Ask the Infrastructure Experts! Lesia's Q & A Corner
The Chatter Around DVI
DVI (Digital Video Interface) seems to be the new popular option in digital devices such as projectors and LCD displays. I have noticed with this popularity DVI has become the question of the day regarding video connections on the plant floor. As we enter, the digital world we must become more educated with new technology such as DVI and learn what it has to offer.
VGA (Video Graphic Array) which was brought to us by IBM in 1981 offered a 16-color display with a maximum resolution of 640 (pixel) by 480 (pixel) but also included lower resolutions. By 1991, high-resolution VGA-compatible adapters had come to the market. These early chips used a VGA programming model but had more memory and could support resolutions such as 800x600 or 1024x768 in 16 colors, or 640x480 in 256 colors. This became the standard and is still today but VGA seems to be going to the way side little by little in the commercial markets now that the media and PC industries are pushing for the digital age. DVI's ability to improve image quality and transfer rates does make it the best successor to analog.
There still is the question of what is DVI and what does it offer? DVI is a specification created by the Digital Display Working Group to accommodate analog and digital interfaces with a single connector. Due to the lack of standardization for digital interfacing DVI was born.
DVI is designed to carry uncompressed digital video data to a display. It is partially compatible with the High-Definition Multimedia Interface (HDMI) standard in digital mode (DVI-D), and VGA in analog mode (DVI-A). The DVI interface uses a digital protocol in which the desired illumination of pixels is transmitted as binary data. When the display is operating 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 diminished by its adjacent pixels as well as by electrical noise and other forms of analog distortion.
DVI uses an interface known as Transition Minimized Differential Signaling (TMDS). A single DVI link consists of four twisted pairs of wires RGB and 1 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 there is no support for only transmitting changed parts of the image. This means that the whole frame is constantly re-transmitted.
With a single DVI link, the largest resolution possible at 60 Hz is 2.75 megapixels (including blanking interval) this gives us our standard 4:3 ratio (1915x1436 pixels), 5:4 ratio (1854 x 1483 pixels) or widescreen 8:5 ratio (2098 x 1311). The DVI connector therefore has provision for a second link, containing another set of RGB 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, allowing resolutions up to 4 megapixels at 60 Hz. The DVI specification mandates a fixed single link maximum pixel clock frequency 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 data pairs carry binary data at ten times the pixel clock reference frequency, for a maximum data rate of 1.65 Gbit/s x 3 data pairs for a single DVI link.
Like modern analog VGA connectors, the DVI connector includes pins for the display data channel (DDC). DDC2 (a newer version of DDC) allows the graphics adapter to read the monitor’s extended display identification data (EDID). If a display supports both analog and digital signals in one input, each input can host a distinct EDID. If both receivers are active, analog EDID is used.
Now that that we have a better understand of DVI, you need to realize that in Industrial Manufacturing VGA remains the standard but when we start to mix consumer displays with your industrial application such as ACP thin clients and terminal services we run into the VGA versus DVI dilemma. Despite its age, VGA still offers full HD resolution of 1920x1080. In a manufacturing environment, there is no inherent advantage to using DVI. VGA still offers the quality display image you need for your application. When shopping for consumer display you just need to make sure they have VGA I/O since a lot of the larger LCD’s only have DVI. If you find yourself in this issue, there is the option of a VGA to DVI converter as well.
In addition, Advantech currently offers a thin client with DVI and Arista is working on a new thin client with DVI for 2010; therefore, we should have less issue in the New Year around DVI and how it works.
For more information on VGA, DVI, thin clients and/or display options for your industrial application please contact Lesia Bell at 443.683.8061 or firstname.lastname@example.org
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