信息天堂

原文 

无庸置疑，手机抢了越来越多便携设备的饭碗，尤其如MP3和数码相机等。DAB/DMB播放功能也不例外，LG、三星等韩国厂商更是早就有了相应的产品。

“电视手机”的优势与劣势几乎同样明显，而这也是手机在抢占其他产品市场时所遇到过的

它的优势在于集成化高，便携性强。现在社会人们对手机的依赖性越来越大，手机已经成了随身必带的物件，使用DAB/DMB手机并不会需要我们多带一样东西在身上，这对于部分经常在外旅行者非常重要。

而它的缺点主要有两点：一是DAB/DMB手机因为屏幕较大而机身尺寸也略嫌宽大，可能会不符合国人的喜好；二是接收及播放DAB/DMB节目毕竟是一件耗电的事情，这对于“以电为本”的手机是一件可怕的事情，如果因为多看了几分钟节目而少接了一个重要电话，那可真是得不偿失。

除此之外，价格昂贵也是电视手机的一大问题。目前这类产品的价格基本上都在四五千元，距离主流手机价位还有一定的差距，而且估计短时间内不会下调

随着移动数字电视引起越来越多的关注，关于DAB/DMB和DVB-H优劣的讨论不绝于耳。但有一个问题值得注意，业界许多人对于DAB/DMB了解不够正确，例如不断听到有人说DAB/DMB不能分时，DAB/DMB带宽窄是主要缺点。其实DAB/DMB分时特性比DVB-H好得多，而DAB/DMB带宽窄是其在移动数字电视方面应用的主要优点之一。只有把这些基本概念搞清楚了，关于DAB/ DMB和DVB-H优劣的讨论才有意义。

DAB/DMB移动性好效率高

长期以来，数字电视的焦点是在家用数字电视(固定位置接收)上，总认为家用数字电视标准确定后，可在此基础上兼容/修改为移动数字电视，包括数字电视手机。这种思路其实是不对的。固定位置接收的数字电视强调的是大屏幕、高分辨率、大数据量，而手持式移动数字接收是有限的数据量、中低分辨率、小屏幕、低功耗，因此手持式移动数字电视的标准与固定信号接收的家用数字电视是不可以兼容的。
正是上述原因，在数字电视标准DVB-T的基础上推出了DVB -H，以改善移动特性，降低功耗。但是与基于数字广播标准DAB的数字多媒体广播标准DMB相比，DAB从十年前制定时就以200公里/小时为主要指标，换句话说，DAB首先就是为手持移动而诞生的，因而它的移动特性更好。DAB在COFDM技术的基础上，采用了时间交织和分等级防错保护，使得DAB移动特性达到近乎完美的境地，而DVB-H只采用了COFDM技术。DAB在全球许多国家经过十年的运行，已被证明是可靠的，可以说是国际上在移动通信领域为数不多的一开始就比较完善的标准。

DAB/DMB的数据带宽为1.2Mbps，频谱带宽为1.5MHz；而DVB -H的数据带宽为4Mbps以上，最高可达27Mbps(在移动电视应用中，可实现的数据带宽为4Mbps-5Mbps)，频谱带宽为5MHz -8MHz。很多人曾认为数据量大是DVB-H标准的优势之一。其实不然，数据量大必然导致运算量大和功耗大，DVB-H虽具有时分功能，可以较大程度地降低功耗，但DVB-H的同步方式和节目复用方式使得它仍需要从完整的数据帧中提取同步信号和节目信息。而DAB/DMB的数据带宽仅为1.2M，同样具有时分功能(国内业界曾流传DAB不具有时分功能的说法是不对的)。当用户选好节目后，DAB解码器只要对需要的DAB符号解码，因而时分作用十分有效。根据所选节目的码流DAB解码器的用功比率可降到全功率的20%-50%。这些因素综合起来，使得DAB解码的运算量小很多，在同等工艺和设计技术条件下做比较，DAB解码芯片的功耗是DVB-H的1/4。这就使得目前的DVB-H的基带处理芯片要靠采用0.13微米甚至0.09微米工艺来降低功耗，大大提高了芯片成本和开发成本。而西芯微电子一款新出的DAB/DMB基带处理芯片用0.18微米工艺制作，其功耗极低，只有25mW-35mW。

