|Deep understanding of the HDMI interface
What is an HDMI interface?
The English full name of HDMI is "High Definition multimedia", the Chinese meaning is the HD multimedia interface. The HDMI interface provides up to 5Gbps of data transmission bandwidth, which transmits uncompressed audio signals and high-resolution video signals. It also eliminates the need for a number/modulus or modulo/number conversion prior to signal transmission, guaranteeing the highest quality audio and video signal transmission. The advantage of applying HDMI is that only one HDMI cable is required, and the audio and video signals can be transmitted simultaneously, rather than requiring more than a few wires to connect, and that the wireless can achieve a higher quality of transmission due to digital/analog or analog/digital conversion. For consumers, HDMI technology not only provides clear picture quality, but also simplifies the installation of home theater systems due to the same cable used in audio/video.
2002 April, Hitachi, Panasonic, Philips, Silicon Image, Sony, Thomson, Toshiba a total of 7 companies set up the HDMI organization began to develop a new dedicated to digital video/audio transmission standards. The end of 2002, the high-definition digital multimedia interface (high-definition digitally multimedia Interface) HDMI 1.0 Standard promulgated. HDMI is compatible with DVI on the pin and is only used in different packages. Compared to DVI, HDMI transmits digital audio signals and adds support for HDCP, while providing better DDC optional functionality. HDMI supports 5Gbps data transfer rates up to 15 meters long enough to handle a 1080p video and a 8-channel audio signal. And because a 1080p video and a 8-channel audio signal demand less than 4gb/s, HDMI also has a very dayu amount. This allows it to connect DVD players, receivers and PRR with a single cable. In addition, HDMI supports EDID, DDC2B, so the device with HDMI has "Plug and Play" features, the signal source and display devices will automatically "negotiate", automatically select the most appropriate video/audio format.
Traditional AV composite and chromatic aberration interface both need separate audio and video data cable to transmit the signal, the same as the digital interface of the DVI interface does not support audio transmission, currently only HDMI has the ability to transmit audio and video signals on a single data line, so people are also accustomed to HDMI called "high-definition line."
What is HDCP protocol?
HDCP is the abbreviation of High-bandwidth Digital Content protection, which can be called "HDCP data Protection". HDCP Technology is a collaboration between Hollywood and the semiconductor giants Intel, it can be used in the video card, DVD player and other transmission end, as well as the display, TV, projector, the receiving end. is an important anti-piracy technology for high-definition movies and TV programs, the display that does not support HDCP protocol does not play the copyrighted high-definition program properly.
The HD movie program behind the DVD uses HDCP and AACS anti-piracy technology, both Blu-ray and HD DVDs Use this anti-piracy technology, and high-definition televisions (HDTV) are also used. With the use of HDCP and AACS anti-piracy technology, movie programs can only play on the device that supports HDCP, otherwise only see the black screen display or low-quality display (only about One-fourth of the normal definition), also handy lost the value of HD. AACS is a cryptographic technology that is used in HD DVDs and Blu-ray discs to protect the video content of the disc from being copied and played elsewhere.
The HDCP protocol is used to prevent the video content from being completely copied in the process of transmission. This technique does not allow digital signals to be recorded illegally, but to encrypt digital signals so that the non-legal recording method cannot achieve the original high-resolution quality. Blu-ray players, for example, cannot record a clear program while playing a high-definition disc while playing a disc on a computer without clearly recording the program on the Monitor. HDCP from the beginning to the end of the protection of the video signal, that is, the entire play system each link must support the HDCP protocol, if the display does not support HDCP protocol, then can not play normal HD programs, only to see the black screen or low-quality programs. To support HDCP protocol, it is necessary to use DVI, HDMI and other digital video interface, traditional VGA and other analog signal interface can not support HDCP protocol. When using the analog signal interface such as VGA, the screen will be reduced to a low quality, or the hint can not play, thereby losing the meaning of HD, to prevent piracy. It should be stated that the HDMI interface is embedded with the HDCP protocol, and the display with the HDMI interface supports the HDCP protocol. But not the LCD with DVI interface Support HDCP protocol, must pass with the corresponding hardware chip, through the certified display.
