|
|
The future of interfacing video signals from a signal source to a video display is going digital.
|
|
||||||||||||||
|
Glen Kropuenske SENCORE, Inc. Application Engineer 1.800.736.2673 or 1.605.339.0100 |
The new Digital Visual Interface (DVI) has skyrocketed in popularity with the increasing number of computer flat panel displays, non-CRT data projectors and HDTV displays. It’s no longer just for computer flat-panel monitors.
|
Tom Schulte, CET SENCORE, Inc. Application Engineer 1.800.736.2673 or 1.605.339.0100 |
||||||||||||||
|
This article explains the basics of the new DVI signal and interface connector and shows how to test video displays equipped with a DVI input, using the Sencore “VideoPro.”
|
||||||||||||||||
|
Why Digital – The History |
||||||||||||||||
|
|
In an analog CRT display, red, green and blue electron beams each scan their respective color phosphors on the CRT face. The intensity of the electron beams determines the level of red, green or blue light output. The electron beam intensity is determined by the analog voltage level applied to the control elements of the electron gun. The varying voltage levels during the beam scanning process produces a display image. Repeated scanning produces a new and changing video on the display.
A non-CRT display (LCD, plasma, DLP) is made up of rows and columns of cells (picture elements or pixels), each containing a red, green, or blue section. Each cell section can be identified and addressed with specific red, green, blue on/off instructions to recreate a displayed image. Periodically and systematically updating each pixel creates new and changing video on the display. Digital processing circuitry handles the job of addressing each pixel with on/off commands and clocking the activity.
|
When CRT computer monitors and TVs dominated the market until the middle 1990s, it was accepted that these analog driven displays required an analog input signal. Therefore, computer makers employed computer video cards with digital-to-analog converters, which converted the digital data representing the video picture to an analog output video. Standard scanning formats VGA, SVGA, VESA, and MAC formats specified the analog signal scanning frequencies, pixels and sync timing.
The introduction of LCD flat-panel computer monitors forced industry experts to rethink the connection between a computer and a display. Since an LCD flat panel display uses digital signals to drive the LCD panel, the conventional analog VGA, VESA, or MAC input signals required an analog-to-digital conversion before they could be displayed. It made sense that a direct digital connection would eliminate signal degradation caused by the D/A and A/D conversions. Plus, a digital signal interface would be less susceptible to noise, crosstalk, and signal losses between the source and display. |
||||||||||||||
|
|
|
|||||||||||||||
|
|
||||||||||||||||
|
Figure 1. A digital interface signal to a display that digitally produces a display image improves picture quality and eliminates unnecessary circuit complexity and cost. |
|
|||||||||||||||
|
|
||||||||||||||||
|
DVI – Transition Minimized Differential Signaling (TMDS) |
||||||||||||||||
|
|
DVI uses a standard developed by Silicon Image, called Transition Minimized Differential Signaling (TMSD), to transmit data to a display. TMDS produces a digital signal optimized to reduce the number of high to low logic transitions on the line. This reduces EMI (Electromagnetic interference), which allows for faster signal transfer rates and decoding accuracy. The differential circuitry in TMDS produces a complementary signal balanced on two wires, with a limited amplitude, enabling use of twisted pair wires instead of more expensive coaxial cables.
TMDS consists of a transmitter that encodes and serially transmits a digital data stream over the twisted pair wires to a TMDS receiver. Video and sync information are serialized and sent over three sets of twisted pair wires, one set for red, green and blue data channels. An additional pair of wires is used to transmit the clock signal needed to decode the signal.
TMDS uses an encoding algorithm, in two stages, that converts 8 bits of data representing pixel values into a 10 bit transition-minimized, DC-balanced character. The first stage produces the transition-minimized nine-bit code word from the eight bit pixel input value. The code word is derived from sequential XOR or XNOR logic functions of a known bit sequence. The ninth bit of the code word identifies which logic function was used so the decoder can reverse the process.
The second stage performs an approximate DC balance on the transmitted data stream by selectively inverting the eight data bits of the 9 bit code word. A tenth bit is added to the code word to indicate when the inversion has been made. The encoder determines when to invert the next TMDS character, based upon the running disparity between the number of ones and zeros found in the current code word. If too many ones were contained in the previous word, and the current word has more ones than zeros, the code work is inverted.
The TMDS receiver synchronizes itself to the character sets contained in the data to recover and decode the red, green and blue signal data. The TMDS clock channel carries a character-rate frequency reference. The receiver/decoder produces a bit-rate sample clock based upon this reference, which is adjusted in phase for each of the data streams, enabling proper decoding. |
|||||||||||||||
|
|
||||||||||||||||
|
Figure 2. Block diagram of a TMDS encoder and decoder used for DVI. |
|
|||||||||||||||
|
|
The DVI specification calls for at least one TMDS "link," which consists of three data channels (RGB) and one clock control channel. According to the DVI specification, a TMDS link may operate at up to 165MHz. A single 10-bit TMDS link offers 1.65Gbps of bandwidth, which is enough for a 1920 x 1080 resolution refreshed at 60Hz on a digital flat panel. The resolution easily handles current HDTV resolution formats.
