What is DVI?

 

     DVI (Digital Video Interface) is a video connector designed by the Digital Display Working Group (DDWG), aimed at maximizing the picture quality of digital display devices such as digital projectors and LCD screens.

     Previous standards such as Video Graphics Array (VGA) were designed exclusively for CRT-based devices and hence did not take into consideration 'discrete time'. In such standards, the source, while transmitting each horizontal line of the image, varies the output voltage to represent the desired brightness level. This CRT responds to these voltage levels changes by varying the intensity of the electron beam as it scans from one end of the screen to the other.

In digital systems, the brightness value for each pixel need to be selected so as to display the image properly. The decoder achieves this by sampling the input signal voltage at regular intervals. This technique has some inherent problems. Because these are purely digitial signals, there will be some level of distortion if the sample is not taken from the center of the pixel. Also, there is also the possibility of crosstalk interference.

     DVI takes an entirely different approach. With DVI, the required brightness level of each pixel is transmitted in a binary code. This way, every pixel in the output buffer of the source device will correspond directly to one in pixel the display device. DVI is free from the noise and distortion inherent in analog signals.

     The data format used in DVI is based on the Panel Serial format that uses TMDS (Transition Minimized Differential Signaling). Each DVI link consists of four twisted pair of wires (one each with a color code of red, blue and green, and one for a clock signal) to transmit 24 bits per pixel. The clock signal is virtually the same as that of the analog video signal, while the picture is sent electronically line by line with blanking intervals separating each line and frame, sans any packetization. Also, DVI uses no compression, and if a changed part of an image is to be transmitted, then DVI has no choice but to retransmit the entire frame once again.

     For a single DVI link, the maximum possible resolution for 60 Hz is 2.6 megapixels. Therefore, DVI connectors are provided with an option to connect a second link if there is a need for more that 2.6 megapixels of resolution. In fact, the DVI specification limits the cutoff point of each link at 165 MHz. Every display mode which may require less than this can use the single link mode, while those exceeding the set limit must switch to the dual link mode. The second link can also be switched on when more than 24 bits per pixel are required, in which case the second link transmits the LSBs.

     DVI is the only standard that uses both digital and analog transmission options in the same connector. Most of the competing standards, such as Open LDI and LDVS, are based exclusively on digital technology.As an extension to the above mentioned point, DVI connectors are classified into DVI-D (DVI Digital), DVI-A (DVI Analog), and DVI-I (Digital and Analog). If there is a provision for a second link, that connector is referred to as DVI-DL (Dual Link).

     Some DVD players and television sets come with DVI/HDCP connectors, which, even though are physically same as the DVI connectors, also have the additional ability to transmit a HDCP signal (encrypted) using the HDCP protocol for copyright protection.