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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。

提高DSRC道路信标定位精度的

【2006-07-10】 来源： 点击次数：826

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一、引　　言 　　近年来，随着经济的发展，社会对于交通的需求日益增加。然而，城市交通拥堵、交通事故频发、交通环境恶化等一系列问题却愈来愈严重，正在成为现代化城市建设的瓶颈，因而各国交通运输领域竞相研究和开发出智能交通系统（Intelligent Transporta－tion System，简称ITS）。ITS是一个将先进的信息技术、电子通信技术、自动控制技术、计算机技术以及网络技术等有效、综合地运用于整个交通运输管理体系而建立起的一种在大范围内全方位发挥作用的、实时、准确、高效的交通运输综合管理和控制系统。为了发挥ITS的功能，实现ITS对车辆的智能化、实时、动态管理，国际上专门开发了适用于ITS领域道路与车辆之间的通信协议，即专用短程通信（Dedicated Short Range Communication，简称DSRC）协议。DSRC是ITS的基础设施之一，是一种无线通信系统，它通过信息的双向传输将车辆和道路有机地连接起来。系统主要包括3个部分：车载单元（On－Board Unit，简称OBU）、路旁单元（Road－SideUnit，简称RSU）以及专用短程通信协议。从通信的角度看，无论OBU还是RSU，都只不过是一种以无线方式（红外或射频／微波）交换数据的收发信设备。RSU作为信标通常有3种类型［1］：定位信标、信息信标和单独通信信标。这些信标被广泛地应用于车辆定位和导航、车辆自动识别、商业车辆运营、交通管理和车－车的相互通信中。 　　一个设计优良的作为定位信标应用的DSRC系统工作原理如下所述：RSU所在位置的坐标信息及其ID码通过预置参数读写接口存储在其数字单元的E2PROM中，并被长期保存。RSU平常处于扫描工作状态（接收－休眠－接收），消耗电流极小。当OBU工作时，可按一定的时序并以一定的数据速率不断发射唤醒信号。当承载OBU的车辆进入RSU的信号覆盖区内时，RSU即可收到此唤醒信号，并经MCU判明是属于同一系统的OBU后，启动唤醒电路，然后发射应答信号，收发双方进行无线握手。握手成功后，RSU即按通信协议发送本站的坐标和ID数据。数据传输完毕，OBU发送接收毕信号，RSU重新进入扫描状态。 　　在DSRC定位信标系统中，最重要的技术指标是定位精度。通常沿道路方向的定位精度要求不大于±5 m。但是由以上定位信标的工作过程可见，只要OBU进入RSU的信号覆盖区，OBU即可接收到坐标数据。而众所周知，信号覆盖区的范围与发射机的发射功率、接收机的接收灵敏度、收发天线的波束形状（方向图）等因素有密切的关系，特别是由于强方向性天线在尺寸、成本上所受到的限制，仅仅依据接收机进入覆盖区是否收到信号来确定信标的位置是很难达到定位精度要求的。 　　本文根据雷达振幅测角的最大信号原理［2］，提出了DSRC道路信标定位的最大信号法，并从硬件和软件方面讨论了工程上实现该方法的一般途径。根据这一方法所设计的数字信标机能够显著地提高定位精度。目前该方法已在某大型ITS工程中获得实际应用。 二、信标定位的最大信号法基本原理 　　为简化起见，不妨假设问题为一维情况，即车辆沿道路直线行驶，那么可绘出车辆行进中OBU与RSU的瞬时相对几何关系如图1所示。图中x坐标轴的方向为道路方向，r为OBU天线与RSU天线之间的距离，θ为两天线连线与坐标轴之间的夹角。GRSU（θ）、GOBU（θ）分别为OBU天线和RSU天线的方向性函数，h为RSU离地面的高度，v为车辆行驶速度。假定车辆从RSU被唤醒处开始计时，即t＝0，且行进到RSU正下方速所需时间为t＝t0，那么在观测的任一时刻t到车辆行进至RSU正下方所需要的时间Δt＝t－t0。 　　由图可知： 类似于二次雷达原理，设RSU的发射功率为Pt，λ为工作波长，则OBU在t时刻接收到的功率Pr（t）为 　　由（3）式计算的车辆行进中OBU接收到的信号强度随时间变化的曲线如图2所示。为了简化问题，假设了天线的增益函数具有正弦分布方向性，即 号强度由－70．8 dBm迅速达到最大值－42．5 dBm。实际上，由于电波传播受环境影响的复杂性，信号随时间变化的曲线不可能那么理想，但是如果对其求统计平均，则理论与实测的结果基本上是一致的。图3给出一组由典型测试的数据绘制的曲线以作比较。测试地点在成都市茶店子东街，收发天线为增益2 dBi的平板天线，其半功率主瓣宽度约为100°，车辆行驶的速度尽量保持在36 km／h（10 m／s）左右，其它测试条件均与图2计算时相同。图3中横坐标单位为记录数据的个数，每个数据的采样周期为25ms，因此10 s应采样400个。在最大值附近，由于信号超出自动场强测试仪的最大输入电平（－49 dBm）约几个分贝，因而出现平顶。   三、最大信号法的工程实现 　　如上所述，最大信号法是在一定的天线方向性基础上，利用RSU覆盖带内场强有最大值这一物理特性进行信标定位的一种技术。但如何将其在工程上实现，仍有许多待解决的问题。以下归纳出几个主要方面。 1．天线的辐射特性 　　根据上面的分析计算可知，天线的辐射特性对系统的性能将有较大的影响。首先是波束宽度问题。