Сандық бейне интерфейсі: DVI стандарты

Standard for transmitting digital video to a display

Digital Visual Interface (DVI) is a video display interface developed by the Digital Display Working Group (DDWG). The digital interface is used to connect a video source, such as a video display controller, to a display device, such as a computer monitor. It was developed with the intention of creating an industry standard for the transfer of uncompressed digital video content. DVI devices manufactured as DVI I have support for analog connections, and are compatible with the analog VGA interface by including VGA pins, while DVI D devices are digital only. This compatibility, along with other advantages, led to its widespread acceptance over competing digital display standards Plug and Display (P&D) and Digital Flat Panel (DFP). Although DVI is predominantly associated with computers, it is sometimes used in other consumer electronics such as television sets and DVD players.

An earlier attempt to promulgate an updated standard to the analog VGA connector was made by the Video Electronics Standards Association (VESA) in 1994 and 1995, with the Enhanced Video Connector (EVC), which was intended to consolidate cables between the computer and monitor. EVC used a 35 pin Molex MicroCross connector and carried analog video (input and output), analog stereo audio (input and output), and data (via USB and FireWire). At the same time, with the increasing availability of digital flat panel displays, the priority shifted to digital video transmission, which would remove the extra analog/digital conversion steps required for VGA and EVC; which released the Plug & Display (P&D) standard in 1997. Because P&D was a physically large, expensive connector, a consortium of companies developed the DFP standard (1999), which was focused solely on digital video transmission using a 20 pin micro ribbon connector and omitted the analog video and data capabilities of P&D. DVI instead chose to strip just the data functions from P&D, using a 29 pin MicroCross connector to carry digital and analog video. Critically, DVI allows dual link TMDS signals, meaning it supports higher resolutions than the single link P&D and DFP connectors, which led to its successful adoption as an industry standard. Compatibility of DVI with P&D and DFP is accomplished typically through passive adapters that provide appropriate physical interfaces, as all three standards use the same DDC/EDID handshaking protocols and TMDS digital video signals. DVI made its way into products starting in 1999. One of the first DVI monitors was Apple's original Cinema Display, which launched in 1999.

DVI's digital video transmission format is based on panelLink, a serial format developed by Silicon Image that utilizes a high speed serial link called transition minimized differential signaling (TMDS).

Digital video pixel data is transported using multiple TMDS twisted pairs. At the electrical level, these pairs are highly resistant to electrical noise and other forms of analog distortion.

A single link DVI connection has four TMDS pairs. Three data pairs carry their designated 8 bit RGB component (red, green, or blue) of the video signal for a total of 24 bits per pixel. The fourth pair carries the TMDS clock. The binary data is encoded using 8b/10b encoding. DVI does not use packetization, but rather transmits the pixel data as if it were a rasterized analog video signal. As such, the complete frame is drawn during each vertical refresh period. The full active area of each frame is always transmitted without compression. Video modes typically use horizontal and vertical refresh timings that are compatible with cathode ray tube (CRT) displays, though this is not a requirement. In single link mode, the maximum TMDS clock frequency is 165 MHz, which supports a maximum resolution of 2.75 megapixels (including blanking interval) at 60 Hz refresh. For practical purposes, this allows a maximum 16:10 screen resolution of 1920 × 1200 at 60 Hz.

To support higher resolution display devices, the DVI specification contains a provision for dual link. Dual link DVI doubles the number of TMDS data pairs, effectively doubling the video bandwidth, which allows higher resolutions up to 2560 × 1600 at 60 Hz or higher refresh rates for lower resolutions.

For backward compatibility with displays using analog VGA signals, some of the contacts in the DVI connector carry the analog VGA signals. To ensure a basic level of interoperability, DVI compliant devices are required to support one baseline display mode, "low pixel format" (640 × 480 at 60 Hz).

Like modern analog VGA connectors, the DVI connector includes pins for the display data channel (DDC), which allows the graphics adapter to read the monitor's extended display identification data (EDID). When a source and display using the DDC2 revision are connected, the source first queries the display's capabilities by reading the monitor EDID block over an I²C link. The EDID block contains the display's identification, color characteristics (such as gamma value), and table of supported video modes. The table can designate a preferred mode or native resolution. Each mode is a set of timing values that define the duration and frequency of the horizontal/vertical sync, the positioning of the active display area, the horizontal resolution, vertical resolution, and refresh rate.

The maximum length recommended for DVI cables is not included in the specification, since it is dependent on the TMDS clock frequency. In general, cable lengths up to will work for display resolutions up to 1920 × 1200. Longer cables up to in length can be used with display resolutions 1280 × 1024 or lower. For greater distances, the use of a DVI booster—a signal repeater which may use an external power supply—is recommended to help mitigate signal degradation.

