Digital set-top boxes applied to video-on-demand systems

[Abstract]: Modern communications to digital, broadband, intelligent, integrated development, the rapid development of digital technology has been CATV network, telephone network and data network closely linked to provide a variety of different types of information services, video on demand is one of them. This paper introduces the digital set-top box in VOD system, including the key technology, relevant standard and concrete realization method of STB.

1. Introduction

Our CATV network has become one of the three information networks including telephone network, data network, its coverage is wide, the scale is huge, it is the world's most. At present, China's existing cable TV stations more than 2000, cable TV users more than 80 million, and is growing at 5 million a year. The existing CATV network is limited to analog television broadcasts, with hundreds of MHz of broadband resources not yet utilized. Developing and utilizing the broadband resources of CATV Network, realizing the multimedia information service such as interactive/non-interactive digital video and data has become the urgent need of the information age, and VOD service is one of the interactive digital video information services. As the client receiving device of VOD system, digital STB has been developed rapidly.

Digital STB may become the mainstream of computer and consumer electronics in the next 10 years or more, so many companies, including Ibm,st,lsi,philips,sony, have entered the field, and some international or regional organizations have developed relevant standards and agreements. This paper will discuss the key technologies and implementation methods of digital set-top box used in VOD system, and introduce relevant standards and protocols. 2, the overall plan and related standards

After the user chooses the program according to the main menu, the digital set-top box transmits the user's program request information to the center control system through the uplink channel, the center control system sends the corresponding program to the CATV network according to the user request information. Then the digital set-top box receives the signal on the CATV network, completes the function of the lower frequency, demodulation, channel decoding, multiplexing, decompression, Video coding and audio DAC, and sends the analog video/audio output signal to the TV set, thus realizes the interactive video/data information service.

At present, the most influential digital broadcasting standards are DVB,DSS and large Alliance DTV, which DVB for satellite broadcasting, cable television and terrestrial broadcasting in different forms are divided into dvb-s,dvb-c and DVB-T. This scheme uses similar standard with DVB-C, the source code video uses the MPEG-2 standard, but the audio uses the Musicam standard, the service multiplex uses is the MPEG-2 system layer specification, the channel code uses the rs+ interleaving way, but the modulation method uses the QAM modulation.

The MPEG-2 standard has developed a common coding standard for active images and their accompanying sound, which is adapted to various applications such as storage media, distribution transmission and communication. The MPEG-2 standard has 9 items, the digital set-top box mainly involves 3 items: The CD13818-1 system, has stipulated the system coding layer technical request; Cd13818-2 video, specify the technical requirements of video coding; Cd13818-3 Audio, specify the technical requirements for audio coding. One of the most notable features of MPEG-2 video standards is its versatility, which is not dependent on specific applications, which makes MPEG-2 the current widely used video compression standard. MPEG-2 Audio Standard is MPEG-1 based on the development of multi-channel coding system, its 1th, 2 layers called musicam surround sound. The data flow types defined by the MPEG-2 system layer are transport flows (TS) and program flow (PS). Where the transfer flow is applied to the transmission and storage of coded data for one or more programs in an environment where there is a risk of serious error, the VOD system uses the MPEG-2 transfer stream to video, Audio and data signals are grouped and reused. RS code is a kind of forward error-correcting code commonly used in digital transmission, it is a good performance of the packet linear code, under the same coding redundancy, RS code has the strongest error-correcting ability. With RS code as the outer code, multilevel trellis code as the cascaded code of the inner code, combined with the complete data interleaving, it provides a powerful forward error correcting ability for digital transmission. Orthogonal amplitude modulation (QAM) can be considered as a combination of amplitude modulation and phase modulation, and QAM modulation has a high frequency band utilization, that is, it can transmit high digital rate on the same bandwidth. The system must transmit a large number of image information, the digital rate is high, in order to effectively use the frequency band resources, the use of QAM modulation method is more reasonable.

3, the hardware realization of the digital STB

Digital STB's hardware function is realized by 8 function modules, including High-frequency Head, QAM demodulation, MPEG-2, MPEG-2 decoding, Video coding, audio DAC, user control Interface and modem, the following will introduce each function module principle and concrete realization.

3.1 High Frequency head

Because the low-frequency signal is not suitable for long-distance transmission, the signal must be modulated before it is converted to high-frequency signal before it is sent out. The CATV network transmits is the 45mhz~860mhz high frequency signal, the digital set-top box must first carry on the frequency conversion to the high-frequency signal, realizes by the high frequency head. The system through the I2C bus can be tuned to High-frequency head, control high-frequency head to output if signal, one of the first intermediate frequency of 36.15MHz, the other way for 7MHz of the second if, the bandwidth is 7MHz, the system uses the second if signal.

3.2 QAM demodulation and channel decoding

As described in the overall scheme, the system channel coding adopts rs+ interleaving method, and the modulation method adopts QAM modulation. Therefore the digital set-top box must complete the QAM demodulation system, the solution interleaving and the RS (in the de-Solomon) decoding.

We use LSI Company's l768s to achieve the above functions. L64768 mainly includes four modules: ADC module (analog-to-digital transform), QAM core module, FEC core module and microprocessor and host interface module. It mainly completes the QAM demodulation, the solution interweave and the Reed-Solomon decoding function. The second intermediate frequency signal from the high-frequency head is transformed into an external or internal modulus transformation to a digital signal of 8bit of each sample point (the second frequency with a sampling rate of four times times), then the signal is divided into synchronous and orthogonal signals. L64768 has the characteristic of automatically obtaining the baud rate, when the actual baud rate is reached, the loop lock is realized by measuring the average power of the input sample point, and the AGC of the High-frequency head is controlled by an external passive low-pass filter to achieve the stability of the output signal power of the high-frequency head L64768. The interleaving device resets the signal order in a determined manner, with a 204-byte block size. The Reed-Solomon decoder in L64768 fully complies with the CCIR723 standard recommended by CCITT, which implements the (204,188) Reed-Solomon decoding, thus completing the function of forward error correction. After QAM demodulation and channel decoding, the signal is sent out in the form of MPEG-2 transfer stream, and is reused by the subordinate module.