5.1.2 Operating Modes Overview
The HDMI link operates in one of three modes: Video Data Period, Data Island period, and Control period. During the Video Data Period, the active pixels of an active video line are transmitted. During the Data Island period, audio and auxiliary data are transmitted using a series of packets. The Control period is used when no video, audio, or auxiliary data needs to be transmitted. A Control Period is required between any two periods that are not Control Periods.
An example of each period placement is shown in the following figure.
Hình minh họa trong link gốc, trang 71 (trang này đánh số 55/156)
Video Data Periods use transition minimized coding to encode 8 bits per channel, or 24 bits total per pixel.
Data Island Periods are encoded using a similar transition minimized coding, TMDS Error Reduction Coding (TERC4), which transmits 4 bits per channel, or 12 bits total per TMDS clock period.
During Control Periods, 2 bits per channel, or 6 bits total are encoded per TMDS clock using a transition maximized encoding. These 6 bits are HSYNC, VSYNC, CTL0, CTL1, CTL2 and CTL3.
Near the end of every Control Period, a Preamble, using the CTLx bits, indicates whether the next Data Period is a Video Data Period or a Data Island Period.
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Mã:
Table 5-1 Encoding Type and Data Transmitted
-------------------------------------------------------------
Period |Data Transmitted |Encoding Type |
--------------|-------------------|-------------------------|
Video Data |Video Pixels |Video Pixels Coding |
| |(8 bits converted to 10 bits)|
|------------------|---------------------------|
|(Guard Band) |(Fixed 10 bit pattern) |
--------------|-------------------|--------------------------|
Data Island |Packet Data |TERC4 Coding |
|- Audio Samples |(4 bits converted to 10 bits)|
|- InfoFrames | |
| | |
|HSYNC, VSYNC | |
|------------------|---------------------------|
|(Guard Band) |(Fixed 10 bit pattern) |
--------------------------------------------------------------|
Control |Control |Control Period Coding |
|- Preamble |(2 bits converted to 10 |
|- HSYNC, VSYNC | |
---------------------------------------------------------------
5.2 Operating Modes
5.2.1 Control Period
Control Period is used for transmission of the Preamble. The Control Period is also used by the
Sink for character synchronization.
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5.2.2 Video Data Period
Video data periods are used to carry the pixels of an active video line.
Each Video Data Period is preceded by a Preamble, described above.
Following the Preamble, the Video Data Period begins with a two character Video Leading Guard
Band. There is no Trailing Guard Band for the Video Data Period.
During active video periods, 24 bits of pixel data are encoded using TMDS transition minimized
encoding during each TMDS clock period.
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5.2.3 Data Island Period
5.2.3.1 Data Island Overview
Data Islands are used to carry packets of audio sample data and auxiliary data. This auxiliary
data includes InfoFrames and other data describing the active audio or video stream or
describing the Source.
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5.2.3.5 Data Island Error Correction
To improve the reliability of the data and to improve the detection of bad data, Error Correction
Code (ECC) parity is added to each packet. BCH(64,56) and BCH(32,24) are generated by the
polynomial G(x) shown in Figure 5-5.
G(x)=1+x^6+x^7+x^8 (127 count repetition cycle).
Chi tiết các gói dữ liệu trong Data Island Period xin xem trong link gốc:
5.3 DATA ISLAND PACKET DEFINITIONS ......................62
5.3.1 Packet Header ...............................................62
5.3.2 Null Packet ...................................................63
5.3.3 Audio Clock Regeneration Packet ........................64
5.3.4 Audio Sample Packet .......................................64
5.3.5 InfoFrame Packet ............................................66
5.3.6 General Control ..............................................67
5.3.7 Audio Content Protection Packet (ACP) ................68
5.3.8 ISRC Packets .................................................68
5.3.9 One Bit Audio Sample ......................................72
5.3.10 DST Audio ...................................................73
5.3.11 High-Bitrate (HBR) Audio Stream ......................74
5.3.12 Gamut Metadata Packet ..................................74
) . Tất cả gồm 255 bộ quy tắc như thế. Xác suất để tín hiệu suy hao đúng quy tắc phức tạp thế này là vô cùng nhỏ (cỡ xác suất truyền hình ảnh xe đạp thành thuyền buồm). Mà đây lại chính là cách mà TV, VGA hay phần mềm chơi video chỉnh màu sắc, sáng tối (do đó hiện tượng bị tối hay khác tông màu nhiều khả năng do chỉnh màu sắc khác nhau trên 2 ngõ input khác nhau gây ra). Suy hao thực tế (nếu xảy ra) thể hiện trên hình ảnh không ổn định hoàn hảo được như thế mà sẽ có dạng không có quy tắc ổn định như nhiễu hay 1 dạng mất ổn định hình ảnh nào đó.