To keep the specification as flexible as possible, a second TMDS link may be used. This link must operate at the same frequency as the primary link, meaning that to obtain 2Gbps of bandwidth, each link must operate at 100MHz (100MHz x 2 x 10-bits).
|
|||||||||||||||
|
DVI Connectors |
||||||||||||||||
|
|
The DDWG felt that the transition from analog to digital monitor interfacing should be gradual, with capability to support analog VGA/VESA formats for some time. With this in mind, they created two connector versions; a DVI-D (digital) version, and a DVI-I (integrated) version; 1. DVI-Digital (DVI-D) to support only digital displays. 2. DVI-Integrated (DVI-I) to support digital displays and be backward compatible with analog displays; contains both analog and digital interfaces.
|
|||||||||||||||
|
|
The two DVI connector styles provide manufacturers with the flexibility to support digital devices while remaining backwards compatible with analog devices. DVI is also backwards compatible, through the use of adapters, with two earlier digital formats that used TMDS technology; the Plug and Display (P&D) and Digital Flat Panel (DFP) standards
The digital DVI-D connector may have 24 pins that can accommodate a dual link TMDS connection. If a single link TMDS connection is used, only 18 of the 24 pins are used.
Figure 3. DVI specifications define a DVI-D (Digital only) connector and DVI-I (Integrated digital and analog) connector.
|
|
||||||||||||||
|
|
The DVI-I receptacle adds additional connection pins to the connector (Figure 3). Connections C1 through C5 provide red, green, blue, and sync signals for an analog connection to a display. Pin 8 of the main body is used for vertical sync. An analog connection is possible with DVI connectors and cables that contain these additional pins. A DVI-I to VGA (15 pin HD) adapter can be used to feed analog signals contained on a DVI connector or cable to the analog inputs of a display with a traditional VGA plug.
You should be aware that with two variations of these connectors there is a possibility of non-campatibility. Using a DVI-D connector/cable will not work to transport analog signals. Also, using a DVI-D connector/cable with only 18 of the possible 24 pins could not be used for dual link DVI.
|
|||||||||||||||
|
“VideoPro” Integrated DVI Generator & DVI Formats |
||||||||||||||||
|
|
The Sencore “VideoPro” models VP401 or VP403 integrate a DVI digital signal generator with a multimedia analog signal generator. The “VideoPro” produces all possible analog signal types, including composite video (NTSC or PAL), component video (YPrPb), and RGB video. The VP401 adds DVI to the basic analog signals and the VP403 adds DVI plus ATSC digital RF signals.
DVI is output from the VP401 or VP403 through the standard DVI-I connector. DVI signals are in a single link format and utilize TMDS channels D0, D1 and D2. Generator analog output signal are output through the DVI connector using pins 8 and C1 – C5. The “VideoPro” comes equipped with a DVI-to-VGA adapter and VGA to BNC connector cable, permitting easy interface to all analog RGB or YPrPb display inputs.
|
|||||||||||||||
|
|
|
|
||||||||||||||
|
Figure 4. The VideoPro Multimedia generator outputs all analog and DVI signal outputs through the standard DVI connector. |
||||||||||||||||
|
|
The DVI output signals from the “VideoPro” include different resolution formats for testing the DVI inputs found on many types of video displays and projectors. Computer monitors with DVI capability display standard VESA formats. Consumer TV and HDTV-compatible video displays are capable of displaying HDTV formats in various resolutions. Some HDTV displays and projectors are capable of displaying both computer monitor formats and HDTV formats.
The DVI standard for HDTV/SDTV signals specifies a luminance range from a digital 16 value (black) to a digital 235 value (white). The DVI standard for VESA PC signals specifies a luminance range from a digital 0 value (black) to a digital 255 value (white). The VP401 and VP403 provide these proper luminance ranges with the HDTV/SDTV-DVI and VESA/Mac-DVI Signal Type selections.
|
|||||||||||||||
|
|
|
|
||||||||||||||
|
Figure 5. The VP401/403 provides both HDTV/SDTV – DVI and VESA/MAC – DVI signals.
|
||||||||||||||||
|
Testing a Display with a DVI Input |
||||||||||||||||
|
|
A multimedia monitor displays video signals from many different sources. The video signal may originate from an antenna, cable system set-top-box, satellite receiver, ATSC receiver/decoder, VCR, DVD player, digital video recorder (DVR), video game or computer. The Sencore VP401 or VP403 substitutes for all these signal sources, providing a test signal for all testing, alignment or troubleshooting applications.
To test the DVI inputs of a display, connect the DVI test lead (Sencore Supplied Part #39G1060) between the VP401 or VP403 “VideoPro” generator and the DVI input of the monitor. Select either the HDTV/SDTV-DVI or VESA/Mac-DVI selections in the Signal Type menu. Use the HDTV/SDTV-DVI when testing HDTV/SDTV monitors. Use the VESA/Mac-DVI signal type when testing computer monitors, data projectors and multi-media projectors.
Select a signal format from the Format menu that provides a signal resolution within the range of the display. Select the DVI Input from the monitor’s input menu. The monitor should decode the DVI signal and display the video test pattern. Note: Many multimedia displays are capable of receiving and decoding both HDTV/SDTV and VESA/Mac signal formats. It is important to test a display at each of the formats the display is designed to accept.
|
|||||||||||||||
|
Figure 6. The VP401/VP403 tests the video display by providing a DVI input at the proper resolution(s).
|
|
|||||||||||||||
|
The future is digital. For additional information on the Digital Visual Interface (DVI) or on the Sencore VP400 family of multimedia generators, please click on http://www.sencore.com/forms/vp403.asp or call 1-800-736-2673.
|
||||||||||||||||
|
1.800.736.2673 or 1.605.339.0100 |
||||||||||||||||