从提高定位精度和节省功率的观点看，天线波束应设计得尽量窄一些，而从OBU－RSU双方通信（如RSU的休眠－唤醒、多个设备组网时双方握手的需要）的观点看，天线波束应设计得尽量宽一些。这只能在设计中兼顾两者的技术指标加以解决。但一般而言，可将RSU天线设计为窄波束，OBU天线设计为宽波束。同时尽量提高接收机的灵敏度，并在可能时适当提高OBU的发射功率以增大通信距离。其次，应注意天线的极化特性。水平线极化和圆极化都是可行的设计方案，但不可采用垂直极化。因为一般情况下，为了避免遮挡，RSU都安装在灯柱上或建筑物高处，当车辆行驶到RSU的正下方时，垂直极化天线将出现波束零点。根据以上分析，具有低轮廓结构的微带天线可能成为设计者的首选方案。 2．场强信号的数字化接收 　　为了对信号进行检测和判断，首先需要将接收到的射频信号数字化。在全数字射频接收（软件无线电）技术中，这不成为问题。然而在该技术尚未得到普及时，传统的模拟－数字相结合的技术方案仍然是主要的解决手段。图4给出一种数字场强接收的原理框图。射频信号经过变频（通常为二次变频）和中放后分为两路，一路经过解调器解调出信标的数字信号，另一路则经过整流和低通滤波后进行A／D变换，从而变成与射频输入强度成正比的“接收信号强度指示”（Received Signal Strength Indication，简写为RSSI）数字信号。   　　器件方面，采用性能优良的高集成度IC芯片可以简化硬件的设计。例如，MOTOROLA公司低成本窄带调频接收芯片MC3371／3372就集成了图4中除A／D变换以外的所有功能部件。值得注意的是，近年来推出的许多性能更优、功能更强的IC或IC组件为设计者提供了更为有利的硬件设计平台。如ATMEL公司的SMART RF和CSI公司的BLUE－TOOTH模块，都能提供6位或8位的串行数字RSSI输出功能，此外还具有休眠－唤醒以及组网等多方面的功能，通过对这些芯片的二次开发，可以实现高精度信标定位系统的技术要求。 3．判决时延内车辆行程的计算 　　由于OBU要对最大信号进行判决，不可能在收到最大信号时立即输出RSU数据，而是有一个时延△t，因而设计者需要考虑△t时间内车辆行程△x的计算问题。以下给出一个实用的方案。如图5所示，OBU在进入RSU场强覆盖区内时，虽然可以接收到RSU数据信号，但是并不立刻输出数据，而是利用其RSSI功能，在每个采样周期内不断地检测RSU的场强幅度值，形成E（x）函数曲线。当E＝E0时，车辆位置对应于RSU所在的坐标x0。由于RSU发送的是x0处的经纬度以及道路方向等数据包，如果此时OBU输出该数据包，就实现了道路的定位。但是由于OBU检测出E0时已经有了一定的延时，因而不可能在x0输出数据，而是在车辆经过x0之后的某处x1输出数据。这样，如果能够计算出△x＝x1－x0的数值，那么OBU就能够对RSU的坐标数据进行修正，计入△x的贡献，保证所修正的数据仍然是精确的。但是为了计算△x，就需要知道车速，并在x0和x1对应的时间t0和t1区间内对时间积分。实际工程中△x可以这样简单地计算得到：通常几乎所有的车辆都配备了里程表，可以预先对车辆的里程脉冲进行校准，即计算出里程脉冲标定系数c（单位脉冲时间内车辆行进的距离，此值一经校准便不会轻易改变），然后在△t内通过计数得到里程脉冲总数N，则△x＝c×N。 4．最大信号法的搜索流程 　　由系统的物理特性可知，只要找到RSU覆盖区域的最大信号，就可以确定RSU所在的位置，因而不必对接收场强数据进行复杂的数学处理，如数据平滑和曲线拟合等。而只要采用简单的最大值搜索就可解决问题，这就极大地简化了程序设计。数字RSSI最大信号搜索的基本过程如下： 　　OBU进入覆盖区内，其微处理器MCU在通过串行中断口收到RSU数据信号的同时，还检测到RSSI数字场强值E1，此外还通过外部中断口收到OBU的里程脉冲信号。MCU将RSU数据和E1存入存储器，并将里程脉冲计数器清零。在下一个RX周期内OBU检测到另一个场强值E2，MCU对E1和E2进行比较，若E2＞E1，则将E2更新E1，里程脉冲计数器不计数；若E2≤E1，则不更新E1，同时对里程脉冲信号计数。若检测中发现场强小于某一门限值Emin，则OBU终止搜索，并从MCU并行口输出RSU数据和偏离场强最大值的里程脉冲的个数。 　　为了避免信号最大值的漏检情况发生，需要保证检测过程发生在一个有效区间，这一区间长度d与标定系数c和里程脉冲计数器累加的个数n的关系为：d＝c×n。如果d小于某一规定值dmin，则即使场强值低于门限，也不应终止搜索。     以上步骤的软件流程示于图6。   四、结束语 　　提高DSRC道路信标定位精度是当今ITS技术应用上的一个重要课题。特别在高楼林立的城市环境中，由于GPS系统受遮挡的影响不能正常工作，此时若单纯依靠基于陀螺传感器的推算定位技术，必然导致不可忽略的累积误差，而采用最大信号法定位的短距离信标技术在这些情况下可以给出较为精确的位置基准，或对误差予以纠正，因此该法在车辆导航与定位中有较大的实用价值。在实际环境下对采用该法研制成的数字信标机原理样机的测试表明，其定位误差均值为3．6 m，方差为1．1 m，完全能够达到预定的最大误差为±5 m的定位精度要求。经过对现存的系统误差进行软件校正后，定位误差的均值还可以进一步减小。最后应该指出，本文所提出的工程实现方案仅仅是原理上的，实际研制过程中，还存在诸多方面的技术细节，对此，作者将另文讨论。 参考文献 ［1］　［美］赵亦林著，谭国真译．车辆定位与导航系统［M］．电子工业出版社，1994． ［2］　丁鹭飞，耿富录编．雷达原理（第二版）［M］．西安电子科技大学出版社，1995．

