HDMI (Highdefinitionmultimediainterface), as a new generation of digital multimedia interface, can transmit high-speed uncompressed digital audio, video data, The HDMl1.2 pixel clock is up to 165MHz, the data rate reaches 4.95Gbps, while the HDMl1.3 pixel clock is up to 340MHz and the data rate reaches 10.2Gbps. At present, HD 1080p data bandwidth is 2.2GBPS,HDMI can be used in high-definition image transmission. In fact, the current high-definition image transmission is also the majority of the choice of this interface. HDMI has also become an interface standard for consumer electronics, and is widely used in digital video cameras and digital television.
EDID is an important part of the HDMI interface, which is the data specification of the display format of PC display in VESA organization. The port and receiver of the HDMI interface are EDID (extendeddisplavidentificationdata) to negotiate the image format transmitted by both parties. The sending end determines whether the receiver is an HDMI device by reading the EDID data at the receiving end. EDID has developed to many versions now, EDID1.3 is currently widely used in data format.
This article unifies the HDMI specification, through analyzes and sets up the EDID data, causes the HDMI equipment to send and receive correctly to negotiate the high definition image format, realizes the high definition image transmission.
HDMI Introduction
An HDMI system consists of a transmitter and a receiving device, as shown in Figure 1, an HDMI system chart. Through 3 TMDs channels, 1 TMDs clock channels, HDMI transceiver to ask audio, video data and auxiliary data transmission.
TMDS (transitionminimizeddifferentialsignaling) means a differential signal that minimizes the transmission. This technology is already in use in DVI devices. TMDS uses advanced coding algorithms to put 8bit data (R, G, b in each of the roadbed color signal through the minimum conversion code for 10BIT data (including line-field synchronization information, clock information, data de, error correction, etc.), after the DC balance, the use of differential signal transmission data, the receiving end after receiving the signal, The 10bit data is then decoded into 8bit data. Compared with LVDS and TTL, it has better electromagnetic compatibility performance, and can realize long-distance and high quality digital signal transmission with low-cost special cable.
The DDC (Displaydatachannel) channel is used to exchange some configuration information between the HDMI sending and receiving terminals. The sending end passes the DDC channel, reads the EDID data stored in the EEPROM by the receiving end, obtains the information of the receiving end, confirms the setup and function of the terminal display, and determines the transmission of the audio and video data between the receiving end and the receiver.
The CEC (CONSUMERELECTRONICSCONTR01) channel is an optional channel. Through the CEC channel, we can realize some advanced control functions between audio and video devices, such as supporting bidirectional communication between video source and digital TV, realizing single key press at the same time, automatic power, automatic signal routing, remote Control and other functions.
E-EDID Data structure
E-edid is a data structure defined by the VESA organization, which is a specification of an optimized display format for a PC display, which is stored in a dedicated EEPROM memory in the display, where the data structure is accessed by the 128BYTE,PC host and the monitor through the DDC channel. To determine the display properties of the display, such as resolution, aspect ratio, and so on. This data structure is used by HDMI, and in the HDMI specification, the DDC channel is also used to access EDID storage to determine the capabilities and properties of the display device.
The HDMI specification stipulates that the first 128Byte of the EDID must be in accordance with the EDID1.3 data structure, and the second 128Byte must be in line with the EIA/CEA-861B ceaedid time series extension structure.
1) E-EDID Data structure
As shown in Figure 2, it is a detailed description of the EDID1.3 data structure and its fields.
2) CEA-861B data structure
As shown in Figure 3, this is a detailed description of each field in the CEA861B data structure.
The EIA/CEA861B specification stipulates that the VSDB signal should be included in the first sequential extension segment of the Ceaedid data transmission. is Hdmivendorspecificdatablock (HDMIVSDB). This is a eia/cea-861bvendorspecificdata block of data. Contains a 24-bit HDMI data qualifier (ieeeregistrationidentifier) 0x000c03, an HDMI approved, licensed data value. In order to determine whether the receiver is an HDMI device, the HDMI source device needs to detect the EDID data transmission of the receiving device in the time series, whether the existence of this vsdb block (by the HDMI equipment manufacturer according to the agreement developed and provided). Any HDMI device will automatically respond to a hdmivsdb, which is a reasonable length of data representation, including and IEEE Registration, registration identifier: 0X000C03, as long as the HDMI source device to receive this identifier related data response, the receiving device is identified as HDMI equipment. Otherwise, the HDMI specification specifies that any receiving device will not return the response of the HDMI data identifier that contains this hdmivsdb, which is a reasonable length, when e-edid data transfer. will be considered to be connected to a DVI device.
Setup of Edid in HDMI HD image transmission
The HDMI interface has hot plug function, the sending device detects that the HPD signal is low, reads the EDID data to confirm whether the receiving device has changed.
The HDMI specification stipulates that the sending device is to detect whether the first Ceaedid extension block of the receiving device contains HDMIVSDB, whether the HDMIVSDB contains an IEEE data identifier 0X000C03, and only the device containing the data identifier will be considered an HDMI device. Otherwise, it is treated as a DVI device.
The edid of an HDMI device usually consists of two modules, the first is the EDIDl-3 data module, and the second is the CEA861B module, which must include the data designator 0X000C03 in this 861B module.
The following take the actual work of EDID as an example, to Edid to do some explaining. As shown in Figure 4.
In Figure 4, 0x00-0x7f, this is the first paragraph of EDID, EDID1.3 data structure, from the last two bytes, the 0x7e address of the data 01H indicates that there is an extended segment, 0x7f 81H is the first edid of the validation field, the first paragraph edid all 128Byte add up to 0.
0x00-0x07 field, is the edid of the data header, 0x12-0x13 place of the 0103 indicates edidversion1,revision30x36-0x47 field, is the first detailed time series description, 011d80 .... This is a detailed descriptor for the 1080i. 0x48-0x59, this is the second detailed sequence description, 8c,0a ... This is a detailed descriptor for the 480p. Starting from the 0x5a is a non temporal description block. 000000FC00 indicates that the next data is the description of the monitor, where the 48444d49 indicates HDMI. The next 00000000fd00 indicates that the monitor can receive a signal range descriptor, which is required in the GTF standard. FC, FD sign data block must be 13 bytes, if not 13 bytes, then the last useful byte followed by 0AH, the remaining fill 20H, such as here 0x66-0x68 data.
0x80-0xff, this is the second segment of EDID, also the cea861b module. As you can see from the diagram, there are identifiers that contain 0x000c03. EDID the second paragraph must contain this character, otherwise the device will be treated as a DVI device. With the first EDID module, 0xFF 1A is the second segment of the EDID checksum: means, the second EDID all 128Byte add up to 0. Unlike the EDIDL.3 of the first paragraph, in the CEA861B module, the unused fields can be populated with 0. So from the graph, the second paragraph EDID data has a lot o.
The 19H of the 0x83 field indicates that the beginning of the video format detail descriptor begins at 0x19. The 0x85 data 85H indicates that the 1080i image format is an intrinsic feature of this receiving device, 5 of which is a short descriptor in the 1080i video format.
Conclusion
HDMI with its excellent performance is widely used in High-definition image transmission, HDMI both ways to transmit images, depending on the receiver EDID data structure content. The EDID data determines the properties of the receiving-end display device. The sending end relies on the EDID read from the receiving device to determine the properties of the monitor and decide how to transmit the image. If the EDID is set incorrectly, the system may not recognize the HDMI device and cannot transmit the image in HD format. Therefore, the EDID setting is critical.
Link: http://blog.csdn.net/linweig/article/details/5518971
