Electronic engineering department of Tsing-Hua University Visible professor Dong
The sound is that the mankind accepts the important media of information, are regarded as a way to transmit information, the sound broadcasts and occupies important position. The present sound has two kinds to broadcast: Amplitude modulation broadcasting and FM broadcasting. Look about the expectation broadcasting from the audience, these two kinds of radio ways have the following deficiency:
1.The sound quality does not meet the demands. With the constant improvement of life quality, people hope the sound quality of radio can reach the level of CD, the present sound can not reach the requirement of the such one in such aspects as bandwidth, dynamic range and interference level while broadcasting quality.
2.It is single to broadcast the business. With the requirements for various information, people expect to accept the business of the data through radio, though can convey the data the radio RDS of wireless data in FM broadcasting at present ,But it can’t meet the demands to convey capacity and quality of the data.
3.Receiving quality can’t guarantee, are especially moving the situation received.
In order to overcome these shortcomings, raise the sound quality of the sound radio, receive quality and increase the business of the data greatly, it can only be the digitization of the public address system, need to develop application of technology of digital audio frequency in radio in a more cost-effective manner.
Basic composition and key technology of the system
It is very similar to composition of the digital communication system, we can say too, digital audio broadcasting is a one point to many point digital communication system.
The key technology of digital audio broadcasting mainly has three respects, the compressed code of the digital audio frequency signal, wireless transmission and group’s network technology of the high-speed data signal.
1.Compressed code of the digital audio frequency signal
After carrying on with linear PCM digitization of the audio frequency signal, its yard of rate is about 700kbit/S. For example the sample rate of CD is 44.1kHz, samples each time and is indicated by 16bit word length, its yard of rate is 705.6kbit/S, one yard of rate of stereoscopic acoustical signals of No. one reaches 1411.2kbit/S. So high yards of rate need very wide frequency band during transmission, need very great memory capacity in memory, this will limit the application of the digitized audio frequency greatly, it is very difficult to realize digital audio broadcasting. In order to solve this problem, need adopting and compressing code technology, on condition that basically guarantee to be close to CD tone quality, reduce one yard of rate effectively.
Apply to digital radio including digital TV and digital audio broadcasting Audio frequency of China compress the intersection of code and technology have multi-type, they based on the intersection of people and the intersection of ear and sense quality, come, realize the purpose of reducing one yard of rate. In code scheme these, spend people frequency, time of ear, cover the effect and localization characteristic to the voice. Frequency it covers effects to be until frequency close to and when intensity have obvious two signals of difference appear at the same time, the signal that the ears of people can only feel of high intensity, and the signal low in intensity will be covered; Time it covers effects to be to appear when inside a time block before or after the signal that intensity better than it as one the intersection of intensity and weak signal, weaker signal will be covered, obviously hidden our signal not to need conveying, thus has reduced one yard of rate; When the ears of people make a reservation to the sound source, insensitive to signal directionality of low frequency, the direction in the high-frequency signal is mainly judging from the feeling that is held to the bag in place with a net, these characteristics are used for reducing one yard of rate of the stereo code. Certainly, deal with a part in the primitive audio frequency signal of losses like this, but because this part is imperceptible or very insensitive, so it will not basically influence the quality of the sound, thus has already guaranteed the tone quality, have reduced one yard of rate. The compressed code scheme of applying to digital audio broadcasting is mainly LayerⅡ in MPEG-1/2, and MPEG AAC,etc., make a yards of rate of stereo No. reduce to 192-96kbit/S, and may continue reducing, reduce the requirement for transmitting bandwidth and memory capacity greatly, should compress the development which encodes technology to be the foundation that the digital audio broadcasting can realize.
2.Wireless transmission of the high-speed data signal
Sound broadcast can transmit through wired channel with wireless channel, convey the signal to the receiver through the intersection of electromagnetic wave and spread in space mainly, the spread characteristic of the channel of space is not steady as the cable, optic fibre, satellite channel far, especially in cases it is received that the urban environment is neutralized and moved, because multipath propagates the alternative decline of frequency formed and delays time to expand, and the intersection of shade and effect that building’s sheltering from form, make it seriously affect not to receive quality, and the public address system often works in such an environment. In order to guarantee radio quality, must improve in the urban environment and move and receive the transmission quality of the signal in the situation, the channel code and modulating some designs in the digital audio frequency public address system is mainly to solve the problem that in this respect. Most digital audio frequency public address systems adopt error-correcting code, perpendicularity to divide and reply using OFDM frequently at present The association interweaving with the time, frequency, OFDM is dividing replying and using technology frequently of a kind of high spectral utilization ratio, it vary many way, walk abreast data transmission at a high speed serial data, help, resist the intersection of frequency and the intersection of alternative and decline and delay time, expand, and it can realize by IFFT and FFT, enable, send and receive the apparatus to simplify. The experiment indicate, transmission systems adopted may satisfied with the reliable transmission of the broadcast program under moving, portable and fixed condition of work.