那么DVB-H的4Mbps-5Mbps数据带宽在传送高分辨率、大数据量节目时是否有优势呢？回答仍然是否定的。DVB-T才要高分辨率、大数据量，手持移动没有这样的要求。通常一套DAB立体声节目的带宽为80Kbps-128Kbps，一套DMB电视节目(QCIF分辨率)的带宽为384Kbps-512Kbps，因此DAB/DMB的1.2Mbps数据带宽是合理的，DVB-H的4Mbps-5Mbps数据带宽显得过大。

DVB-H从DVB-T演变而来，却继承了DVB-T的4Mbps-5Mbps带宽和数据同步结构和节目复用方式，这是不合理的，也是其技术方面不占优势的主要原因。

况且DAB/DMB可以共用1.5MHz频谱带宽，意味着DMB可以在已有的DAB平台上播出，例如拿下2-3套DAB立体声节目就可以播放一套DMB电视节目，因此DMB可以利用全球已有的1000多个DAB发射台。而DVB-H与DVB-T不能共用，要分别占用5MHz-8MHz频谱带宽。DVB-H也没有其他现成的平台可以兼容使用。

DAB/DMB商业运行性价比高

DVB-H的5MHz-8MHz频谱带宽同样引起了商业运行方面的问题。DVB-H和DVB-T工作在400MHz的频段，几乎都被模拟电视占据了，要拿到5MHz-8MHz频谱带宽相当困难。DVB-H和DVB-T要一起上的话就要10MHz-16MHz频谱带宽，就更为困难。

DAB/DMB工作在200MHz附近的Ⅲ波段，拥挤程度相对好一点，或工作在1.4G的L波段。况且DAB/DMB只需要1.5MHz频谱带宽并能共用。频段问题就容易解决得多。

DVB-H的数据带宽过大也给媒体运行商带来麻烦。DAB/DMB的1.2Mbps数据带宽可以安排6套左右的节目(2套电视节目，2-4套音频节目，还可有文字传送)，节目数量适中;DVB-H的4Mbps-5Mbps数据带宽要安排约20套节目，在起步期间是很浪费的。

还有一个不容易注意的事实是DVB-H实际上笼罩在DVB-T的阴影下，数字电视运营商的注意力还主要在DVB-T上(技术的原因是两者不能兼容);而DAB运营商则把DMB视为新的推动力，心甘情愿地让DMB唱主角(技术的原因是两者能兼容)。这一贬一宠便使得二者的境遇有天壤之别。
从工程量和成本方面分析，DAB台可以直接播放DMB节目，因此DMB全球已有的1000多个DAB发射台都可以用来播放DMB节目。DAB/DMB发射台建台成本低，为几十万元量级，发射功率可以很低，1千瓦，甚至500瓦。因此DMB的建台成本本身就低，而且被消化、分摊了。显然运营商介入DAB/DMB门槛低、风险低。

2004年下半年DAB/DMB在全球趋热，原因之一是众多数字移动标准中，DAB最成熟，且已有现成的台站和网络设施可用。DAB/DMB是实现手持式移动数字接收工程量最小、成本最低、性能最好的技术。随着越来越多的DAB/DMB开播，DVB-H的机会将越来越小。