The data content protected by HDCP technology on the computer platform will be verified by the COPP driver (authentication Output Protection protocol) in the operating system first to verify the video card, only the legitimate video card can achieve the content output, and then to authenticate the display device key, Only HDCP-compliant devices can ultimately display the content that the video card transmits. In the process of HDCP transmission, both the sender and the receiver store an available key set, which are secret stores, both the sender and the receiver are encrypted and decrypted according to the key, and a special value KSV (video encryption key) is added to the operation. At the same time HDCP each device will have a unique KSV serial number, the sender and the receiving end of the password processing unit will check the other's KSV value, to ensure that the connection is legitimate. HDCP encryption process for each pixel processing, so that the picture becomes irregular, unrecognized, only after confirming the synchronization of the sending and receiving end can reverse processing, complete the restoration of data. During the decryption process, the HDCP system will make a connection confirmation every 2 seconds while the sender and receiver sync identifiers are sent every 128 frames to ensure the synchronization of the connection. In order to deal with key leaks, HDCP has specifically established a "revoke key" mechanism. The key set KSV value for each device is unique, and the HDCP system will compare and find it in the revocation list after receiving the KSV value, and the KSV appearing in the list will be illegal, leading to the failure of the authentication process. The revocation key list will be included in the multimedia data for HDCP and will be updated automatically.
It can be seen that in order to play copyrighted high-definition programs on your computer, whether it's HDTV, Blu-ray, or HD DVD, the monitor and graphics cards are required to support the HDCP protocol. But manufacturers to the product to play HDCP logo, you need to pay a certain amount of certification fees, but also to increase the hardware chip, obviously raised the cost, at present only some products through certification. Because of the popularity of high-definition programs, HDCP is a foregone conclusion, so support HDCP protocol devices will be more and more.
Full decryption of HDMI interface technology
Mention the interface of digital products, we can enumerate a large, what s terminal, AV terminal, chromatic aberration component interface, VGA interface, DVI interface ... When it comes to HDMI digital interface, consumers are not unfamiliar, as the latest generation of digital interface, HDMI has been widely used in a variety of digital products, whether it is flat-screen TV, DVD player, HD player, projector, digital camera, LCD display, as well as Blu-ray DVD and HD DVD, HDMI digital signal interface is indispensable.
Consumers of the advantages of the HDMI interface are very understanding, the author is not prepared to introduce, why the HDMI interface has these advantages may not be clear, the HDMI interface in the privacy of the data technology has won the praise of many enterprises, then in the end it has what characteristics, the following will give everyone to solve the mystery.
Basic transmission principle of HDMI
The HDMI (high-definition multimedia Interface), also known as the high-definition multimedia interface, is the first digital interface that supports all digital high-definition, multichannel audio, and intelligent format and control command data that is transmitted over a single cable without compression. The HDMI interface was developed by Silicon Image USA, a joint team of eight renowned consumer electronics manufacturers, including Sony, Hitachi, Panasonic, Philips, Thomson, Toshiba, etc. HDMI's earliest interface specification, HDMI1.0, was published in December 2002, and the current highest version is the HDMI1.3 specification released this June.
HDMI originates from the DVI interface technology, which is mainly based on the TMDS signal transmission technology of the American Crystal Image company, which is why the HDMI interface and the DVI interface can be converted to each other through the adapter. The American Crystal Image Company is the only IC design and manufacturing company in the HDMI eight initiator, is the leader in the field of high-speed serial data transmission technology, because the TMDS signal transmission technology mentioned below is what they developed, so here is a little mention.
TMDS (Transition minimized differential signaling) is also known as the minimized transmission differential signal, refers to the original signal data is converted to 10-bit through an XOR and XOR logic algorithm, the first 8 is the data from the original signal after the calculation, the 9th Bit indicates the way of operation, the 10th bit is used to correspond to the DC balance (dc-balanced, that is, in the encoding process to ensure that the channel DC offset to zero, level conversion to achieve a different logical interface between the matching), the converted data transmission by differential drive. This algorithm reduces the overshoot and undershoot of the transmitted signal transition process, and the transmitted data tends to be DC balanced, which reduces the electromagnetic interference of the signal to the transmission line, and improves the speed and reliability of the signal transmission.
In general, the HDMI connection consists of a pair of signal sources and receiver, sometimes a system can also contain multiple HDMI input or output devices. Each HDMI signal input interface can be based on the standard receive connector information, the same signal output interface will carry all the signal information. The HDMI cable and receiver includes three different TMDS data information channels and a clock channel that supports video, audio data and additional information, and video, audio data, and additional information is transmitted over three channels to the receiver, while the pixel clock of the video is transmitted via the TMDS clock channel. After the receiver accepts this frequency parameter, it restores the information passed by the other three data information channels.
