DVI

Overview

This TekSpek explains what DVI is and how it works.

The Technology

As you'll all likely know by now, DVI is the current standard for connection of a PC or other display generator to a digital display output. You'll recognise the multi-pin connector and know that almost all modern LCD displays have the corresponding input connector, for feeding from your PC. But do you know how DVI works? This TekSpek seeks to teach you how.

How it works

It's pretty simple to conceptualise and therefore visualise DVI. Think about an LCD display panel and its pixel array. DVI encodes the value of each pixel in that array as a set of binary values for the brightness of the RGB subpixels (each pixel is made up of three subpixels, one for each standard transmission colour), and transmits it serially to the input device. The device then decodes that binary data stream and uses it to drive the panel's pixel array.

DVI stream data isn't compressed at all, so that means the relationship between the DVI link bandwidth and screen resolution is a fixed one. Put another way, if you only have the ability to send so many pixels in a fixed time, since DVI requires that all values for all pixels be presented to the input device in specified time intervals, there's a limit to the size of the display you can drive.

DVI therefore introduces the possibility for multiple transmission links. Each DVI link has the same bandwidth, so it's up to the transmitter and receiver to decide how they're going to use it, and whether they want to use multiple links for moving the data. A single DVI link runs at a maximum of 165MHz, transmitting 24 bits per cycle or around 0.5GB/sec of transmission bandwidth. That's enough for a 1920x1200 display at 60Hz easily enough, or a full 1080p HDTV signal, with a little room to spare on both counts.

Add a second link and you can drive higher resolutions (the classis example is the 30" LCD panel from the likes of Dell and Apple, at a staggering 2560x1600 pixels), or even push more bits per pixel if you want to drive higher dynamic range displays. It's a simple binary data stream and thus can be manipulated in a number of ways.

DVI in pictures

There's really nothing interesting about looking at connectors, but there's enough difference in DVI to have a quick look at the two biggest variants.

First, the basic connector itself.

The block of four pins on the left carry an analogue representation of the signal, should the transmitting system provide it. However it's the main 8x3 pin block that we're interested in. Here's why.

Single-link DVI only uses two 3x3 pin blocks for I/O, so you can tell whether your DVI cable supports dual-link just by looking at the pins it provides. Dual-link uses them all, like so.

DVI variants

Lastly we'll talk about the different variants of DVI. There are three main variants, A, I and D. A just carries an analogue signal (counter intuitive we know!), D just digital and I carries them both. D and I can also be single- and dual-link as we've shown.

Summary

Hopefully grasping the basic technical details of DVI was simple. Per-pixel data is transmitted over a fixed bandwidth link (or links) by a transmitter, encoded as a binary stream, which is decoded by the DVI receiver and used to drive the display device. You can double up on links to drive higher resolutions or carry more data per-pixel, and as the industry standard you'll find it on almost all consumer display hardware on the market today, be that input or output.