什么是信标(Beacons)

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.

 

　　数据广播是近年来在国际上发展非常迅速的一项业务，是继声音广播与电视广播后的第三种广播类型。调频多工数据广播是利用调频广播频谱的社会公德部分，增设数据信道进行点对点、点对面的数据广播方式。开办调频多工数据广播业务，具有投资省、见效快、效益好、应用广的特点。因此，广受国外广播部门的青睐。 　　1、调频多...

RDS-TMC简介  （参见文档 RDS-TMC通讯方案.doc）

RDS是于1984年由欧洲广播联盟（EBU）制定的数据广播系统的欧洲规范，1986年国际无线电咨询委员会（CCIR）通过了有关RDS的643号建议书，1990年正式通过和出版“实施RDS的准则”EN50067。自此欧洲各国纷纷开设RDS的广播业务。
与中波相比，RDS城市交通信息广播的主要特点是利用现有的调频广播资源，通过广播信号里插入数字码实现，只需少量的投资即可建成广播发射端。它与音频信号是分开的，丝毫不会干扰收音，也不会影响收音机音质。当收音机检测和解调这些数字码后，便能提供相应的功能。
  RDS接收机的调频波段在87.5～108.0 MHz范围，相邻电台波段间隔至少100 kHz，在57 kHz上加载副载波数据。数据内容可以包括电台类型、节目类型、交通公告、广告信息、标准时间、天气预报等，同时提供了开放式数据接口，为特殊要求用户提供数据文本应用通道。
 
  TMC（Traffic Message Channel，交通信息频道）是一个数字编码系统。TMC能产生连续的交通信息流，如交通拥塞或事故，可报告出事地点与时间结果。信息包括了一定地域范围内的交通状况。将TMC信息与地图导航结合到一起，提高了车辆导航对前方路况预测的准确性。同时，在很多地区，建立DGPS数据的FM 负载波广播服务，提供广播电台周围的GPS差分校正数据，大大提高了GPS的位置定位精度。
RDS-TMC是采用RDS技术实现信息发布的应用之一。交通信息在广播前按照标准编码，采用RDS技术发布。车载终端设备可接收该码型信息，并可选择信息的实现方式，如文本、简单图形和语言等。接收RDS-TMC需要一个特别的无线电接收机，其最主要部分就是TMC卡，该卡包含了具体的路线信息等。假如要从甲地到乙地，在离开甲地前，先购买或租用甲地到乙地路线的TMC卡，这样就会收到路线中最新的交通信息。在进入其他国家时，接收机就会自动转到提供TMC信息流的另一个无线电台。
经过20年左右的发展，目前RDS-TMC技术已经成熟，相关产品在全球已经形成了年销售上百亿欧元的产业规模。在我国，该技术起步较晚，目前只有个别地区和单位建立了一些特殊用途的RDS电台，但是作为一种成本低廉、技术成熟、覆盖范围广的无线广播数据系统，其中孕育的市场商机是不可估量的。
 