1.早期发展
声音广播的数字化历程可以追溯到30年前。广播数据系统（RDS）的前期研究工作于1974年开始，整整十年之后该系统才完成其标准化工作（EBU3244）。该系统仅是在已有的调频信道中增加了一定的数据传输能力（1187.5b/s），还不能称为真正意义上的数字广播系统。然而其影响却是深远的，其中定义的许多业务（自动调谐、频率切换、紧急报警、交通信息等）和参数（如数字台标、节目类型等）在后来出现的数字广播系统中被一直沿用至今，特别是其中定义了透明数据通道。这是第一个实际的多工数据广播系统（SCA在出现之初仅是模拟的多工广播系统）。
随着处理器能力的增强以及成本的大幅降低，从1985年开始，日本的NHK开始了新一代调频数据广播系统的研究工作，即数据广播信道系统（DARC）。DARC系统数据传输能力为16kb/s，比RDS系统高出10倍以上。由于传输能力的提高，DARC接收机上采用了分辨率为640×480的LCD液晶屏作为显示。
2.数字声音广播
第一个在世界范围内被广泛采用的真正意义上的数字广播系统是数字声音广播（DAB）系统。DAB的相关技术研究始于80年代末期，由Eureka 147项目支持。在经过大量试验之后，DAB标准草案于1992年完成，并于1995年成为ETSI正式标准。该系统实现了从信源编码、信道编码直到调制方式的完全数字化，采用了新的技术、新的频率和新的业务结构。
DAB是一种可靠的，用于移动、便携、固定接收的多业务数字广播系统，可通过简单的无方向性天线接受。DAB的中心工作频率范围为30MHz到3GHz，宽带1.5MHz，有4种地面传输方式，码率2.048Mb/s，净码率0.6～1.7Mb/s左右。DAB采用单频网方式覆盖，即同频、多点、小功率覆盖。
在DAB系统中，业务构成由复用配置信息（MCI）描述，其中可以包括音频业务和通用数据业务等。根据需要，广播经营者将整个宽带划分为大小不等的子通道，各业务在不同的子通道内传输。各子通道的码率和保护等级可以灵活设置，而数据业务既可以支持同步的流模式，也可以支持异步的包模式，这些都使得DAB在业务构成方面有很大的灵活性。由于在DAB系统中采用了DQPSK调制以及深度交织技术，使得DAB迄今为止仍然是世界上移动接受效果最好的地面广播系统。
3.调幅波段广播数字化
作为最早的广播形式，始于上世纪20年代的30MHz以下的调幅广播以其覆盖范围大、传输距离远、接收机简单、价格低廉等突出优点，一直被作为基本的信息传播技术手段之一，尤其是在地域广阔、人口密度低的地区覆盖以及对外广播等方面是首选的方案，其优势十分明显。但传统的模拟调幅广播也存在着传输质量差、业务单一和易被干扰等缺点，同时在该频段内的频谱过度占用问题也愈演愈烈。利用数字技术对调幅波段广播进行改造已势在必行。目前最为成熟，已成为国际标准的是所谓的DRM系统，DRM是研发该技术的国际组织，Digital Radio Mondiale的简称。
该系统是历经世界上多个国家5年多的研究和试验工作之后完成的。该系统不仅着眼于改善调幅广播频段易受传输信道固有特性干扰的状况，提高广播接收质量及声音质量，节约频谱并降低射频功率等方面的问题，还充分考虑了利用数字广播特点，为传统的广播增加文本、图像、数据等附加的信息广播能力。同时数字广播还可以根据不同的频谱规划以及业务规划，灵活地调整配置，满足规划要求，而不需要对设备进行大规模的改造。因此数字AM广播技术赋予了传统的调幅广播新的内涵，使其具备了应对其他传输技术手段挑战的能力，数字AM广播是传统调幅广播的出路和未来。
4.其他数字广播技术
除上面提到的两种数字广播技术外，数字技术在广播中的应用还包括卫星数字广播、网上广播，以及台内数字化等方面。
由美国世广卫星集团开发运营的World Space系统是最早的卫星数字广播系统。关于该系统的主要技术将在后面予以介绍。该集团创立于1990年，目前已经在Asia Star、Afri Star、Ameri Star等卫星上开办了100余套卫星广播节目，该系统还能够进行多媒体业务广播。
除World Space系统外，目前主要的卫星数字广播系统还包括美国的XMRadio、Sirius等。这两套系统主要覆盖美国，可以提供约100套高质量的声音广播节目。
由于声音压缩技术的发展以及声音广播先天具有的宽带需求小的特点，以因特网作为传输媒介的声音广播业务率先在窄带接入的条件下获得了广泛的应用。网上广播具有投资小、覆盖面积广、无地域国界限制的优势，特别是多播技术的发展与应用，使得网上的声音广播可以满足更多用户的需求。目前，网上的广播业务已经拥有了一定规模的听众群，具有了相当的影响力。
与电视台的数字化进程一样，目前广播电台的数字化已不仅仅满足于数字录音机、数字调音台、数字音频工作站以及台内节目交换手段的简单应用，而在将这些技术融入到全台的信息化管理以及媒体资源共享的建设中。

Introduction

Digital Audio Broadcasting (DAB) is the standard for digital transmission of digital radio programs in the terrestrial realm. In addition, it is also suitable for broadcasting via digital cable and satellite using the frequency range between 30 MHz and 3 GHz.
It was developed in an EU project by the year 2000.

Availability

DAB is broadcast in Germany, in parts of Switzerland, South Tyrol, Belgium and in the United Kingdom. In France, reception is currently possible only in the population centers around Paris and Lyon, as in the Netherlands and Austria.
In Italy, the construction of the DAB network is being expedited primarily by private broadcasters in the metropolitan areas of northern Italy. In Canada, DAB signals are found near the large cities like Ontario and Québec.

