Introduction to LCD TV Video Decoder Technology

LCD, video decoder

In terms of technology, LCD TV is a highly integrated electronic product. It covers algorithms, software, hardware, chips, radio frequency and other fields, and many patents belong to foreign companies or institutions. At present, there is a lack of system integration talents familiar with this field in China. At present, even in advanced countries, because LCD TV or HDTV are still the latest products, there are not many technical talents familiar with this field. Similar to digital STB, however, the technical complexity of the former is much higher than that of the latter, because LCD TV or HDTV not only includes the function of network communication, but also includes OSD (on screen display), zoom controller (scalar), screen drive and other functions. Therefore, even those who have designed STB dare not say that the technology of LCD TV or HDTV is very simple.

Hardware architecture of LCD TV

Figure 1 hardware architecture of LCD TV

Fig. 1 is a schematic diagram of the hardware architecture of LCD TV. Among them, the more important audio and video components are: CPU, video decoder, RF tuner, channel demodulator / decoder and audio codec. In addition, it may also include common I / O interfaces and devices, such as USB, 1394, Bluetooth, WLAN, IrDA, MMC / SD / CF, Ethernet, digital camera, DSL or cable modem, PDA, etc.

Video decoder

Fig. 2 location of mixer decoder

Fig. 2 shows the relationship between an LCD TV / HDTV video decoder and adjacent chips. It has the following main functions:

1. Programmable pixel output port: the YCbCr / RGB ratio of pixels is determined by 8 to 30 bits, such as 4:2:2 or 4:4:4.

2. Support external RF tuner with 10 bit digital fundamental frequency composite video input unit.

3. Automatically identify and process VCR or digital TV signals.

4. Support the output of "vertical blank interval (VBI)" data through the pixel output port, which can be used to transmit "closed caption (CC)" data, such as real-time stock market. Other VBI data can be transmitted through I2C interface, such as copy generation management system (CGMS), wide screen signaling, enhanced definition television (EDTV), CC subtitles, and extended data service (XDS).

5. Support a variety of existing global color formats:

*PAL B/D/I/G/H、PAL60、PAL M、PAL N、PAL Nc

*NTSC M/J、NTSC 4.43

*SECAM B/D/G/K/L

*NTSC square pixel

6. Support all input signals of HDTV, such as 525P, 625p, 720p and 1080i, as well as other high-resolution (hd) and SMPTE (Society of motion picture Engineers) standards. Standard resolution (SD) is also supported.

7. In addition to supporting CVBS, S-Video, YPbPr and RGB, it also supports 5-wire ypbprhv / rgbhv and CSYNC, as shown in Figure 3.

Figure 3 supports multiple video input formats

Path of video stream

Fig. 4 is an architecture of a simple LCD TV or digital TV receiving system. It includes the above hardware architecture and the necessary software.

Figure 4 simple LCD TV system architecture

After the MPEG-2 transport stream (TS) transmitted from the decoder and demodulator passes through the "demultiplexer", different signals such as video, audio, data, subtitle or subtitle... Can be obtained. After passing through the video decoder (e.g. MPEG-2 / 4 or H.264) and audio decoder, the obtained data is the most original data. These data will be sent to the appropriate communication protocol layer by the CPU, such as v4l2, RTP / RTCP / RTSP, HTTP, Java application... And then sent to the Internet through the "return channel", such as Ethernet; Or send it to OSD and audio DAC for LCD and speaker playback.

Yes, digital set-top boxes have similar functions as mentioned above. Like LCD TV, it also has the function of "conditional access (CA)". However, LCD TV screens are built-in, and general digital set-top boxes can only obtain video through a single path. However, LCD TV can obtain video through four different paths, including satellite digital, digital terrestrial, Internet and wired data network. However, as far as the software of interactive TV is concerned, the software of digital set-top box and LCD TV is very similar or even the same.

In the "device driver" layer, in order to play digital TV signals, the I / O port of LCD TV must have LCD driver (capable of setting OSD and scalar) and audio driver. In order to transmit interactive information to the Internet, it also needs Ethernet drivers and DSL (or cable) modem drivers. External devices can also send data to LCD TV for processing through IrDA or I2C, SPI, UART... And other interfaces. In addition, the driver of the video / audio decoder at the receiving end is also located in this layer.

Many engineers do not understand the action path of video data flow of LCD TV or digital set-top box, so it often causes trouble in error elimination. Especially after passing through the video decoder, the signal should be handed over to the central processing unit instead of other components. Similarly, if the signal source in Fig. 4 is replaced by Ethernet instead of RF antenna, the signal decoded by the video decoder shall still be handed over to the central processing unit. In addition, in Figure 4, whether MPEG-2 ts or PS / PES, they can be regarded as the lowest digital data packets, and their status and role are the same as Ethernet packets, which are different from analog signals.

Decoding engine

Due to the diversity of video and audio formats, using a single decoder can not meet so many needs. Therefore, there is a so-called "decode engine", which may be a RISC or DSP, which is responsible for resetting, stopping, loading and starting firmware or "microcode", and handling all work unrelated to stream decoding, such as outputting sound.

Each decoding engine has several decoders, which are responsible for the following tasks: 1. Turn on or off a single stream. 2. Transmit command. For example, initialize, Uninitialize, play, pause, stop, etc. 3. Obtain current status information. For example: playing, paused, etc. 4. Unique characteristics of various streams. For example: data format, etc.

There are many hardware modules in these RISC or DSP, and each module has a specific entity function; These functions can usually be set using register. Generally speaking, these RISC or DSP will include the following hardware modules: 1. Demux engine: divide video and audio at the same time. 2. MPEG engine: it is a video DSP, including a video decoder and a "subpicture decoder". 3. Audio engine: it is an audio DSP, including two audio decoders and a mixer. 4. CPU: CPU core, DMA, interrupt processing unit.

These RISC or DSP software modules use hardware modules to generate "tasks". For example, the following software modules: 1. System block: PLL initialization, DRAM setting, etc. 2. Graphics acceleration engine: use the FIFO controlled by the graphics acceleration command to speed up the processing speed. 3. Subtitle decoding module: subtitle decoding unit of software. 4. I2C: I2C interface of software, which is a programmable input / output port (PIO). 5. Memory management module: DRAM configuration.

Decoding engine driver

Figure 5 driver architecture of video decoding engine

Generally, the driver of video decoding engine will include the following units (as shown in Fig. 5): 1. Application program interface (API) independent of operating system and chip: control the access of buffer and DRAM. 2. Hardware abstraction layer link library: related to hardware, it can be set through XML language to adapt to specific chips. It does not contain APIs related to the operating system, and only calls APIs unrelated to the operating system and chip. 3. Minimum core module: it is above 1. Above, responsible for accessing buffer and processing DMA, and processing all operations of OS core, such as IRQ processing, DMA configuration, etc. 4. Core module: it calls "minimum core module" to access hardware.

Memory configuration

Fig. 6 memory configuration of video decoder

Memory configuration must be specially considered when developing video decoding device. Fig. 6 is a memory configuration of a typical video decoder; Attached table 1 shows the memory size required for decoding various video formats.

Table 1 memory requirements of various video formats

When configuring memory, you must avoid splitting too small and too many. Be sure to configure a large and long-term buffer first; During multi-channel playback, unnecessary configuration or release of buffer space shall be avoided.