About DVI interface, DVI connectors and types of DVI cables
DVI (Digital Visual Interface) is a digital video interface. It was established in September 1998 at the Intel Developer Forum and was launched by the DDWG (Digital Display Working Group) composed of Silicon Image, Intel, Compaq, IBM, HP, NEC, Fujitsu and other companies.
DVI is based on TMDS (Transition Minimized Differential Signaling) technology to transmit digital signals. TMDS uses advanced coding algorithms to convert 8-bit data (each primary color signal in R, G, B) into 10-bit data (including line field synchronization information, clock information, data DE, error correction, etc.) through minimum conversion. After DC balancing, it uses differential signals to transmit data. Compared with LVDS and TTL, it has better electromagnetic compatibility performance and can use low-cost dedicated cables to achieve long-distance, high-quality digital signal transmission. Digital Video Interface (DVI) is an international open interface standard that is widely used in PCs, DVDs, high-definition televisions (HDTVs), high-definition projectors and other devices.
Size of DVI Interface
There are 3 types and 5 specifications of DVI interface, and DVI connector size is 39.5mm×15.13mm.
3 major types of DVI connector is : DVI-Analog (DVI-A) interface, DVI-Digital (DVI-D) interface, and DVI-Integrated (DVI-I) interface.
The 5 specifications include DVI-A (12+5), single link DVI-D (18+1), dual link DVI-D (24+1), single link DVI-I (18+5), and dual link DVI-I (24+5).
DVI-Analog (DVI-A) interface (12+5) only transmits analog signals, which is essentially the VGA analog transmission interface specification. When the analog signal D-Sub connector is connected to the DVI-I socket of the graphics card, a conversion connector must be used. The conversion connector is connected to the plug of the graphics card, which is the DVI-A interface. This plug can also be seen in early large-screen professional CRTs.
DVI-Digital (DVI-D) interface (18+1 and 24+1) is a pure digital interface that can only transmit digital signals and is not compatible with analog signals. Therefore, the DVI-D socket has 18 or 24 digital pins + 1 flat jack.
The DVI-Integrated (DVI-I) interface (18+5 and 24+5) is compatible with digital signal and analog signal interfaces, so the DVI-I socket has 18 or 24 digital pins + 5 analog pins. The 4 extra wires compared to DVI-D are used to be compatible with traditional VGA analog signals. Based on this structure, the DVI-I socket can be plugged with DVI-I and DVI-D plugs, while the DVI-D socket can only be plugged with a DVI-D plug. DVI-I's compatibility with analog interfaces does not mean that the analog signal interface D-Sub plug can be directly connected to the DVI-I socket. It must be connected through a conversion connector. Generally, graphics cards that use this interface will come with related conversion connectors. Considering compatibility issues, current graphics cards generally use DVI-I interfaces, which can be connected to ordinary VGA interfaces through conversion connectors. Monitors with two DVI interfaces generally use DVI-D interface. For monitors with one DVI interface and one VGA interface, the DVI interface generally uses a DVI-I interface with analog signals.
DVI interface is divided into two modes when transmitting digital signals: single link and dual link. Transmission rate of the single-link DVI interface is only half of that of the dual connection, which is 165MHz/s. The maximum resolution and refresh rate can only support 1920x1200, 60hz. As for the dual-link DVI interface, it supports 2560x1600, 60Hz mode, and support 1920x1080, 120Hz mode. LCD monitors must have a refresh rate of 120Hz to achieve 3D effects, so in the 3D solution, if DVI interface is used, a DVI cable with a dual-link DVI connector must be used. In general, if the resolution is within 1920x1200, the output quality of single and dual link DVI interface is the same.
DVI-I interface is an interface type that supports digital/analog conversion. If the monitor has only one DVI interface, it will be equipped with DVI-I interface that supports both digital/analog modes. If the monitor has both DVI and VGA interfaces, it will be equipped with DVI-D interface.