On the other hand, in the USA another standard is used for digital transmission by FM and AM radio broadcasters. In addition, there are also the fee-based offerings from "pay radio" providers.

Availability in Germany

Broadcast coverage in Germany totals 80 percent. Indeed, transmitter power is often insufficient to reach into homes, so outside antennas are often required. However, a ten-fold increase in transmitter power is planned for this year in order to eliminate this problem. About 120 DAB programs can currently be received in Germany.

In sum, DAB can currently be received by approximately 500 million people in more than 40 countries. To date, approximately 12 million receivers have been purchased throughout the world.

Market

In Germany, DAB is currently being promoted and held out as "digital radio" by its lobbyists, which is admittedly not precisely correct in this form, because there several formats and technologies that cover digital broadcasting of radio programs via cable, satellite or terrestrial radio. In contrast to DAB, however, they are primarily suited for stationary reception and not for mobile reception.
Since 2004, a large selection of reception devices has been available to potential users, which was the greatest hurdle at the beginning. So far, 12 million devices have been sold to consumers around the world, of which 3 million are accounted for by the British market alone.

Even the German consumer information centers recommend the selection of a device that supports DAB reception along with the normal VHF standard when purchasing a new radio. This recommendation is also supported by ADAC [General German Automobile Club], which supports the proliferation of DAB in new vehicles and new car radios.

However, the success of DAB is still limited in places. In Germany, the number of DAB listeners amounts to 200,000 - 300,000. In Finland, it was discontinued entirely as a result of the lack of interest. As a result, Sweden discontinued all additional work on its own network as well.

Technology

In DAB, the audio stream is compressed into the MP2 format. By doing so, a data rate of 32 to 256 kbit/s can be achieved, which in principle approximates the quality of a CD, but which is used only by a few broadcasters for cost reasons.
The audio streams are consolidated for transmission with data services into a multiplex with a high data rate.

This signal is modulated using the COFDM method (Coded Orthogonal Frequency Division Multiplex). Compared to an analog signal, it has proven itself markedly less susceptible to interference in practice.

In addition, the COFDM method enables coverage of large areas and stretches using one frequency. As a result, the use of broadcast spectrum by each broadcaster is much less than for analog reception. In addition, it offers cost savings, because each program has to rent fewer licenses for frequency ranges.

In Germany, DAB uses frequencies in Band III (174-230 MHz), mostly the former television channel 12 (223–230 MHz), as well as the range 1452–1492 MHz in the L-band.

Band III is used in the signals broadcast nationwide, and the L-Band is used for the broadcast of "local" multiplexes.

Due to their higher frequency, the frequencies in the L-band require a high transmitter density, which often cannot be ensured, whereby the local stations are often affected by reception problems.

Data services

In addition to the transmission of the radio programs, a number of data services and data types are also established, which can be used via the DAB format:


MOT: MOT is a protocol in which files of any format can be transmitted to a receiver using a push-broadcast method. The files can be transmitted in repeated partial segments, whereby the receiver is provided the option of gathering the parts of the file together.
Here, MOT is either hidden in the data stream or transmitted separately as a data service.

DLS: DLS refers to the transmission of information in the VHF range like the RDS format. Short character strings (up to 128) are sent to the receiver.

IP over DAB: Describes the transmission of IP data packets through the DAB network. Thus, services such as streams, for example, which are placed in the IP protocol, can be transmitted via DAB using the IP protocol.

TMC: As already established in RDS, DAB also supports the transmission of encoded traffic information. It serves as the basis for traffic-related route evaluation in modern navigation devices, for example.

TPEG: TPEG is considered an extension of the TMC standards currently in place. In addition to the information about road traffic, passenger service should also be included here.

As a result of the technical structure of DAB, additional data services can also be implemented without problems.

http://www.dab-digitalradio.ch/?lang=en

What Is DAB?

DAB stands for Digital Audio Broadcasting and is a method for the digital transmission of radio signals. DAB is the transmission technology of the future and will replace FM radio in the medium to long term.
The DAB method was developed in Europe within the framework of EUREKA project 147 and is currently being introduced in a large number of countries. The DAB standard has been adopted by all European countries, Australia, a number of Asian countries (Singapore, Taiwan, South Korea, China and India) and some countries in the New World (Canada, Mexico, Paraguay).
The only exception among leading nations is the USA, which has launched a digital-radio standard of its own called IBOC (In Band On Channel). Information on the IBOC situation in the USA is available from the Federal Communications Division FCC; technical details are available from the iBiquity Digital Corporation.

