Hp LP3065 An Overview of Current Display Interfaces

An Overview of Current Display Interfaces

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

The Future of Display Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Monitor and TV Market Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

VGA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

DVI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

HDMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

DisplayPort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Display Interface Comparison Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Introduction

Concerns over image quality, the near-complete replacement of the CRT by LCDs and other new display
technologies, and growing concerns over content protection are prompting significant changes in display
interface technology. Although PCs will likely continue to use the VGA interface in the short run, long-term,
the future is moving to all-digital.

Various proposals, including improved analog connectors, hybrid digital-plus-analog approaches, and
several all-digital interfaces have been put forth since the early 1990s. One, the Digital Visual Interface
(DVI), which is available in both analog plus digital and digital-only forms, has seen reasonable success
in higher-end PC products, but has not managed to take the majority of the market from the VGA.
Consumer HDTV products have started a migration from the various analog connections to the all-digital
High Definition Multimedia Interface (HDMI) – but HDMI shows no signs of adoption in PCs, except for
connection to TV products. Most recently, a pair of new PC-oriented standards, the Intel Unified Display
Interface (UDI) and the VESA DisplayPort (DP) specification, have received some attention in the press as
possible replacements for VGA. The UDI effort is now defunct, never having appeared on any actual
hardware, while DisplayPort now appears to be the long-term future for the PC industry, and possibly the
converged standard for both PC and CE products.

This paper gives an overview of the four leading display interfaces – VGA, DVI, HDMI, and DisplayPort –
in the PC industry, and includes a brief history of each, their basic characteristics and pinouts, and
provides a comparison of each interface’s features and performance.

The Future of Display Interfaces

Monitor and TV Market Trends

The following trends seem likely over the next 3-5 years in the monitor and TV markets.

• While there will continue to be a trend to larger-sized monitors, the upper end of this market will not

increase significantly in terms of size – the majority of desktop monitors will remain under 30-inch
diagonal. Some increase in resolution (pixel formats) will likely occur, but most monitors will maintain
the current norms – 1680 x 1050 to 1920 x 1200 resolution for widescreen displays in the 20-inch
to 27-inch size range, with the top end of mainstream monitors at about 2560 x 1600 resolution.

• The trend in the TV market will be to larger sizes and a greater percentage of widescreen, HDTV-type

displays, but in this particular application the pixel formats are constrained by television broadcast
standards. The highest-definition format in normal use will remain at 1920 x 1080 pixels.

• There will continue to be growing pressure to provide content protection of copyrighted material

(meaning the prevention of unauthorized viewing or copying of this material). As it is very difficult to
provide adequate protection in the case of analog interfaces, this factor will likely accelerate the
adoption of digital connections – and the displacement of their analog counterparts, such as the
VGA connector in the PC market. Although content protection is of greatest importance in the
consumer products market, the commercial sector may also be affected by this trend.

• There is also a growing desire, primarily on the consumer side of the PC market, for better

interoperability between PC and digital TV products. Over the long term, this may lead to

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convergence on a single digital interface standard for all such products, which would likely pull
commercial users in the same direction.

• LCDs already account for the majority of the PC monitor market, and have a rapidly-growing share of

the TV market. No other display technology is expected to displace the LCD from its top position over
this time period.

These trends and the current state of the market will have the following effects on the predicted future of
display interfaces.

• The long-lived VGA connector will continue on for the foreseeable future, but will continue to lose

market share slowly to the newer alternatives. The VGA connector for now remains the interface of
choice for entry-level products, and will almost certainly be the only connector used with CRT
monitors. The VGA connector will eventually be driven out of the market due to content-protection
issues, but not until some time in the 2010s.

• The DVI connector will continue to be used over the next 2-3 years as one of the PC market’s standard

digital interfaces. But starting in early 2008 DVI will begin to lose market share to the newer, smaller,
more capable, and ultimately less expensive DisplayPort standard. This trend will accelerate over the
next few years. (Support for legacy products will be provided via adapters between dual-mode
DisplayPort products and their older DVI counterparts.)

• The HDMI connector has already displaced DVI, for the most part, in consumer HDTV gear. HDMI

will continue to grow in popularity in the consumer market for both HD and digital SDTV equipment
and start to displace the older analog-only TV interfaces (such as S-Video). HDMI, however, is very
unlikely to see much use as a PC monitor connection or graphics output, except for TV connectivity
purposes and in the near future as a smaller DVI-compatible output for some notebook PCs.

• The DisplayPort connector will start to show up in the PC market in early 2008, and its use will grow

over the next several years. Initially, this growth will be at the expense of the DVI share of the PC
market (that is, the DisplayPort interface will be provided alongside the VGA connector). Eventually
DisplayPort will also displace VGA and become the dominant PC-market interface. DisplayPort brings
advantages in performance, size, and eventually (as volumes mature) cost over the older DVI
standard, and has much better extensibility for the future. (As noted in the DisplayPort section later in
this document, a second-generation DisplayPort spec is expected around 2009 that will provide a
significant capacity increase as well as adding additional features, while maintaining full backward
compatibility with the original version.) DisplayPort is also the only one of these interfaces that is
intended for use as a panel-level (internal) interface, permitting direct-drive monitor products that may
be attractive in some markets.

• In the more distant future, it is at least possible that DisplayPort could also be adopted for CE-market

products, and become the converged, common digital interface used by both CE and PC displays
although HDMI currently shows no signs of decline in its CE-market dominance.

The following sections provide brief overviews of each of these standard interfaces.

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VGA

The VGA connector – named for the Video Graphics Array standard introduced by IBM for the original
Personal Computer products in 1987 has been the most successful PC monitor interface to date in the
computer industry.

In use now for over 20 years, the VGA (also known as the 15HD connector, for15-pin high-density D­subminiature) remains the standard analog video interface of the PC industry, but is beginning to
experience limitations. The D-subminiature connector family from which this connector was originally
selected was never intended to handle very-high-frequency video, and VGA connections can often show
the effects of low bandwidth, overall signal loss, and “ghosting” from impedance mismatches in the
system. The use of cable extenders and switches often introduces additional problems of this nature. The
popularity of the VGA connector continues primarily because it is inexpensive and has an enormous
installed base – and the latter is not a minor concern as the industry tries to transition to newer, more
capable interfaces. The analog section of the DVI-I standard carries VGA-compatible video, can
interoperate with this standard, and will typically provide far better video performance, particularly for
video timings and formats over 1280 x 1024 resolution. However, fully-digital interfaces (currently DVI-D
or the digital section of DVI-I, but soon changing to the DisplayPort interface in PC applications and HDMI
for TV/CE products) are the most probable long-term solutions.

Figure 1 VGA Connector

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Figure 2 VGA Connector Pinout

Table 1 VGA Connector Pinout

Pin Signal

1Red video

2Green video

3Blue video

4 Unused (n.c.)

5Return

6Red return

7 Green return

8Blue return

9 +5 VDC

10 Sync. return

11 Unused

12 DDC Data (SDA)

13 Horizontal sync (TTL)

14 Vertical sync (TTL)

15 DDC Clock (SCL)

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