3.Group’s network technology
With broadcasting the development of the undertaking, frequency resources are in short day by day, must consider saving frequency resources and not producing the contradiction with the own broadcasting station by work while developing the digital audio frequency public address system. In the public address system put forward at present, mainly adopt two kinds of methods: One to organize network frequently only, broadcast the network to use same frequency of signal carrier while being whole, the program load of many ways on this signal carrier, and does not need different cover areas to distribute different frequency of signal carrier, thus has saved frequency resources, Eureka-l47 digital audio frequency public address system is this kind of representative of schemes; Another method is to use quite rate of the same signal carrier with the broadcasting station that is using at present, rely on the frequency to separate and modulate the difference of the way, broadcast two kinds of programs of simulation and figure at the same time, IBOC digital audio frequency public address system of U.S.A. is this kind of representative of schemes.
Digital audio frequency public address system
Digital sound to begin, study from the European Economic Community the eighties in broadcast, mainly there are four kinds of schemes at present:
This is a digital sound public address system of broadband that is studied by the European Economic Community, belong to a project Eureka-l47 that Ulika plans . The intersection of system and bandwidth accounted for this, as 1.5MHz, utilizes the same signal carrier to convey many sets of programs, audio frequency code adopts MPEG-1 LayerⅡ, every the intersection of No. and the intersection of audio frequency and yards of rate of signal can have many kinds of choice, adopt in system l92kbit mainly at present / stereo, the sound quality of broadcasting can reach CD tone quality; The channel code can delete the convolution code to adopt in order to realize the different data to importance use different protection ranks; Adopt OFDM modulation and frequency, time to interweave, the ones that can be guaranteed under moving, portable and fixed condition receive quality; The design of their structure of data makes the system convey the audio frequency and convey the high speed data; Can utilize the frequency of the same signal carrier to make up the radio network, namely the network only frequently, thus save frequency resources greatly.
Eureka-l47 DAB system is own through carrying on the test live for a long time in a plurality of countries, besides the European Economic Community, country and some countries of Asia such as Canada, Australia are carrying on the test, moreover, Germany, France, Britain and Canada are own to pilot stage through entering, our country is own too to set up experiment network in Guangdong and Beijing.
In the strip with In Band On Channel, IBOC frequently study by U.S.A. this ” ” Digital sound public address system, it utilizes amplitude modulation used at present or broadcasting station of the frequency modulation, in the frequency band putting the digital signal in the frequency band the same as analog signal or adjointing to, realize the digitization of mediumwave AM and ultrashort wave FM, the frequency of a signal carrier only conveys a program. This greatest characteristic of planting the public address system is, do not in fact change the operating frequency sum broadcast business of the broadcasting station, not needing new frequency to assign, simulation and figure can be broadcast in several years at the same time, apt to realize the intersection of simulation and transition to reach figure. And each radio station only covers the local country is very important in a lot non- state-run radio stations such as this pair of U.S.A.. Because of adopting digitization, the tone quality of such digital sound public address system has obvious improvement more than the sound qualities of amplitude modulation that use at present, FM broadcasting, three kinds of schemes are carrying on the test at present, the audio frequency codes of two kinds of schemes adopt MPEG AAC, another EPAC adopting Lucent and studying by oneself; Transmission technology adopts OFDM
IBOC system is carrying on the test too, but the progress is not so good as EUREKA-147 system.
Its goal is the satellite public address system of setting up African, South America and Asian, testing, will enter commerce to use, this system will adopt the QPSK modulation and continuous channel code in combination with a code of convolution code at present, audio frequency code adopts MPEG Layer.
Its purpose is to develop long, medium-short wave under 30MHz Digital audio broadcasting, channel code and modulating the scheme to test and assess, their codes of audio frequency adopt MPEG AAC.
Digital audio broadcasting is the technology of the digital audio frequency and synthesis of communication, as the progress of technology and demand of the society are played with, the business content of digital audio broadcasting will be expanded constantly, but audio broadcasting remains its main business lines, the progress of the technology of the digital audio frequency, including new compressed code technology, multi-channeled technology, various sound effect treatment technology,etc. will promote the development of digital audio broadcasting.
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