RDS-TMC是目前欧洲唯一的大规模交通信息解决方案

 

DAB-TMC(TPEG)简介

◆DAB-TMC(TPEG)产生的背景      DAB（Digital Audio Broadcast）即数字广播，目前无线电广播已进入数字化时代，即FM的模拟服务转变成了一系列字节，因而，即使在山区或高楼中，信号也能被接收到，这些字节即使在有干扰情况下也能被识别，它能被分解成1536种不同的频率，如同穿越一个个时间间隔，在短时间失去信号的情况下，接收机仍能恢复原有信号，这个子系统叫做COFDM（Coded Orthogonal Frequency Division Multiplex），号码正交频率分配多路传输。COFDM允许在全国各个无线电站点使用一个频率，这就避免了正在行驶的车辆接收机进行反馈的必要性。     ◆TPEG协议     TPEG是欧洲广播联盟EBU所制定的协议，可透过各种数字媒介，如：互联网络、数字广播、数字电视等，发布交通与旅游信息。同时TPEG是欧洲广播联盟致力于发展“交通与旅游信息”传输协议的研究群体。这个研究群体于1997年成立，总计超过60家来自消费电子产品、电子地图、服务提供、广播等领域的欧洲公司与组织参与制定TPEG协议，不论是道路交通、公共交通、天气状况等信息，都在TPEG制定标准的范畴之内，以提供完整的交通与旅游信息。TPEG的信息具有语言无关（language independent）、传输载质无关（bearer independent）、多模应用（multimodal application）等特性。             TPEG协议共分为三层（同步层、封包层、应用层），对应至OSI七层模型的第三至第七层。TPEG第一层为同步层，对应至OSI的网络层，处理网络同步的问题。第二层为封包层，对应至OSI的第四至第六层，负责将TPEG应用层的数据串连成TPEG stream，TPEG stream可被加密或压缩。第三层为应用层，对应至OSI的应用层。         TPEG应用层标准目前已制定的有：服务与网络信息（TPEG-Service and Network Information；TPEG-SNI），道路交通信息（TPEG-Road Traffic Message；TPEG-RTM）以及公共交通信息（TPEG-Public Transport Information，TPEG－PTI）；TPEG-PTI，TPEG-SNI提供有关服务提供者的各项信息，以及服务内容的信息。TPEG-RTM提供有关道路交通的讯息，如：交通事件、交通阻塞等；TPEG-PTI则提供公共交通工具的相关信息，如：班次信息，班次延误信息等。TPEG-RTM与TPEG-PTI共享用来描述地理信息的TPEG-LOC 协议。另外还有一些正在制定的应用层协议，如：气象资讯（TPEG-Weather；TPEG-WEA）、停车信息（TPEG-Parking Information；TPEG-PKI）等。          交通及旅游信息并非一成不变，它们会因为国家、地点、时间、人物、车辆、环境等因素而有所不同，TPEG所制定的这些标准不一定会完全符合每个道路使用者的需求，因此TPEG标准是可修改的，今后会再加上一些尚在制定中的TPEG应用层标准。因此，若TPEG编解码器设计不恰当，会导致编译码器不适用新的标准而需要重新设计。 ◆RDS-TMC与TPEG之间的差别      无线电广播的发展趋势是从模拟向数字转变，DAB将在今后的市场中得以应用，同时，RDS-TMC， DAB－TMC接收机也像普通收音机一样普及。TPEG具有发送大容量信息的特点。RDS的发送速率是1.1875kbps，而TPEG可以在Internet上运行，有可能达到8000bps，且能发送巨大的信息量，未来发展前景光明。RDS编码信息将随着TPEG的发展而被取代，这是因为TPEG更简洁，无须配备一个复杂的数据库。

