Design Solution 2 for Video Call Based on STi7105 HD Set-Top Box

3.1.2 Audio Coding Technology

Voice communication is the most basic function of videophone. Due to the limitation of network conditions, videophone usually works at a lower bit rate. ITU-T has launched a series of voice compression standards, including G.711, G.723.1, G.

G.728, G.729 and G.729A are widely used in videophones. When videophones communicate with ordinary phones, the G.711 standard is used. G.711 is PCM coding, which only samples and quantizes the voice signal to generate a 64kbps code stream. The voice quality after G.711 coding is high, but the disadvantage is that the bandwidth occupied is also high. G.723.1 can generate two-rate code streams. The high-rate encoder uses the multi-pulse maximum natural quantization (MP-MLQ) algorithm, and the low-rate encoder uses the algebraic code excited linear prediction (ACELP) algorithm. G.729A is a simplified version of G.729. Its complexity is reduced by 50% compared with G.729, and the voice quality is slightly reduced. The code streams encoded by the two standards can be decoded mutually.

When actually selecting a voice codec algorithm, various factors must be considered. A high bit rate can ensure good voice quality, but it takes up a lot of storage space and consumes more resources; while a too low bit rate will affect the quality of the voice and increase latency. Due to the limited audio processing capabilities of the STi7105 chip, we use the G.711 and G.723.1 formats for audio encoding.

3.1.3 Video Coding Technology

Video compression is the core technology in multimedia applications. The low-bitrate video compression standard launched by ITU-T has played an important role in promoting the development and practical application of videophones. H.261 is the first low-bitrate video compression standard launched by ITU-T. The bitrate is P×64kbps, where P is between 1 and 30, and the image format is CIF and QCIF. The basic idea of ​​the H.261 compression coding algorithm is to use predictive coding to reduce temporal redundancy and transform coding to reduce spatial redundancy. The algorithm mainly consists of motion estimation, motion compensation, DCT transform, quantization and Huffman coding.

H.263 is the earliest ITU-T standard for low-bitrate video coding. The subsequent second version (H.263+) and H.263++ added many options to make it more widely applicable. H.263 is a video coding standard developed by ITU-T for narrowband communication channels below 64kb/s.

It is developed on the basis of H.261, and its standard input image format can be S-QCIF, QCIF, CIF, 4CIF or 16CIF color 4∶2∶0 sub-sampled image. Compared with H.261, H.263 adopts half-pixel motion compensation and adds 4 effective compression coding modes.

H.264 is a new generation of video compression coding standard developed by the Joint Video Team (JVT) composed of ISO/IEC and ITU-T. It has strong adaptability to channel delay and can work in low-delay mode to meet real-time business, and can also work in situations without delay restrictions. The codec adopts a scalable complexity design, which can be graded between image quality and coding processing to adapt to applications of different complexities and improve network adaptability. Compared with previous video compression standards, H.264 introduces many advanced technologies, including 4×4 integer transform, intra-frame prediction in the spatial domain, motion estimation with 1/4 pixel accuracy, and inter-frame prediction technology with multiple reference frames and blocks of various sizes.

H.261 and H.263 are widely used in video communication. Compared with H.261, H.263 adds several options, provides a more flexible encoding method, greatly improves compression efficiency, and is more suitable for network transmission. The introduction of the H.264 standard is a milestone in the progress of video encoding standards. Compared with H.263 and H.261, it has obvious advantages, especially the improvement of encoding efficiency, which enables it to be used in many new fields. Due to the limited video encoding capability of the STi7105 chip, we choose the H.263 format for video encoding.

3.2 System Structure

The video call test system is mainly composed of a server, a transmission network and an STi7105 set-top box terminal. The architecture is shown in Figure 2.

Figure 2 System structure diagram

The server is used to control the call process of the entire system. The basic function of the server is to complete the switching of signaling, video, voice and other signals according to the user's requirements to ensure the continuity of audio and video.

The network access uses the existing wired HFC two-way network.

The STi7105 set-top box terminal is a node device that provides real-time, two-way communication functions. It mainly processes various digital signals such as signaling, video, and audio to form a composite digital code stream, and then converts it into a format suitable for transmission in the transmission network and sends it to the channel for transmission. At the same time, the STi7105 set-top box terminal can receive video and audio signals, and send them to the corresponding output device after processing. The STi7105 set-top box terminal mainly includes the following main components: video codec, audio codec, data protocol, control protocol, and demultiplexing module.

3.3 STi7105 Set-Top Box Terminal

STi7105 HD set-top box terminal is a wired HD digital two-way set-top box that fully complies with the DVBC standard. It uses a single-chip decoding chip STi7105 (ST40 CPU@450MHz 32K I 32K DCache), and is equipped with 64M FLASH and 256M SDRAM; built-in Cable Modem circuit board, the cable digital TV signal from the HFC network is first connected to the Cable Modem circuit board, and then looped out to connect to the STi7105 main circuit board; dual-channel network RJ45 interface, one for external interface, one for Cable Modem; dual-channel USB2.0 high-speed interface, HD output supports HDMI, YPbPr HD interface, audio supports Dolby5.1 channel digital audio output and PCM decoding output, can support built-in SATA hard disk to realize local recording and playback time-shift of multimedia data, realize real home theater effect, and support network download and upload services, providing users with comprehensive digital audio and video value-added services. External camera, telephone or Bluetooth headset is used to realize video calls. The structural diagram is shown in Figure 3.

Figure 3 STi7105 set-top box structure diagram