Classification of DVI cables
DVI-A (12+5) Cable
12+5 pin DVI-A cable is the most common DVI cable on the market. DVI-A to DVI-A Analog/Analog cable is used in monitors with a resolution of less than or equal to 1920x1200. Because the image quality of DVI single-channel and dual-channel output is the same within this resolution, there is no need for manufacturers to spend more money to provide dual-channel cables.
Single link DVI-D(24+1/18+1) Cable
DVI-D interface is a pure digital interface that can only transmit digital signals and is not compatible with analog signals. Since it does not transmit analog signals, it cannot convert DVI interface to VGA interface.
Up Angle DVI-D 18+1 Single Link Digital Video Cable
144HZ DVI-D 24+1 Dual Link Monitor Cable
Dual Link DVI-I(24+5/18+5) Cable
DVI-I interface is compatible with digital and analog interfaces. To be compatible with traditional VGA analog signals, it has 4 more signal pins than DVI-D for transmitting analog signals.
Dual link DVI-D 245 male to female extension Cable
Application of DVI interface
In order to achieve high-definition display requirements, scanning generally adopts 1080i@60Hz format (i.e. interlaced scanning, line frequency 33.75kHz, field frequency 60Hz, pixel frequency 74.25MHz). In actual applications, in order to reduce line frequency conversion, all video input formats (such as 480P, 576P, 720P, etc.) are uniformly converted to 1080i@60Hz format output through format conversion (Scale and De-interlace, etc.), that is, multi-frequency normalization. DVI interface discussed in this article is considered based on the above digital TV standards.
It is relatively simple to add a DVI interface to a digital TV. From the perspective of hardware circuit, one is to add a DVI decoding part at the interface, and the other is to provide a data channel at the back end. If the original TV solution has A/D conversion and corresponding post-stage data processing channels, then the data output by the DVI interface decoding can be shared with it, because when the digital signal format is certain, its bit rate, line frequency, field frequency, and clock are consistent.
In actual research and development, special attention should be paid to the isolation of DVI decoding output data signals and A/D conversion output data signals and to avoid mutual interference between front-end channels. Since the sharing of two groups of channels is equivalent to extending the length of the signal line of the digital output pin, it is necessary to interrupt the long-distance digital signal printed line at its characteristic impedance to avoid overshoot, undershoot and ringing of the digital signal. Usually, a resistor of tens of ohms is connected in series on the data line. At the same time, for the output driver, it is necessary to minimize the capacitive load of the digital output pin, but in the signal wiring stage, the capacitive load cannot be accurately calculated. In order to facilitate system debugging, it should be considered to connect the data signal line, the line field synchronization signal line, and the clock signal line to the ground in parallel. The capacitance value is generally tens of pF, depending on the PCB material and signal length. In this way, channel load balance, consistency of data rising edge, falling edge and phase can be achieved, and digital noise interference and jitter can be reduced.
When testing the performance of the DVI interface of a digital TV, the bit error rate index should reach 10-9, that is, one bit error is allowed for every 1 billion bits. Therefore, a certain test time must be guaranteed during the performance test. For example, for VGA@60Hz, 25MHz clock frequency, the test time should be greater than 40s. For 1080i@60Hz, 74.25MHz pixel frequency, the test time should be greater than 14s. At the same time, the interface performance can be judged by subjectively observing the image for more than 1 minute and there is no obvious pixel noise.
There is a +5V voltage in the DVI interface, and the hot plug detection (HPD) voltage is required to be obtained from this voltage. The HPD effective level should be greater than 2.4V, so the HPD series resistance of the receiving device should generally be less than 10kΩ. The receiving device can also use this voltage in the application for system power supply, but the load current should not be greater than 50mA, preferably less than 10mA, to ensure the HPD level requirement. To ensure the normal startup of the interface, the EDID memory power supply is also preferably generated by the +5V of the transmitting end.
To ensure the feasibility of the hardware circuit design, software support is also required. Optimized software flow is the key to ensure the normal operation of the DVI interface system.