DAB-TMC服务构成与信息编码

TMC技术能够为用户提供动态信息，而DAB-TMC和RDS-TMC比较起来，其传输速度更快，传输效率相对也较高，因此，基于DAB-TMC的发送周期较短的特点，使得大范围内统一的交通报告成为可能。与RDS-TMC比较起来，DAB-TMC具有以下几个方面的优点： 周期要短很多，因此DAB接收器获取信息的时间被大大缩短，也意味着接收的信息量也更大。 根据选定区域，非常容易就能实现信息过滤。 在FIBs（Fast Information Block） 中，在FIG（Fast Information Group）5/1中，DAB-TMC信息和其他的FIG被组合在一起，每个FIB的32字节的数据就有两个字节的循环误差核对，这样保证了很强的纠错能力。此外，3/1特殊的编码方式也为FIC(Fast Information Channel)提供了额外的错误保护功能。FIC数据不是被逐行扫描的，因此TMC数据的获取速率很高，FIC中的数据网络传输速率是每秒32Kbits。 CODFM(Coded Orthogonal Frequency Division Multiplex)的调制程序使得DAB-TMC在多路径环境下具有很强的生命力。     现有文件中定义的DAB-TMC可以通过DAB中的快速信息频道（FIC）来传输TMC信息。依据ISO14819-4中协议进行编码的系统信息和用户信息在DAB-TMC中，负载在FIG5/1中。     在DAB中，每项服务可以包括几个服务Component，最主要的即主要服务Component ，一般大多为音频方式，但数据服务同样可以作为主要Component；其他的被称之为次要Component内容，一般是可以选择的。下图1是一个关于服务结构的示例，标为ALPHA1广播的服务包括两个部分：主要音频广播部分和次要数字广播部分，而数字部分则被用来传输DAB-TMC信息。音频信息通过主要信息频道（MIC）里的子频道携带，而TMC信息则由快速信息频道(FIC)中的快速信息数据频道（FIDC）负载。     一个服务Component可以被不同的服务共享，而服务通过改变结构，也可以更改自己的服务Component。如图中所示，ALPHA1和ALPHA2共同拥有ALPHA-TMC Component，而ALPHA2可以选择不同的音频部分作为其Component。     交通信息在FIG5/1中编码，图2显示了其中TMC信息区域的结构。FIG的类型成分检验区域用来识别能共存于一个多元体中的8种不同类型的信息。下面是D1和D2的定义：     D1 这个1比特的标识用来指示FIG中是否包含着37位或者16位的TMC信息：        0――――37位TMC信息        1――――16位TMC信息     D2  总是被标定为0     37位TMC信息必须包括以下的一种：     TMC用户信息：ISO14819－1 (4)中定义的包含位置和事件编码等参数。     TMC调谐信息：由ISO14819－1 (4)中定义，当信号变弱时，TMC产品需要调换接收器。     加密管理组：由ISO14819－1 (6)中定义，包含一些诸如SID(Service Identifier)、ENCID(Encryption Identifier)、LTNBE(Location Table Number before Encryption)等的加密参数。     16位TMC信息：必须包括ISO14819－1(4)中定义的系统信息。

IMPLEMENTING TPEG AND MULTIMEDIA SERVICES FOR DIGITAL

参见文档IMPLEMENTING TPEG AND MULTIMEDIA SERVICES FOR DIGITAL.pdf

INTRODUCTION
For the past few years much work has been done in both the Digital Television and Digital Radio arenas to develop new types of service that will fully exploit the benefits of digital transmission. As part of its drive towards new services, the BBC has been at the forefront of this work and this is reflected in the launch of several new types of service on both Digital Radio and Digital Television.
In the UK, Digital Terrestrial Television (DTT) now offers the capability to deliver MHEG 5 based services, notably BBC Digital Text launched earlier this year. Complementing this, the BBC has also launched two pilot data services on its Digital Radio platform: a ‘TPEG’ service for providing Traffic and Travel Information (TTI) and an HTML based service analogous to BBC Digital Text.
BBC Travel, the BBC’s ‘TPEG’ service is the result of the work of the Transport Protocol Experts Group (TPEG) of the European Broadcasting Union (EBU), and is now entering a validation phase in preparation for full services. At the same time, Eureka and WorldDAB have provided the transport protocols to allow TPEG data to be carried using the Digital Audio Broadcasting (DAB) Digital Radio standard, as well as developing the Broadcast Web Site (BWS) application to the point where Radio can genuinely compete in a multimedia environment.
Tracking the work of technical specification and standardisation (in which the BBC has been deeply involved), the BBC has developed the equipment and infrastructure required to effectively implement, manage and test these and future services.

TPEG
The TPEG protocol was conceived by the EBU with a number of key goals in mind. The first, and most obvious, of these was to develop a protocol for delivering TTI and other information for the support of Intelligent Transportation Systems (ITS) in a rich and flexible manner. In addition to this, however, was the desire to create an open specification, free of Intellectual Property Rights (IPR) issues and license fee considerations, for a broadcast system that could easily be adapted to a wide range of potential digital bearers.
As with any initiative aimed at developing new broadcasting standards, it was very important for the work of the TPEG project to have support from all appropriate industry sectors. Fortunately, TPEG has easily achieved this with involvement from information providers, broadcasters, map makers and, importantly, receiver manufacturers.