The country with the widest availability of DAB is the UK. About 85% of UK households can receive DAB, and the number of DAB radio stations is now more than 400. A recent survey shows that more than a third of the UK population knows about DAB technology, and in May of this year, sales of DAB receivers exceeded sales of VHF radios for the first time.

In Switzerland, the number of radio stations is currently limited to 14, but DAB reception is already available to 60% of the population, and there are specific extension plans both as far as availability and the number of programmes is concerned.

How DAB Works

First, the analogue acoustic signals must be digitised, i.e. converted to series of zeros and ones, and then processed in the following three steps for optimal transmission:

Any sounds that are out of the range of human hearing are filtered out before transmission. The method used to achieve this is called MUSICAM (Masking Pattern Adapted Universal Subband Integrated Coding And Multiplexing) and results in a significant reduction of the transmitted data without impairing the listening experience.

Next, a so-called multiplexer attaches supplementary data to the signal. Since transmission is digital, it is irrelevant whether it is sound, text or images that are broadcast, so with DAB it is possible to relay not only audio signals, but also to forward to the receivers' displays useful supplementary information (e.g., song titles and interpreters) in dynamic text form, so called Programme Asscociated Data (PAD). One example of PAD are Dynamic Labels (information like song titles etc. that is transmitted in dynamic text form to the display of the DAB receiver). MOT (Multimedia Object Transfer) even makes it possible to transmit multimedia data (e.g., images from CD booklets).
Another option is to provide data services that are independent of radio broadcasting, so-called NPAD (Non Programme Associated Data), e.g., information on the arrivals and departures at an airport.
In the multiplexer, the signal can be combined with the digitised signals of other radio studios and converted to a uniform data stream (also called "package" or "ensemble").

In a final step, the digital data stream is split into small units and chronologically nested. At the same time, error protection is added, which ensures that erroneous and missing packages can be reconstructed or extrapolated. This error protection level is assigned a specific value (usually 3 or 4). The lower the value, the better the protection level.
From the broadcasting locations, these small units can be transmitted in the form of a single frequency block at 1.5 MHz. The individual packages are distributed across up to 1,536 "sub-frequencies" or carrier frequencies. This modulation procedure (which is called COFDM - Coded Orthogonal Frequency Division Multiplex) greatly reduces interference.

The receivers are equipped with built-in Viterbi decoders which put the digital signal in the correct chronological order and check the signal for transmission errors. Finally, the digital data is converted back into analogue sound that can be heard by the user.

Advantages of DAB

DAB offers great advantages over today's VHF, medium-wave, long-wave and short-wave transmission:

• With DAB, ten radio programmes can be broadcast on a single frequency.

• As long as a given DAB receiver can pick up the signal sent out by a broadcasting station (even if the signal is very weak), sound reproduction is ensured. There is none of the fading (weakening of the sound) that is typical of VHF reception; the DAB signal is reproduced at a constant volume. If the signal is too vague to be interpreted by the receiver, reception is interrupted completely or the device switches over to the appropriate VHF frequency (if possible).

• Interference such as that which can be caused by power lines are filtered out by the DAB receiver. Such noises should simply not occur; as indicated above, reception is either excellent or not available.

• Interference caused by stations that are on close frequencies – a phenomenon that is typical of VHF – does not occur in connection with DAB.

• If the signal is reflected by natural obstacles or buildings, the reception quality of DAB is improved due to the multiplication of the signal, whereas VHF reception is considerably worsened in such cases.

• In addition to audio signals, DAB offers a wide range of supplementary services, including the transmission of song titles, interpreters, images, CD booklets, etc.

• Many newer receivers have attractive auxiliary features that offer a completely new radio experience: a pause button that allows the user to stop the programme a nd restart it at the same place, the possibility to record favourite programmes and a programming function that allows specific programmes to be flagged for recording before they are broadcast.

2005-12-13 19:48 金陵客 TMC交通信息广播频道技术悄然入沪

12月8日，欧洲智能交通协会（ERTICO）在上海举行新闻发布会。会上，他们重点介绍了合作推荐项目——交通信息广播频道（TMC），将欧洲领先的智能交通技术带入中国。
TMC是无...