introduction
The concept of digital television has gradually become popular. Most of the existing digital television receivers can only receive analog signals that meet the digital video signal standard through the component interfaces. This is still an analog signal interface, but the video signals conform to digital television video. Signal standards (such as ITU-R 601, CEA/CEA-861-A/B, and other digital video sampling, quantification, and coding standards that meet the requirements of digital TV's line frequency, field/frame rate, synchronization, clarity, etc.), Therefore, if this type of television is to be digitally processed in the system, it is necessary to add A/D conversion, Scale, De-interlace, and other functional circuits before the video signal processing, which will reduce the signal quality to some extent. The definition of the interface is difficult to really meet the display standard of HD digital television (horizontal resolution is greater than 720 lines).
The digital video interface receives digital signals that have been processed (compressed or uncompressed), which is beneficial to improve the performance of the entire receiver system regardless of signal quality or post-processing considerations. DVI (Digital Visual InteRFace) is a digital video interface that is widely used at present, and it is mainly used in the PC industry at first. DVI supports single-pixel RGB 24-bit data, the transmitted digital signal is not compressed, the single-link transmission rate can reach 4.9Gbps, and the fidelity of 1080i digital high definition television with a data transmission rate of 1.78Gbps can be achieved, especially In LCD, DLP and other display devices, do not need any D / A conversion and processing, reducing signal loss, can be applied to digital television, flat-panel TVs and other products.
DVI overview and working principle
DVI is a high-speed digital signal transmission technology invented by the DDWG (Digital Display Working Group) and has two different DVI-D and DVI-I interfaces. DVI-D only has a digital interface, and DVI-I has a digital and analog interface. The current application is mainly DVI-D.
DVI is based on TMDS (Transition Minimized Differential Signaling) technology to transmit digital signals. TMDS uses advanced encoding algorithms to encode 8-bit data (each channel's primary color signal in R, G, and B) into 10 bits using minimum conversion. (Including line and field synchronization information, clock information, data DE, error correction, etc.) After DC balance, differential signals are used to transmit data. It has better electromagnetic compatibility performance than LVDS and TTL, and can be used for low-cost applications. Cables enable long-distance, high-quality digital signal transmission. TMDS technology connection transmission structure shown in Figure 1.
Figure 1 TMDS connection transmission structure
The DVI digital signal transmission has two methods: Single Link and Dual Link. For single connection, only the 1/2, 9/10, and 17/18 pins shown in Figure 1 are used for transmission. The rate can reach 4.9Gbps, dual connection can reach 9.9Gbps.
Application of DVI Interface in Digital TV
â— Analysis of basic program argument
Digital TV sets generally meet the requirements of high-definition display.
Figure 2 DVI interface block diagram
From the schematic diagram, it is relatively simple to add a DVI interface to a digital TV. Considering the hardware circuit, one adds the DVI decoding part at the interface, and the second one provides a data channel at the back end. If the original TV program has A/D conversion and Corresponding data processing channels at the later stage, then the data output from the DVI interface can be shared with it, because in the case of a certain digital signal format, the code rate, line frequency, field frequency, and clock are the same.
In actual research and development, special attention should be paid to the isolation of DVI decoding output data signals, A/D conversion output data signals, and avoidance of front-end channel mutual interference. Because the sharing of the two groups of channels is equivalent to extending the length of the signal lines of the digital output pins, it is necessary to interrupt the long-distance digital signal printed lines at their characteristic impedance to avoid overshoot of the digital signals. Undershoot and ringing, typically a few tens of ohms of resistance are wired in the data line. At the same time, for the output driver, the capacitive load of the digital output pin needs to be minimized, but in the signal wiring stage, the capacitive load cannot be accurately calculated. In order to facilitate the system debugging, the data signal line and the line field should be considered. Synchronous signal line, clock signal line to ground parallel capacitor, according to the PCB material, the signal length is different, the capacitance value is generally tens of pF, so you can achieve the channel load balance, data rising edge, falling edge and phase consistent, reduce Digital noise interference and jitter.
When the digital TV DVI interface performance test is performed, the error rate index should reach 10-9, that is, one billion bits is allowed to have a bit error, so a certain test time must be guaranteed during the performance test.
Figure 3 DVI interface system workflow
For DVI interface applications in digital television, flat-panel TVs, more critical is the EDID (Extended Display Identification DATA) programming, HDCP (High-bandwidth Digital Content Protection) function. These are all new applications for digital TVs. After EDID and HDCP are implemented on digital TVs, the DVI interface is the real digital TV interface.
In application research, two schemes are proposed for the adaptation of HDCP in digital television.
Modify the HDCP specification. One method is to cancel the recognition once every two seconds, that is, the initial recognition is normal. To prevent the digital signal output from being connected to other illegal devices, it can be determined whether it is an authorized device by detecting the HPD (hot swap detection). Once the HPD is detected to be low, it is considered that the physical connection is interrupted at this time and the signal output can be terminated immediately. The second type is: when the receiver is powered down, the receiver randomly sends a special code to the sending device that requires temporary termination of the transmission. When the receiver resumes Power On, the special code is transmitted again. After the device is verified, the receiver can restart. Transmit digital signals. The above proposal was proposed as a later revision of the specification.
When designing the chip, it is considered that the HDCP processing system and the TMDS decoding processing system are properly handled within the chip. When the decoder is Power Down, the HDCP can also work normally.
The application of DVI to digital TV is one of the methods to improve the definition of TV. As long as the design is reasonable, EDID and HDCP are developed and expanded according to the corresponding standards, then the combination of DVI interface and digital TV is feasible and effective.
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