For the application research of DVI interface on digital TV and flat-panel TV, the key is EDID (Extended Display Identification DATA) programming and HDCP (High-bandwidth Digital Content Protection) function. These are all new applications for digital TV. Only after EDID and HDCP are implemented on digital TV, DVI interface is a real digital TV interface. There is still a difference between the broadcast of the signal and the signal. How to use the correct method to test the video signal output by DVI interface for accurate broadcast-level indicators, there is no perfect method at present.
How to choose DVI interface and what are the advantages of DVI interface
The advantages of the DVI interface are mainly reflected in the following two aspects:
1. Fast speed: DVI transmits digital signals. Digital image information does not need to be converted and will be directly transmitted to the display device. Therefore, the tedious conversion process from digital to analog to digital is reduced, which greatly saves time. Therefore, it is faster and effectively eliminates the phenomenon of ghosting. Moreover, when using DVI for data transmission, the signal is not attenuated, and the color is purer and more realistic.
2. Clear picture: The computer transmits binary digital signals inside. If the VGA interface is used to connect the LCD monitor, the signal must first be converted into R, G, B three primary color signals and line and field synchronization signals through the D/A (digital/analog) converter in the graphics card. These signals are transmitted to the LCD through the analog signal line. The corresponding A/D (analog/digital) converter is also required to convert the analog signal into a digital signal again before the image can be displayed on the LCD. In the above-mentioned D/A, A/D conversion and signal transmission process, signal loss and interference will inevitably occur, resulting in image distortion or even display errors. The DVI interface does not need to perform these conversions, avoiding signal loss, and greatly improving the clarity and detail expression of the image.
In edge fusion systems, capture cards for dual-channel VGA signals or dual-channel DVI signals are basically used. Next, we will explain the relevant issues that should be paid attention to when choosing a DVI dual-channel capture card from the following aspects: ① Is DVI a true digital signal architecture? Compared with VGA capture cards, the development difficulty of DVI digital signal architecture capture cards is much higher, so we must pay attention to whether the selected card has a simple VGA to DVI interface module on the card, rather than a true DVI architecture pure digital capture card. ② Due to the limitations of development technology, the technical threshold for DVI capture cards, including VGA capture cards, to capture independent graphics card output and non-independent graphics card output will be very high. When choosing such products, you must pay attention to whether it is supported. ③ Whether it has onboard cache, this is very important. Many key performances of the capture card need onboard cache to improve.
④DVI dual-channel capture card is not a simple copy of two cards. It really needs to combine two DVI capture cards into one, with two cards in one slot, to capture two DVI signals at the same time, and the parameters must be independently adjustable and cannot affect each other. It is not necessary for the two cards to be completely set to the same to be used, and one cannot be set to affect the other. You should also pay attention to distinguish when choosing.
⑤The application of software should be relatively simple, because the DVI dual-channel capture card is used to capture non-standard image signals, and many parameters need to be debugged, so the design of the software should be conducive to smooth debugging by beginners and quickly achieve the best results.
Some misunderstandings about DVI interface
1. Myth: Image quality of DVI interface and HDMI interfaces is very different.
Under current circumstances, at the same resolution, there is no obvious difference in the image quality of DVI interface and HDMI interfaces, but with the development of HDMI interface, it can provide more functions than DVI interface.
2. Myth: HDMI interface can transmit audio signals, but DVI can not.
Both DVI and HDMI interfaces transmit TMDS signals. TMDS actually only needs 4 pairs of wires, and the other pins are auxiliary. The difference is that DVI signal does not contain audio when encoded, while HDMI signal is packaged with audio. When the manufacturer encodes the audio signal and video signal into TDMS format, the DVI interface can also transmit audio signals.
Some special DVI monitor cables
90 Degree up angle Dual Link DVI 24+1 Male To DVI 24+5 Female extension Cable
left angle DVI 24+1 male to DVI 24+5 female extension Cable
Down angle DVI 24+1 to Right angle DVI 24+1 Cable