One of the purposes of DVI stream encoding is to provide a DC balanced output that reduces decoding errors. This goal is achieved by using 10 bit symbols for 8 bit or less characters and using the extra bits for the DC balancing. Like other ways of transmitting video, there are two different regions: the active region, where pixel data is sent, and the control region, where synchronization signals are sent. The active region is encoded using transition minimized differential signaling, where the control region is encoded with a fixed 8b/10b encoding. As the two schemes yield different 10 bit symbols, a receiver can fully differentiate between active and control regions. When DVI was designed, most computer monitors were still of the cathode ray tube type that require analog video synchronization signals. The timing of the digital synchronization signals matches the equivalent analog ones, so the process of transforming DVI to and from an analog signal does not require extra (high speed) memory, expensive at the time. HDCP is an extra layer that transforms the 10 bit symbols before transmitting. Only after correct authorization can the receiver undo the HDCP encryption. Control regions are not encrypted in order to let the receiver know when the active region starts.

DVI provide one TMDS clock pair and 3 TMDS data pairs in single link mode or 6 TMDS data pairs in dual link mode. TMDS data pairs operate at a gross bit rate that is 10 times the frequency of the TMDS clock. In each TMDS clock period there is a 10 bit symbol per TMDS data pair representing 8 bits of pixel color. In single link mode each set of three 10 bit symbols represents one 24 bit pixel, while in dual link mode each set of six 10 bit symbols either represents two 24 bit pixels or one pixel of up to 48 bit color depth. The specification document allows the data and the clock to not be aligned. However, as the ratio between the TMDS clock and gross bit rate per TMDS pair is fixed at 1:10, the unknown alignment is kept over time. The receiver must recover the bits on the stream using any of the techniques of clock/data recovery to find the correct symbol boundary. The DVI specification allows the TMDS clock to vary between 25 MHz and 165 MHz. This 1:6.6 ratio can make clock recovery difficult, as phase locked loops, if used, need to work over a large frequency range. One benefit of DVI over other interfaces is that it is relatively straightforward to transform the signal from the digital domain into the analog domain using a video DAC, as both clock and synchronization signals are transmitted. Fixed frequency interfaces, like DisplayPort, need to reconstruct the clock from the transmitted data.

The DVI specification includes signaling for reducing power consumption. Similar to the analog VESA display power management signaling (DPMS) standard, a connected device can turn a monitor off when the connected device is powered down, or programmatically if the display controller of the device supports it. Devices with this capability can also attain Energy Star certification.

The analog section of the DVI specification document is brief and points to other specifications like VESA VSIS for electrical characteristics and GTFS for timing information. The motivation for including analog is to keep compatibility with the previous VGA cables and connectors. VGA pins for HSync, Vsync and three video channels are available in both DVI I or DVI A (but not DVI D) connectors and are electrically compatible, while pins for DDC (clock and data) and 5 V power and ground are kept in all DVI connectors. Thus, a passive adapter can interface between DVI I or DVI A (but not DVI D) and VGA connectors.

HDMI is a newer digital audio/video interface developed and promoted by the consumer electronics industry. DVI and HDMI have the same electrical specifications for their TMDS and VESA/DDC twisted pairs. However HDMI and DVI differ in several key ways. HDMI lacks VGA compatibility and does not include analog signals. DVI is limited to the RGB color model while HDMI also supports YCbCr 4:4:4 and YCbCr 4:2:2 color spaces, which are generally not used for computer graphics. In addition to digital video, HDMI supports the transport of packets used for digital audio. HDMI sources differentiate between legacy DVI displays and HDMI capable displays by reading the display's EDID block. To promote interoperability between DVI D and HDMI devices, HDMI source components and displays support DVI D signalling. For example, an HDMI display can be driven by a DVI D source because HDMI and DVI D both define an overlapping minimum set of supported resolutions and frame buffer formats. Some DVI D sources use non standard extensions to output HDMI signals including audio (e. g. ATI 3000 series and NVIDIA GTX 200 series). Some multimedia displays use a DVI to HDMI adapter to input the HDMI signal with audio. Exact capabilities vary by video card specifications. In the reverse scenario, a DVI display that lacks optional support for HDCP might be unable to display protected content even though it is otherwise compatible with the HDMI source. Features specific to HDMI such as remote control, audio transport, xvYCC and deep color are not usable in devices that support only DVI signals. HDCP compatibility between source and destination devices is subject to manufacturer specifications for each device.