Digital Radio and TPEG
From its inception, the DAB Digital Radio standard was developed with mobile reception in mind. The combination of parameters for the Coded Orthogonal Frequency Division Multiplex (COFDM) modulation scheme ensure that a robust digital signal can be delivered to mobile receivers in a Single Frequency Network (SFN), which has been
reflected in the network planning for the BBC’s national Digital Radio multiplex. This suitability for mobile reception makes Digital Radio an ideal partner for a TTI service using TPEG since the question that needs to be answered is:
“How do I get from where I am to where I want to be - given the state of the road network at this precise moment?”
Part of this question can be answered by navigation systems and another part can be answered with positioning systems such as the Global Positioning System (GPS). The picture can only be completed, however, with an accurate and timely TTI service that can be delivered to where it is needed - i.e. to a vehicle.

Relating TPEG to other TTI protocols for Radio
An examination of the emerging market for navigation products that incorporate some form of TTI reveals a number of candidate protocols that could be used without ‘re-inventing the wheel’. In the UK and elsewhere, several commercial closed user group systems have been set up using GSM channels, but these are generally client-server systems where a user terminal requests information. In such cases, the protocols rely on bi-directional communication and are not suitable for use in broadcast environments. A more promising avenue is the Traffic Messaging Channel (TMC) application for the Radio Data System (RDS), which is part of many analogue FM radio broadcasts. Unfortunately, there are some obstacles that make it undesirable for some broadcasters to use a TMC based protocol for TTI in Digital Radio. TMC was developed to use a very narrow channel offered by FM/RDS where events are coded into 37 bit messages and one message can be sent per second. With such restrictions, the coding of TMC messages has had to be made extremely efficient but this has necessarily led to several restrictions. One of the principle restrictions imposed on TMC is the need to use a location coding table in the receiver to allow a 16 bit location code to be converted into its geographical representation. This leads on to the other problem with using TMC, as the location coding scheme involves the use of proprietary technology for which licenses must be paid. It is difficult for public service broadcasters like the BBC to support proprietary technology in this way, and the license fee payable by receiver manufacturers has led to a very slow development of the receiver market.
DAB Digital Radio has the potential to deliver bit rates that are orders of magnitude greater than those available with FM/RDS and so it was recognised that there was a need for a richer, more flexible system for higher bit rate broadcast digital bearers such as Digital Radio. At the same time, a great deal of investment has been made in the development of RDS/TMC services and decoders. The TPEG project has sought to be as compatible with TMC as is possible, while at the same time making best use of the available bit-rate.

The basics of TPEG
While TPEG is generally aimed at delivering information to support ITS applications, there are a number of different classes of information that are being considered. This, combined with the desire to have a single TPEG stream be able to carry a variety of different types of information, potentially from a variety of service providers led to the concept of a TPEG multiplex. The need to be readily portable between different broadcast digital bearers suggested the use of a simple asynchronous stream format, and so a framing protocol layer for transporting TPEG data was developed, within which a multiplex of different TPEG services can be delivered.
Recognising the importance of supporting subscription services as well as free to air services, the framing protocol layer provides support for conditional access layers. Clearly, future navigation systems will be able to take information from a variety of sources, and it is important that TPEG is able to ensure that both public service broadcasters and commercial service providers can compete in the normal way. Such an open market for TTI guarantees that the overall winner will always be the end user.

The Road Traffic Message application
While it is intended that TPEG should ultimately support delivery of a wide range of types of information, the initial focus of the TPEG project was the ability to describe the state of a road network. As a result of this, the first TPEG application that was developed was the Road Traffic Message (RTM) application. Within the RTM application, a carousel of messages is broadcast where each message describes an event on the road network. For example, a message might describe an accident, road works or even escaped animals! Each message can be tagged with an expiry time and a version number which can be used to ensure that receivers always maintain an up to date record of all messages.

A hierarchical approach to event coding
A great deal of TPEG’s flexibility derives from the use of a hierarchical event coding scheme that allows a variable level of detail to be used with the RTM application. This is extremely useful for three reasons: First, it allows receivers to easily filter out the information that is appropriate to the receiver implementation. Second it allows service providers to trade off detail level against number of messages or bit rate, which allows the RTM application to scale well according to the size of the channel. Third, it makes content generation simpler as more levels of detail can be supplied as more information becomes available. For example, when an accident occurs, the first reports received by a TTI provider will probably enable them to say that an accident has occurred at a particular location. Quarter of an hour later, the TTI provider may know more about the incident and so can add further detail to the RTM event description that indicates that the accident involved a two vehicles. Later still the TTI provider may know enough about the incident to say that the accident involved a bus, an articulated lorry and a deer and that the lorry is now blocking all 6 lanes and there is a 10 mile tailback.
While the hierarchical coding structure is not something that is common to RDS/TMC, the basic ‘data dictionary’ for describing events is the same. This means that there should be a high degree of compatibility between origination systems for TMC services and origination systems for TPEG. For example, although the actual coding of information about vehicles is different, the concept of a ‘heavy goods vehicle and trailer’ is common to both and has the same semantic definition in both.
A universal coding scheme for location coding Another important feature of the TPEG RTM application is the use of a location coding scheme that is independent of any proprietary mapping system. The TPEG RTM application uses the concept of Intersection Location (ILoc) codes to represent locations which consist of a longitude and latitude, together with up to three 5 character strings giving the first 5 characters of the names of the roads that define the intersection. Using longitude and latitude allows the coordinates of the location to be easily transformed to any desired coordinate system. The three road names make it easy for navigation systems to precisely locate the incident when there are minor discrepancies in the map database, due to the resolution or quality of the map data.

