors can be plugged into an analog VGA receptacle.

Figure 3. Comparison of DVI-D and VGA Connectors

Copy Protection In February 2000, the DDWG released version 1.0 of a new copy protection method that is designed to provide a protected DVI link between a host and display device. High-bandwidth Digital Content Protection (HDCP) addresses copy protection issues for high-definition content such as DVD movies that are transmitted through the DVI link. This is a high priority for the movie industry in particular because of concerns about pirating of movies. HDCP is designed to provide authentication and key exchange to verify that a display device is authorized to receive protected content, encryption of each pixel as it is transmitted from the PC to the display, and “renewability.” Renewability is the ability to identify and exclude compromised devices. The process is designed to be dynamic. If the security of a display device is compromised, a licensing administrator adds the device to a revocation list. The updated list is distributed to host systems so that they can detect newly compromised display devices. The distribution method has not Dell Patent Pending As DVI-enabled flat-panel displays with both digital and analog interfaces proliferate, PCs will need a way to identify at bootup which interface is being used in order to correctly initialize the display. There is no standard way to do this today. Currently, PCs use the VESA EDID standard to initialize the display. Under this standard, the display transmits a single EDID file to the PC. The file contains display characteristics such as identification, interface type (either DVI or VGA), timing, capabilities, color parameters, and so forth. The PC uses this information to initialize the display on the VGA or the DVI-D interface. But new dual-interface displays may be attached to either the VGA or the DVI-D interface on the PC. So, how will PCs handle this situation? The methods will vary until an industry standard is established to address the issue.

Figure 2. DVI-D and DVI-I Interfaces and Cable Connectors

Figure 3 compares the 24-pin DVI-D and the 15-pin legacy VGA receptacle connector.

Meanwhile, Dell has filed a patent application disclosing a method for solving the problem. One example of the method involves specifying two EDID files, one containing the VGA characteristics of the display and the other, the DVI characteristics. The patent application also describes how the appropriate EDID file is selected and transmitted to the PC. Using this method, the PC initializes the display using the correct EDID file. This solution is currently implemented on Dell systems equipped with a dual-interface graphics card and a dual-interface flat-panel display.

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The Future of DVI

than traditional CRTs, and current single-channel DVI configurations will not support CRTs larger than 19 inches because of bandwidth restrictions. Dell continues to monitor digital CRT technology for relevance to customers.

The DVI adoption rate will be closely tied to the availability of flat-panel displays. Shortages still plague the industry and, as a result, these displays remain relatively expensive. As supply issues ease and prices are reduced, DVI is expected to eventually replace the legacy VGA interface. Meanwhile, they will coexist. Current DVI graphics cards are equipped with both a 15-pin legacy VGA connector and a single-link DVI-D connector. Dual-link configurations are expected soon. In the future, PCs may contain only a DVI-D connector for digitalonly displays or a DVI-I connector that can accommodate digital displays or analog displays with the newer analog connector. In addition, there is a trend toward integrating DVI support into the graphics chip, thus reducing its cost.

Although the initial appliDVI on Dell™ Systems cation of DVI is between Dell offers DVI-enabled flat-panel displays a PC and a monitor, the and associated add-in graphics cards. specification is actually The displays and graphics cards are equipped with both VGA and single-link display-technology indeDVI-D interfaces. These are available pendent. Thus, it could with Dell Dimension™, OptiPlex™, be used as the standard WebPC™, and Dell Precision™ WorkStaconnection between any tion systems. device and display type. For instance, DVI could provide the link between a settop box HDTV receiver and an HDTV monitor or television. It is too early to tell whether DVI will become the standard interface between these types of consumer devices, but DVI is poised to be the standard digital link between PCs and PC displays.

yet been finalized. Version 1 of the specification was released at the February 2000 Intel® Developer Forum and implementations are being developed.

Digital CRTs that use a single-link DVI interface are beginning to appear on the market. These analog monitors are modified to use the DVI interface by moving the digital-to-analog converter from the video interface to the monitor itself. It is not yet clear how successful these monitors will be in the market. They are more expensive

For More Information • •

Digital Content Protection, LLC: http://www.digital-cp.com Digital Display Working Group: http://www.ddwg.org

Information in this document is subject to change without notice. © 2000 Dell Computer Corporation. All rights reserved. Trademarks used in this text: Dell, the DELL logo, Dimension, Dell Precision, WebPC, and OptiPlex are trademarks of Dell Computer Corporation; Intel is a registered trademark of Intel Corporation; Microsoft and Windows are registered trademarks of Microsoft Corporation; VESA is a registered trademark of Video Electronics Standards Association. Other trademarks and trade names may be used in this document to refer to either the entities claiming the marks and names or their products. Dell Computer Corporation disclaims any proprietary interest in trademarks and trade names other than its own.

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