HDIVI Transmitter Solution for Audio/Video Information Playback System

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INTRODUCTION <br> Most playback systems on the market now use independent audio/video interfaces, such as audio using RCA terminals, coaxial terminals, etc.; video uses S-terminal, color-difference terminal, DVI interface, etc. These interfaces are bulky (such as DVI, not suitable for use on portable devices), and the interfaces are independent and have too many wires. In addition. Many of the traditional interfaces are analog, such as RCA, S-terminal, they are difficult to reflect the advantages of today's digital products.

High-definition multimedia interface HDMI shows its unique advantages in all aspects: small size, high bandwidth; not only can transmit uncompressed digital video images from SDTV to HDTV resolution, but also can transmit 2-8 channels of digital audio at the same time, interface friendly The E-EDID structure of the receiving device can be read through the display data channel (DDC), and the audio/video format supported by the receiving end is automatically determined; the HDCP broadband digital content protection mechanism is adopted to prevent eavesdropping of the illegal device. These meet the new consumer demand for digital products, so the HDMI interface is more widely used in various digital products.

1 Playback system playback system, mainly to read audio/video information received from external memory cards (SD, MMC, etc.) or through wired (USB, etc.), wireless (wireless network card, etc.) transmission, after decoding The audio/video interface is transmitted to audio-visual equipment (such as TV) to play back audio/video such as digital photos and digital cameras.

Seen from Figure 1. The audio/video interface is an integral part of the playback system. The traditional video interface mainly adopts S terminal and color difference terminal; the audio connection mainly adopts RCA terminal and coaxial terminal. These traditional interfaces are no longer suitable for high-quality, uncompressed digital video and multi-channel digital audio transmission with the rapid development of digital technology. The new HDMI interface is set to meet the higher demands of consumers.

2 HDMI interface
2.1 Introduction to HDMI HDMI is a new interface following DVI (Digital Video Interface). It not only supports uncompressed digital video transmission from SDTV to HDTV, but also supports 2 to 8 channels of digital audio transmission; it is equipped with many other auxiliary functions such as HDCP (Wideband Digital Content Protection) and intelligent connection configuration.

HDMI has many advantages:
â—‡ Small size, more suitable for portable devices;
â—‡ It is able to transmit audio/video simultaneously on a single cable, and the interface is more friendly;
â—‡ High bandwidth, capable of transmitting high-definition video up to l080 p and up to 8 channels of audio, and supporting RGB and YCbCr two pixel encoding formats, providing higher color depth;
â—‡Use minimal variation differential coding TMDS to provide robust transmission mechanisms and reliable data recovery capabilities;
â—‡Provides intelligent connection function, enabling the device to automatically discover each other and automatically identify the resolution and data format required by the target device;
â—‡With HDCP encryption, it can prevent eavesdropping of illegal devices;
â—‡Good compatibility and full compatibility with DVI.
The HDMI connection structure is shown in Figure 2.

The HDMI interface has three independent communication channels: TMDS, DDC, and CEC. The TMDS channel is used to transmit audio/video data and auxiliary data; the DDC is used to read the E-EDID data structure of the receiving end in the automatic configuration; the CEC is an optional channel to support some advanced user functions, such as Infrared remote control, etc.

2.2 HDMI Signal Encoding HDMI has three operating modes: control cycle, video data cycle and data island cycle. The control period is used to transmit the pilot information; the video data period is used to transmit the pixels of the active video line; the data island period is used to transmit the audio sample data packet and the auxiliary data packet. The periodic allocation of data on a line period with video information is shown in Figure 3.

In 3 cycles, different coding is used on the TMDS channel: control period, 2 bits of valid information per channel, only 4 coding combinations; data island period, 4 bits of valid information per channel, 16 kinds of coding combinations, so the coding is relatively simple. The lookup table is available; the video data cycle is relatively complex, using TMDS encoding. TMDS encoding is a special encoding method that converts 8-bit characters into 10-bit characters. First, the 8-bit character, with the lowest bit unchanged, encodes the remaining 7 bits corresponding to the previous encoded data by XOR or XNOR minimum change ( The least change before and after encoding) is a 9-bit character (what is the conversion of the 9th bit flag, O means XNOR, l means XOR). Then, according to the number of data O and l that have been transmitted and the number of 0 and 1 data to be transmitted currently, it is determined whether to invert the 8 data bits of the 9-bit information generated in the first step (if already Transmit more l, and the current data is more than 0, then reverse), converted into a 10-bit DC balance code (whether the 10th bit flag is inverted, 1 means reversed, O means no reverse turn). Finally, the encoded data is serialized and sent serially on the TMDS channel in differential form.

