An overview of the most common signal transmission standards for digital sources and display devices.

DIGITAL VISUAL INTERFACE
Digital Visual Interface (DVI) and High Definition Multimedia Interface (HDMI) are based on a common signaling scheme for video known as Transition-Minimized Differential Signaling (TMDS). A DVI TMDS link consists of three serial data channels, one for each color (red, blue, and green) plus a fourth channel carrying a pixel rate clock that keeps the three color channels synchronized. Each DVI link is essentially a digitized RGB raster, with support for 8 bits per color, and video blanking intervals comparable to analog RGB. Horizontal and vertical sync information is carried on the TMDS blue data line. All TMDS data and clock lines are differential (balanced) and carried on twisted pairs within DVI cable assemblies.

To support different resolution requirements, the DVI specification provides for one or two video links per connector, commonly known as single link or dual link, respectively.

The maximum pixel rate for single-link DVI is 165 MHz (4.95 Gbps), which is more than sufficient for WUXGA 1920x1200 and HDTV 1080p/60, with a color depth of 8 bits per color. Higher resolutions and greater color depths can be supported by use of dual-link DVI, which handles pixel rates up to 330 MHz and resolutions as high as 3840x2400.

The DVI specification also provides for two additional lines of communication. The Display Data Channel (DDC) is a serial connection for Extended Display Identification Data (EDID) and High-bandwidth Digital Content Protection (HDCP) communication. The Hot Plug Detect (HPD) pin allows for implementation of hot plug detection, which allows a PC, for example, to detect the presence of a display without user intervention.

The DVI specification provides for two types of connectors: DVI-D, the standard connector, and DVI-I, which can carry analog RGBHV as well as digital signals.

HIGH DEFINITION MULTIMEDIA INTERFACE

The HDMI format incorporates the TMDS video functionality of DVI and extends it to carry digital audio and control information. The most common HDMI connector is the 19-pin Type A, which contains a single TMDS link plus DDC and HPD lines. A 5-volt power supply line is also provided. In addition, HDMI connectors incorporate the Consumer Electronics Control (CEC) line, which is used for integrated control of multiple devices within an AV system. At this time, CEC control protocols are proprietary to each equipment manufacturer, and there is no CEC compatibility between manufacturers. However, there are implementation guidelines for CEC and manufacturers are beginning to work together to develop standardized control.

Unlike DVI, the HDMI specification has evolved through several revisions. The current version is HDMI 1.3, released in August 2006. Compared to previous versions, HDMI 1.3 specifies a twofold increase in the maximum TMDS single link clock rate to 340 MHz (10.2 Gbps). The increased bandwidth of HDMI 1.3 enables up to 16 bits per color (also known as Deep Color, an extended color space), the latest high resolution surround sound audio formats for Blu-ray disc, and video resolutions up to WQXGA 2560x1600. Version 1.3 also mandates the inclusion of HDCP, a digital rights management scheme that prevents the copying of digital video and audio content. The next version, HDMI 1.4, was recently published for developers.

DISPLAYPORT
between sources and displays that is being positioned as a low-cost alternative to HDMI for PC equipment manufacturers. DisplayPort uses a digital video transmission scheme that differs from TMDS and is therefore not directly compatible with HDMI and DVI. However, the 20-pin DisplayPort connector, with characteristics similar to the HDMI Type A and Type C connectors, can be used to pass HDMI signals, provided that the device supports HDMI.

DisplayPort video and audio signals are carried on four lanes of differential wires, with each lane running at either 1.62 Gbps or 2.7 Gbps for a maximum data rate of 10.8 Gbps.

As with HDMI 1.3, DisplayPort is capable of supporting Deep Color, multichannel high resolution audio, and video resolutions well beyond WUXGA 1920x1200 and HDTV 1080p/60. Analogous to the DDC channel for HDMI, DisplayPort connectors provide for a differential AUX channel for EDID communication. HDCP is supported within DisplayPort. DisplayPort also incorporates its own digital rights management similar to HDCP, called DisplayPort Content Protection (DPCP).

SDI — SERIAL DIGITAL INTERFACE
SDI is a set of video standards defined by the Society of Motion Picture and Television Engineers (SMPTE) for serial transmission of video and audio over standard RG59 or RG6 coaxial cable. SDI standards encompass a variety of data rates from 270 Mbps to 2.97 Gbps per link and are primarily used on professional broadcast and video production equipment, with secondary use in live events, rental and staging, medical imaging, digital cinema, and telepresence cameras and recording devices.