Video and audio signal transmission
The source encoding format of the HDMI input includes video pixel data, control data, and data packets. The data packet contains audio data and auxiliary information data, and HDMI in order to obtain sound data and control data of high reliability, the packet also includes a BCH error correction code. HDMI data processing can be in many different ways, but ultimately in each of the TMDS channel contains 2-bit control data, 8-bit video data and 4-bit packets. The data transmission process of HDMI can be divided into three parts: video data transmission period, island data transmission period and control data transmission period.
HDMI data transmission, HDMI has three TMDS data information channel
Video data transmission period, the HDMI data line transmits the video pixel signal, the video signal is encoded, generates 3 channels (namely 3 TMDS data Information channel, each road 8 bits) altogether 24 bits of video data stream, enters into the HDMI emitter. The 24-bit pixel video signal is transmitted through the TMDS channel, the signal encoding of 8 bits per channel is converted to 10 bits, a minimized signal sequence is transmitted at each 10-bit pixel clock cycle, the video signal is modulated to transmit the TMDS data signal, and finally is received in the receiver.
The island data transfer period, audio data and auxiliary data will appear on the TMDs channel, each of which is a group of 4 bits, constituting a 4-bit packet mentioned above, and the same as the video data, is modulated to 10-bit a set of TMDS signal after the issue. The video data transmission period and the island data transmission period both begin in a guard band Protection band, Guard band is composed of 2 special characters, so the purpose of the design is to explicitly limit the control data transmission period after the jump is the video data transmission period.
HDMI data transmission period: Left to right respectively for the control data transmission period, the island data transmission period, video data transmission period
Control the data transfer period, between any of the above two data transmission cycles, each TMDS channel contains 2-bit control data, the total 6-bit control data is HSync (line synchronization), VSYNC (field synchronization), CTL0, CTL1, CTL2 and CTL3. Each TMDS channel contains 2-bit control data, using a 2-bit to 10-bit encoding method, at the last stage of each control cycle, a file header consisting of CTL0, CTL1, CTL2, and CTL3, indicating whether the next cycle is a video data transfer period or an island data transfer period.
The transmission of island data and control data is the blanking period of video data transmission, which means that when transmitting audio data and other auxiliary data, it does not occupy the bandwidth of video data transmission, and does not have a separate channel to transmit audio data and other auxiliary data. This is why an HDMI cable can simultaneously transmit video and audio signals.
HDMI high-pitched video bandwidth
HDMI data processing can be in a number of different ways, that is, HMDI supports a variety of ways of video coding, through a reasonable allocation of 3 TMDS data information channels, can transmit the RGB digital chroma component 4:4:4 signal, The 4:2:2 signal of the YCBCR digital chromatic aberration component can also be transmitted, which can meet the transmission needs of the 24-bit video signal.
HDMI per tmds channel video pixel stream rate is generally between 25mhz~165mhz, the HDMI1.3 specification has raised this limit to 225MHz, when the video format rate is less than 25MHz, the pixel repetition will be used to transfer, that is, the pixel in the video stream is reused. With a frequency of up to 165MHz per TMDS channel, 3 TMDS channels transmit r/g/b or Y/CB/CR encoded 24-bit pixel video data, The maximum bandwidth can reach 4.95Gbps, the actual video signal transmission bandwidth is close to 4Gbps, and now the highest standard HD video format The bandwidth required for the 1080p is only 2.2Gbps, so HDMI has enough bandwidth not only to meet the needs of today's HD video, but also to provide support for higher definition video formats over a long period of time.In addition to the high video signal bandwidth, HDMI also adds support for audio signal transmission in the protocol, forming the industry's first single-wire multimedia interface protocol. HDMI audio signals do not occupy an additional channel, but are used together with other ancillary information to form a packet, using 3 TMDS channels in the video signal transmission of the blanking period, in the form of island data transmission. Even when transmitting a 1080p (60HZ) video signal, it can provide up to 8 high-quality audio signals per sample frequency of 192kHz, in contrast to the two-channel signal of the CD audio format 44.1kHz, and the latest Dvd-audio audio formats 96kHz 6-channel signal , it dwarfs it.
HDMI interface and protocol