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DVI:

The Digital Visual Interface (DVI) is a video interface standard designed to maximize the visual quality of digital display devices such as flat panel LCD computer displays and digital projectors. It was developed by an industry consortium, the Digital Display Working Group (DDWG). It is designed for carrying uncompressed digital video data to a display. It is partially compatible with the High-Definition Multimedia Interface (HDMI) standard in digital mode (DVI-D).

Standards Group:

The Digital Display Working Group (DDWG) is an open industry group lead by Intel, Compaq, Fujitsu, Hewlett Packard, IBM, NEC and Silicon Image. The objective of the Digital Display Working Group is to address the industry's requirements for a digital connectivity specification for high-performance PCs and digital displays. See Recommended Links to right-->

DVI’s Reign

DVI emerged as the preferred connector because it’s purely digital. This means the integrity of the video signal remains true through any transmission, even long distances. You may have heard that DVI is limited to 5-meters in distance, according to the DVI 1.0 specifications. But with solid soldering techniques, using low capacitance, thicker DVI cables and fiber optic interface technologies, the distance of DVI transmission is not limited and can extend as far as 500-meters from the source to the display.

HDCP: No More Piracy

With the advent of DVI, however, came HDCP (High-bandwidth Digital Content Protection), a standard "key" encoded into the DVI signal to prevent video data from being pirated. HDCP was strongly endorsed by the entertainment industry. If a source device is HDCP coded and is connected to a HDTV display or projector via DVI without the proper HDCP decoding mechanism, the picture is relegated to "snow" or in some cases, a very low (480P) resolution. In order to see HDTV with HDCP compliance, both the source and display devices must be equipped with DVI connections that can enable HDCP using "software key" decoding.

Overview:

The DVI interface uses a digital protocol in which the desired illumination of pixels is transmitted as binary data. When the display is driven at its native resolution, it will read each number and apply that brightness to the appropriate pixel. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device, whereas with an analog signal the appearance of each pixel may be affected by its adjacent pixels as well as by electrical noise and other forms of analog distortion.

Previous standards such as the analog VGA were designed for CRT-based devices and thus did not use discrete time display addressing. As the analog source transmits each horizontal line of the image, it varies its output voltage to represent the desired brightness. In a CRT device, this is used to vary the intensity of the scanning beam as it moves across the screen.

However, when using digital displays (such as LCD) with analog signals (such as VGA), there is an array of discrete pixels and a single brightness value must be chosen for each. The decoder does this by sampling the voltage of the input signal at regular intervals. When the source is also a digital device (such as a computer), this can lead to distortion if the samples are not taken at the center of each pixel, and there are also problems with crosstalk.

Basic features of DVI interface are:

The data format used by DVI is based on the PanelLink serial format and named Transition Minimized Differential Signaling (T.M.D.S.). A single DVI link consists of four twisted pairs of wire (red, green, blue, and clock) to transmit 24 bits per pixel. The timing of the signal almost exactly matches that of an analog video signal. The picture is transmitted line by line with blanking intervals between each line and each frame, and without packetization. No compression is used and DVI has no provision for only transmitting changed parts of the image. This means the whole frame is constantly retransmitted.

With a single DVI link, the largest resolution possible at 60 Hz is 2.6 megapixels. The DVI connector therefore has provision for a second link, containing another set of red, green, and blue twisted pairs. When more bandwidth is required than is possible with a single link, the second link is enabled, and alternate pixels may be transmitted on each. The DVI specification mandates a fixed single link cutoff point of 165 MHz, where all display modes that require less than this must use single link mode, and all those that require more must switch to dual link mode. When both links are in use, the pixel rate on each may exceed 165 MHz. The second link can also be used when more than 24 bits per pixel is required, in which case it carries the least significant bits.

The DVI interface uses a digital protocol in which the desired illumination of pixels is transmitted as binary data. When the display is driven at its native resolution, it will read each number and apply that brightness to the appropriate pixel. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device, whereas with an analog signal the appearance of each pixel may be affected by its adjacent pixels as well as by electrical noise and other forms of analog distortion.

Previous standards such as the analog VGA were designed for CRT-based devices and thus did not use discrete time display addressing. As the analog source transmits each horizontal line of the image, it varies its output voltage to represent the desired brightness. In a CRT device, this is used to vary the intensity of the scanning beam as it moves across the screen.

However, when using digital displays (such as LCD) with analog signals (such as VGA), there is an array of discrete pixels and a single brightness value must be chosen for each. The decoder does this by sampling the voltage of the input signal at regular intervals. When the source is also a digital device (such as a computer), this can lead to distortion if the samples are not taken at the center of each pixel, and there are also problems with crosstalk.

DVI Connector and Pin Out:

DVI carries the same type of signal as the DFP connector; however, the connector is not the same physically or in terms of pin out. The DVI connector comes in two forms, a 24-pin version (DVI-D, digital-only) and a 29-pin (DVI-I, digital and analog) version. The 29-pin version allows an analog signal to also be carried.

Like modern analog VGA connectors, the DVI connector includes pins for the display data channel, version 2 (DDC 2) that allows the graphics adapter to read the monitor"s extended display identification data (EDID).

There are three types of DVI connections:

1. DVI-D (Digital) is a digital ONLY connector.

2. DVI-A (Analogue)

3. DVI-I (Integrated Digital/Analogue)

Single link DVI uses a 165MHz transmitter to provide resolutions up to 1920x1080 at 60Hz. For higher resolutions, starting with 1920x1080 at 85Hz and upwards to 2048x1536 and 2560x1600 pixels, dual link DVI is required (two transmitters).

DVI-D - True Digital Video DVI-D format is used for direct digital connections between source video (namely, video cards) and digital LCD (or rare CRT) monitors. This provides a faster, higher-quality image than with analogue, due to the nature of the digital format. All video cards initially produce a digital video signal, which is converted into analogue at the VGA output. The analogue signal travels to the monitor and is re-converted back into a digital signal. DVI-D eliminates the analogue conversion process and improves the connection between source and display.

DVI-A - High-Res Analogue DVI-A format is used to carry a DVI signal to an analogue display, such as a CRT monitor or an HDTV. Although some signal quality is lost from the digital to analogue conversion, it still transmits a higher quality picture than standard VGA.

DVI-I - The Best of Both Worlds DVI-I format is an integrated cable which is capable of transmitting either a digital-to-digital signal or an analogue-to-analogue signal, but it will not work transmitting a digital-to-analogue or analogue-to-digital signal.

SINGLE AND DUAL LINKS:

The Digital formats are available in DVI-D Single-Link and Dual-Link as well as DVI-I Single-Link and Dual-Link format connectors. These DVI cables send information using a digital information format called TMDS (transition minimized differential signaling). Single link cables use one TMDS 165Mhz transmitter, while dual links use two. The dual link DVI pins effectively double the power of transmission and provide an increase of speed and signal quality; i.e. a DVI single link 60-Hz LCD can display a resolution of 1920 x 1080, while a DVI dual link can display a resolution of 2048 x 1536.

Like any other format, DVI digital and analogue formats are non-interchangeable. This means that a DVI-D cable will not work on an analogue system, nor a DVI-A on a digital system.

There is a length limitation of 15-foot (4.6 m) in DVI cables. For longer distances, to eliminate the video degradation, the use of a DVI booster is recommended. DVI boosters may or may not use an external power supply.

Table 1

DVI Connector Pinout