Hardware Reference
In-Depth Information
Silicon Image developed. Unfortunately, both of these interfaces had relatively low-resolution
support (1280×1024 maximum) and were only implemented in a handful of video cards and monitors.
As such, they never really caught on in the mass market and were overshadowed by the DVI, which
become the first truly popular digital display interface standard.
DVI
DVI was introduced on April 2, 1999 by the Digital Display Working Group (DDWG). The DDWG
was formed in 1998 by Intel, Silicon Image, Compaq, Fujitsu, Hewlett-Packard, IBM, and NEC to
address the need for a universal digital interface standard between a host system and a display.
Unlike the P&D and DFP interfaces that came before it, DVI gained immediate widespread industry
support, with 150 DVI products being shown at the Intel Developer Forum in August 1999, only four
months after DVI was released. Since then, DVI has become the most common interface for digital
video connections between PCs and displays. DVI also allows for both digital and VGA analog
connections using the same basic connector.
DVI uses Transition Minimized Differential Signaling (TMDS), which was developed by Silicon
Image (
www.siliconimage.com
)
and trademarked under the name
PanelLink
. TMDS takes 24-bit
parallel digital data from the video controller and transmits it serially over balanced lines at a high
speed to a receiver. A single-link TMDS connection uses four separate differential data pairs, with
three for color data (one each for red, green, and blue data) and the fourth pair for clock and control
data. Each twisted pair uses differential signaling with a low 0.5V swing over balanced lines for
reliable, low-power, high-speed data transmission. A low-speed VESA Display Data Channel
(DDC) pair is also used to transmit identification and configuration information, such as supported
resolution and color-depth information, between the graphics controller and display.
TMDS is designed to support cables up to 10 meters (32.8 feet) in length, although the limits may be
shorter or longer depending on cable quality. Several companies make products that can amplify or
re-drive the signals, allowing for greater lengths.
Figure 12.5
shows a block diagram of a single-link
TMDS connection.
Figure 12.5. A single-link TMDS connection.
Using TMDS, each color channel (red/green/blue) transmits 8 bits of data (encoded as a 10-bit
character) serially at up to 165MHz. This results in a raw throughput of 1.65Gbps per channel. There
are three color channels per link, resulting in a maximum raw bandwidth of 4.95Gbps per link.