2.3 HDMI supported audio/video format 1 video. HDMI can support RGB4:4:4, YCbCr4:4:4, YCbCr4:2:2 three pixel encoding formats. When transmitting video, you must follow the number of pixels of the video line specified by a certain video format, the number of lines of the field (effective number and total number), and the position, polarity, duration, and so on of the two synchronization signals. The basic video format timing supported by HDMI is 640×480 p@59.94/60 Hz, 1280×720 p@59.94/60 Hz, 1 920×1 080 i@59.94/60 Hz, 720×480 p @59.94/60 Hz, 720(1440)×480 i@59.94, 60 Hz, 1 280×720 p@50 Hz, 1 920×1 080 i@50 Hz, 720×576 p@50 Hz, 720 (1440) × 576 i@50 Hz.
2 audio. HDMI uses a "packet" structure to transmit audio data in the data island cycle, and the packet structure is in IEC60958 or IEC61937 package form. HDMI supports at least two channels of L-PCM with IEC60958 structure. The sampling frequency can be 32 kHz, 44.1 kHz, 48 kHz, and the sampling depth is 16 bits or more. It is allowed to transmit L-PCM in any packet format of IEC60958 or IEC61937. Or encoded audio data at a sampling frequency of 44.1 kHz, 48 kHz, 88.2 kHz, 96 kHz, 176, 4 kHz or 192 kHz. The audio sampling package has two layouts: the first layout sends the audio data of channel 1 and channel 2 in each sub-package, and only supports 2 channels of audio; the second layout is on the sub-package [O~3]. The audio data of channel [1, 2] to channel [7, 8] is respectively transmitted, and 4 to 8 channels of audio are supported.

2.4 HDCP Content Protection HDMI uses HDCP for three main purposes:
First, HDCP can be used to verify that the receiving device is authorized to receive encrypted content. The source first exchanges the device key through the DDC channel and the receiving end, verifies the receiving device, and generates a common key, which is equivalent to establishing an encrypted channel. Secondly, during the transmission process, HDCP is encrypted at the source end and decrypted at the receiving end to prevent the encrypted content from leaking during transmission. The source (or the receiving end) respectively generates a 24-bit pseudo-random data stream (by XOR) in the encryptor to encrypt (or decrypt) the input 24-bit data by using a public key, as shown in FIG.

Also, HDCP is able to identify and "revoke" unauthorized devices to prevent the bulk distribution of illegal keys. The HDCP has an update function: The Digital Content Protection Authority LLC places the Key Selection Vector (KSV) of the compromised device into the "Revocation List" and sends it to the subsequent source device via the System Update Message (SRM). Because the source detects the list during the verification process, subsequent source devices can automatically block the receiving device that has been "revoked".

3 Playback system HDMI transmitter implementation playback system uses Atmel's multimedia processing chip AT76C114 as the signal source, HDMI interface controller uses Silicon Image's Sii9030.

The AT76C114 is a dedicated multimedia processing chip based on the ARM946E-S core. It not only has powerful audio/video processing capabilities (supports MP3 decoding, supports JPEG codec, MPEG-1 and MPEG-4 codec), but also is equipped with a USB controller, flash memory card (such as SD, MMC, etc.) controller, etc. Peripheral function. Its video output supports analog composite video signals (CVBS), discrete video signals (Y/C), component video signals (Y/PrPb), and digital (from 480i/480p for SDTV to 720p and 1080i for HDTV) Video format; pixel encoding supports YCbCr and RGB output. The audio output supports I2S and S/PDIF interfaces. The AT76C114 functional module diagram is shown in Figure 5.

The Sil9030 is an HDMI Transmitter that complies with HDMI 1.1, HDCP 1.1 and is compatible with DVI l.0. It supports DTV's various video format inputs, supports YC and RGB pixel encoding formats; audio supports I2S and industry standard S/PDIF inputs; integrates PaneLink TMDS encoding core; integrates HDCP content protection encryption engine; supports the same I2C The interface accesses the Sii9030's registers and DDC channels; it supports detection of new display devices, audio FIFO overflow and other interrupt outputs. The SiI9030 function module diagram is shown in Figure 6.

The AT76C114 is connected to the Sil9030 hardware as shown in Figure 7. The video connection uses a 16-bit YCrCb (4:2:2) pixel encoding format; the audio uses I2S; SCK and SDA are the I2C connections that the AT76C114 accesses the Sii9030 register. The external interrupt INT2 of the AT76C114 is used to process the interrupt request issued by the SiI9030.

All initialization configurations are implemented through the I2C interface. Because the AT76C114 does not have a hardware I2C interface, the implementation uses a general-purpose input/output port (GPIO) through software emulation. The initial configuration is as follows:

First, the AT76C114 sets the HDCP controller register of the SiI9030 through the I2C interface, drives the DDC channel logic controller inside the SiI9030 to perform the authentication process with the receiving device, and establishes an encrypted channel. If the authentication is successful, the E-EDID of the receiving device is accessed through the DDC channel. Determine the receiving format of the best audio/video for the display device. Then, according to the format determined above, the AT76C114 sets the audio/video format of its own output and the format of the SiI9030 receiving (capturing), including: the video pixel clock and the polarity and resolution of the two sync signals; and the audio I2S clock regeneration parameters. N/CTS, sampling rate, sampling depth, sample package layout, etc. Finally, the audio/video output and HDCP encryption are activated.

Conclusion
The playback system HDMI transmitter solution can be applied to multimedia products such as MP4 and set-top boxes. The series of advantages of the HDMI interface will make it more and more widely used in the increasingly mature consumer electronics field of digital multimedia technology.