Validating TPEG with the ‘BBC Travel’ service
For the BBC, TPEG is ideal. The BBC Travel unit currently deals with up to 1200 events in the UK road network, with many information sources ranging from police control rooms to the general public. The output from the travel unit takes a number of forms - Internet, Teletext and, naturally, scripts for radio and television travel bulletins. Radio is the only way to deliver this to a vehcle but radio travel bulletins are necessarily limited to a tiny fraction of the total information available. Out of the 1200 or so events that are known about, only 4 or 5 will be mentioned in each bulletin. By delivering all of the BBC content directly to mobile receivers in a form that can be interpreted electronically, TPEG allows the BBC to provide a valuable public service based on content that we already own.
With such compelling arguments in favour of using TPEG with Digital Radio, the BBC decided to launch a pilot TPEG service with three basic aims in mind:
. To gain experience with using TPEG

. To promote and validate the TPEG standard

. To allow receiver manufacturers access to a stream of TPEG data for development purposes

The pilot TPEG service has been running since early February and uses live content generated by the BBC Travel Unit to create a TPEG stream that is both broadcast using Digital Radio and accessible over the Internet. The provision of the Internet service illustrates the bearer independence of TPEG and also allows receiver manufacturers to develop TPEG decoders against
the service, even if they cannot receive the BBC’s Digital Radio signal.
Simply providing a stream of TPEG data is useful for developers but does not demonstrate the capabilities of the protocol. In order to overcome this, the BBC has also developed an example TPEG receiver that has been written in Java. The receiver shows a variety of ways in which the TPEG service can be used, including showing the events on a map. Writing the receiver in Java means that it can be delivered as part of a normal web site for decoding the Internet version of the service, as well as being used to decode the Digital Radio service.

How the pilot TPEG service works
The TPEG service starts with BBC Travel Unit on the seventh floor of London’s Television Centre where events in the UK road network are collated into a single database. This database is capable of
supporting all of the Travel Unit’s outputs and, in particular, can generate a file representation of the complete database as TPEG RTM messages. This RTM file is transferred to the Digital Radio system at Broadcasting House across the BBC Intranet using FTP. Whenever a new RTM file is transferred, the TPEG software for the Digital Radio system reads the new content and spools it into a ‘TPEG multiplexer’. The output of the TPEG multiplexer is a complete TPEG stream that is fed to both the Digital Radio equipment and the Internet. The general arrangement is shown in Figure 1.
Figure 1 Architecture for the BBC TPEG pilot 略

Plans for the future
The TPEG project is continuing to develop new applications for the TPEG protocol and will soon have completed the specification a ‘Service and Network’ application in addition to the RTM application. The Service and Network application will allow the TPEG stream itself to be completely described, including information to allow service following between different TPEG bearers. Once this work has been completed, work is already in preparation to define and application for describing public transport networks to complement the RTM application.
All of this future work will continue to maintain the central ideas of an open, license free specification together with rich and flexible structures for describing problems with the various transport networks.

The TPEG library
All of the software for the BBC TPEG pilot service has been developed using a library of code for manipulating TPEG data structures. The language chosen for writing this library was C++ as TPEG lends itself naturally to object-oriented programming techniques, and the library was structured to allow the development of three types of application: TPEG application sources (such as the RTM spooler in Figure 1), TPEG multiplexers and TPEG decoders.
Having developed the TPEG software library, the BBC has made this software freely available to the TPEG community under a ‘copyleft’ style license, as advocated by the Free Software Foundation. This was done in order to stimulate the development of both TPEG receivers and TPEG services. More information about both the TPEG software library and the BBC TPEG pilot service can be obtained from the BBC’s TPEG web site at http://www.bbc.co.uk/rd/projects/tpeg

THE BROADCAST WEB SITE AND BEYOND
About a year ago the concept of a ‘Broadcast Web Site’ (BWS) was introduced, where a data carousel protocol is used to deliver a set of files for web site. Receivers of various types can then recreate and browse the service to give the same experience as if connected to the Internet - only without the wait or the connection charges. Since then, Eureka and WorldDAB have proceeded to rapidly develop concrete specifications for this application, with the result that we now have a genuinely interoperable standard for multimedia services in Digital Radio.

