Transcript
Dual-link DVI Technical Brief
DVI technology overview DVI, or Digital Video Interface, is a standard for transmitting video data between devices. The standard refers to the transmission protocol, cables and connectors that are used to transmit the data. Because DVI transmits data using a digital protocol, it typically produces a higher quality output than analog devices. DVI can be used for connecting devices such as flat panel display monitors, DVD players, TVs, and video projectors. There are three DVI formats: §
DVI-D – this is an all digital connection in which both the source and display devices use digital signals. This type of interface is typically used between devices with video cards and digital LCD monitors. The all digital interface provides a higher quality image than with analog displays, because the conversion (and subsequent signal degradation) between the analog and digital signal is not necessary.
§
DVI-A – this is a high-resolution analog standard that provides a digital to analog connection, such as a digital video card to a CRT monitor. The quality is not as good as an all digital connection, but provides better picture quality than a standard VGA connection.
§
DVI-I – this connection supports both digital-to-digital connections and analog-to-analog connections with a single integrated cable.
Dual-Link DVI DVI formats can be single-link or dual-link. Both types of formats use a method of transmitting data called Transition Minimized Differential Signaling (TMDS). The TMDS link has three data channels (RGB) and one clock control channel. The difference is that single-link DVI uses one TMDS transmitter and dual-link uses two, which effectively doubles the transmission providing higher signal quality and increased speed. With dual-link systems, the first link is used up to its bandwidth capacity (165 MHz maximum), at which time the second link is enabled. The dual-link DVI connector contains additional pins to provide for the second set of data signals. Some manufacturers refer to these connectors as DVI-DL (dual-link). These graphics show examples of a DVI-D single-link and DVI-D dual-link cable connector. DVI-D Single-link cable connector
DVI-DL dual-link cable connector
Notice the pins in the center of the cable are not present
All three rows of pins are present
The following table compares sample display resolutions for single-link and dual-link formats: This link …
Single-link
Dual-link
With this refresh rate:
With a total data rate of:
Provides this screen resolution
60 Hz
25 MHz
640 X 480
60 Hz
65 MHz
1024 X 768
75 Hz
79 MHz
1024 X 768
60 Hz
108 MHz
1280 X 1024
75 Hz
135 MHz
1280 X 1024
60 Hz
146 MHz
1680 X 1050
60 Hz
154 MHz
1920 X 1200
60 Hz
162 MHz
1600 x 1200
60 Hz
269 MHz
2560 x 1600
Note: Some listed modes use reduced blanking to lower pixel rates. A longer blanking period is required by CRT monitors in order to allow time for the CRT beam to retrace from the end of a current line of scanned pixels back to the beginning of the next line. The blanking period required by LCD monitors is much less than that of CRT monitors. By reducing the blanking time for LCD monitors DVI is able to display the same screen resolution and refresh rate with a lower speed pixel clock. DVI cables and connectors According to the DVI specification, DVI equipment must produce a digital signal at 5 meters (16 feet) in length, but many devices exceed that specification, making longer cable lengths possible. The exact maximum cable length for DVI is therefore difficult to determine. In addition, DVI signalboosting equipment is available to help extend the length of the cable run. DVI hardware has a female connector (shown in this graphic), and DVI cables have male connectors on the both ends. All DVI female connectors have the full three rows of eight holes, whether they are singlelink or dual-link capable devices. Therefore, DVI-DL cables can be used on both singlelink and dual-link connectors. However, only single-link mode will be used if one or more DVI hardware devices or cables support only single-link. Users must check the hardware specifications of their device to verify if the device supports dual-link. Note: All DVI hardware and cables that support dual-link also support single-link. If one or more DVI hardware devices and cables support only single-link, then dual-link mode will not be used.
DVI-D cables fit both DVI-D and DVI-I connectors, but DVI-I cables do not fit DVI-D connectors. Since DVI-DL cables include additional pins to support the second link, the cables can be used on both single-link and dual-link connectors. The DVI-D dual-link cable is therefore considered to be a ‘universal’ cable because of its versatility – it can be used with all DVI-D and all DVI-I hardware, single-link or dual-link.
The following graphics show the various types of DVI connectors: DVI-D Single-link
DVI-A
DVI-I Single-link
DVI-D Dual-link
DVI-A connector types are not interchangeable with other formats.
DVI-I Dual-link
DVI and HDMI High Definition Multimedia Interface (HDMI), the latest in all-digital transmission standards, is backwards compatible with DVI-D. That is, DVI-D sources are compatible with HDMI displays, and HDMI sources will work with DVI displays. Note that although HDMI is compatible with all DVI-D signals, DVI-D does not support the HDMI CEC or audio functions. (Read the HDMI Technical Brief for more information on HDMI.) There are many adapters and cables to interconnect HDMI and DVI equipment. When connecting HDMI and DVI-D components, only the digital video is output and received. Any audio channels must be connected separately. Common Issues DVI is a plug and play technology designed to work with very few problems. Most issues with DVI have to do with cable quality. Inexpensive cables or overly lengthy cable runs can cause image quality issues. Analog image quality can range from excellent to very degraded images, with all variations in between. With digital, however, which is 1s and 0s, the image quality is either perfect or very distorted (for example stuck pixels or no display at all), with no real in-between degradation. For both types of DVI formats, it is important to use the correct quality and length of cable for the screen resolution and refresh rates being employed by the hardware device. If a customer is experiencing image quality issues, ask the customer to verify that the type of cable being used is a high-quality cable and that the cable length is supported by all of the DVI hardware devices. Other customer issues might involve questions about single-link versus dual-link. One cannot tell whether a DVI hardware device supports dual-link by looking at it, because the female connector contains all of the pin holes. The customer must refer to the device’s hardware documentation to find out if it supports dual-link mode.