Specification issues
While the current DAB Digital Radio standard is a mature and established mechanism for robustly delivering high quality audio, it does not provide complete support for other forms of service. This has required the development of additional features for Digital Radio that have been successfully introduced without affecting the established DAB Digital Radio standard in any way.

Application signalling
The first important feature that has been introduced is the concept of application signalling. Within the DAB Digital Radio specification, MPEG-2 audio is carried in specially constructed audio channels whose ‘protection profile’ is tailored to the audio frames but non audio services such as TPEG and BWS services are carried in channels with even protection applied to the data. Receivers are easily able to identify this difference in the way the digital information is delivered, and so distinguishing between an audio service and a non-audio service is straightforward. With the recent advances in the development of non-audio services, however, the need has been recognised for receivers to be able to differentiate between various types of non-audio service. The problem can be expressed simply as: ‘The receiver needs to know where to find the data for a service and what to do with it -i.e. which software to use to decode it’.
The Service Information (SI) structures for DAB Digital Radio are carried in the Fast Information Channel (FIC) using Fast Information Group (FIG) structures. Thus, application signalling has now been defined in the form of a new FIG (FIG0/13). This FIG allows an association to be made between the location of a data service, an application identifier to identify the software needed to decode the service, and a variable amount of application specific data. The application specific data field can be used in any way, as determined by the application specification for the indicated application identifier and, as will be seen, can be used where different profiles or contours for the service are used.

The BWS application and profile specifications
Having introduced the concept of an application identifier as part of the DAB Digital Radio SI, it necessarily follows that the application identifier must refer to some application specification that completely and unambiguously describes the operation of the service. The BWS application is no exception and so considerable effort has gone into the development of the BWS application specification. The BWS specification consists, in large part, of a description of the interface between the Multimedia Object Transfer (MOT) data carousel containing the files for the web site and the presentation engine. For example, the way the BWS decoder resolves Uniform Resource Locators (URLs) within the service content to file names within the carousel is part of this interface description.
One of the peculiarities of the Digital Radio situation is that a wide variety of receiver platforms are being considered for many services. These range from in-car and domestic portable/hi-fi integrated receivers  - some with graphics displays, some without -through to PC card receivers. Clearly, presenting a BWS service on such a wide variety of platforms requires careful consideration as not all types of receiver will have the same presentation capabilities. This leads to the concept of signalled profiles - equivalent to the DAVIC concept of contours -which describe the constraints on the content for a service that must be applied for a given type of receiver. It is not in the interests for broadcasters to be required to support a large number of profiles, so the BWS application has been developed with two profiles currently in mind: An integrated receiver profile and a PC decoder profile. The PC profile is a pseudo profile that imposes no explicit constraints as the presentation capabilities of the decoder are dependent only on the web browser installed on the receiver PC. In addition to the PC profile, however, a basic integrated receiver profile has been developed and both of these profiles are associated with profile identifiers that can be carried in the FIG0/13 application signalling. The result of this is that a scalable service can easily be provided where more capable receivers (such as PC based receivers) present more of the overall service than less capable receivers.

The BBC pilot BWS service
For exactly the same reasons as for the TPEG pilot service, a BWS pilot service has been created by the BBC. Once again, this pilot service makes heavy use of automatic translation of existing content wherever possible. Because the BWS application uses HTML as a content format, the natural source for a large proportion of the service is the BBC’s extensive online service. In particular, it is relatively straightforward take BBC News stories from BBC News Online to make use of one of the BBC’s strongest brand images. With the development of suitable receivers, which are anticipated in the very near future, the stage will be set for developing this pilot service into a full time multimedia service for Digital Radio.

The VIADAB API
An important activity that has recently been initiated is the development of an Application Programming Interface (API) for Digital Radio receivers in a PC environment. This work, as part of a UK Department of Trade and Industry (DTI) funded project, aims to produce a completely open specification that can be adopted by all manufacturers of Digital Radio PC cards without a license fee. The result of this work will provide broadcasters with an unprecedented level of flexibility as service providers will be able to write their own data service decoding software. Also, a ‘virtual sockets’ layer built into the API will allow BWS services to make use of Java functionality within  web  browsers  to  allow  dynamic 
presentations  to  be  created  in an HTML framework. 

The core of the API is a D-COM interface specified in Interface Definition Language (IDL). This makes the API entirely language independent, as well as allowing Digital Radio receivers to be seamlessly integrated with Local Area Networks (LANs).

CONCLUSIONS
The slow take up of Digital Radio has been seen as a problem for a number of years. Much has
been said about the impact that ‘data’ services might have on the appeal of Digital Radio. Now, thanks to extensive work by the Digital Radio community as a whole, the new pilot services offered by the BBC clearly demonstrate that DAB Digital Radio is now in a position to deliver on this potential.

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...