The Definition of Digital TV

     NAB'98 is all about digital television. You don't have to be an industry insider to know this is a very complex and absolutely critical issue.

     On the one hand, there is no doubt that digital is the way forward. On the other hand, several different, apparently incompatible digital television standards are emerging from different regions of the world.With different digital television standards from Europe , Japan and America ,has a vital opportunity to forge a single global standard been missed?A new "standards gulf?

     Broadcasters are all familiar with the "standards gulf?from the last thirty or forty years of NTSC, PAL and SECAM analogue television.It should come as no surprise therefore that the US has chosen a system developed in America, and that Europe has adopted an essentially European system, developed by the international DVB Project.The good vews is the standards gulf no longer has anything to do with the pictures, Now, there is a single global standard for compressing digital TV pictures prior to transmission. It is MPEG-2?, developed by the Moving Pictures Experts Group(MPEG).The bad news is, the standards gulf continues in the sound coding systems, and most importantly, in the RF modulation of the MPEG- ?2 bitstream.Picture Format --MPEG-2 ?

     Digital Broadcasting offers the potential for having more channels, and/or for improving the resolution of TV pictures. The MPEG?2 standard used by both the US and DVB standards supports a number of different picture formats.MPEG ?2 digital compression is used by all proposed digital TV systems to achivev an adequate throughput of the vast amounts of data required by High Definition (HDTV) or multi ?channel standard Definition (SDTV).Beyond the pictures, the US and DVB systems use different systems for coding audio. Both audio systems offer a range of sound qualities from CD?quality stereo to full surround sound.While current digital television plans in Europe are geared towards SDTV, and in the States towards HDTV, the important point to keep in mind is that the question of HDTV or SDTV has nothing whatsoever to do with the RF modulation, or transmission of the MPEG ? bitstream.There are no obstacles for broadcasters who wish to use DVB ?T to carry HDTV. all of the MPEG ?2 compliant formats in the US HDTV standard can be delivered by DVB.

     Modulation and Transmission

     Sending the signals is easy ,it is receiving them which can be difficult. Broadcasting does not happen until you have been modulation and demodulation daking place, relatively error-free.The diffrernce between the American and DVB standards exists largely in their RF modulation technique.The US system uses the singe ?carrier, 8?VSB modulation scheme.

     DVB uses multiple carrier "COFDM modulation to provide the most rugged and flexible delivery mechanism for information available today.

     The reception of degital TV signals can he difficult in dense city environmints, or areas with steep hills or hodies of water.COFDM is particularly suited to environments with cochannel interference and multiple reflections. Having multiple carriers means that when your signal encounters obstacles there is considerable "insurance against loss of picture.Even in the US, there is considerable interest in COFDM, especially among broadcasters who wish to deliver services to inexpensive reception equipment in dense urban areas.When the chips are down...

     The emergence of two partly compatihle digital TV standards is very good news for one group within the manufacturing community ?the MPEG? chip-makers.The same chip ?sets now becoming available for HDTV receivers in the US are equally valid for DVB. Remove the MPEG? SDTV decoder chips from a DVB receiver, replace them with MPEG? HDTV decoder chips and you have DVB HDTV.More DVB ?T ?compliant COFDM chips from more manufacturers are available than for any other digital terrestrial standard, and consumer?type DVB-T receivers have already arrived on the world market.In Europe, audiences in several countries will he enjoying digital terrestrial television before the end of the year.And that says it all.

     数字电视概说 98年的NAB*（注）全部是关于数字电视的。不是一个行业的局内人不可能了解这是一个非常复杂和绝对重要的结果。

     一方面，无疑数字化是未来的方向。另方面，在世界上不同地区形成了几个不同的，明显矛盾的数字电视标准。

     来自欧洲、日本和美国不同的数字电视标准错过了建立一个单一的全球标准的重要的机会。一个新的“标准的鸿沟”

     广播工作者从NTSC、PAL和SECAM模拟电视的过去三四十年中都熟悉了“标准的鸿沟”。

     因此依据国际DVB（DigitalVideo Broadcasting数字电视广播）计划，美国选择了在美洲发展的制式，而欧洲采用了源于欧洲的制式就不足为奇了。

     好的消息是对图象不再有标准的分歧了。现在，对传输之前压缩数字电视图象有一个单一的全球标准，就是活动图象专家组（MPEG）开发的 MPEG—2。

     坏的消息是在声音编码系统，且 最重要的是在MPEG-2比特流的射频调制方面，标准的分歧仍在继续。图象格式——MPEG-2

     数字广播为获得更多的信道和／或改善电视图象的清晰度提供了潜力。被美国的和DVB的标准所采用的MPEG-2标准支持了许多不同的图象格式。

     MPEG-2数字压缩被用于所有被提出的数字电视制式以得到足以处理一个高清晰度电视（HDTV）或多路标准清晰度电视（SDTV）所需的大量数据的能力。

     除图象以外，美国和DVB制式采用不同的音频编码系统。两种音频系统都能提供从CD（激光唱机）品质的立体声到全环绕声的声音质量。

     当前欧洲的数字电视计划调整倾向于SDTV，而在美国倾向于HDTV，要记住重要的是HDTV或SDTV与射频调制或MPEG-2码流的传输毫无关系。

     对希望使用DVB-T（DVB-Terrestrial地面数字电视广播）传送HDTV的广播公司是没有障碍的。所有适应MPEG-2格式的美国HDTV标准都能由DVB传送。调制和传输

     发送信号容易，接收它们可能是困难的。在你获得无误差的调制和解调之前不能实现广播。

     美国和DVB标准之间在射频调制技术方面存在着很大差异。

     美国制式采用单载波，8-VSB（8电平，残留边带）调制的方案。

     DVB使用多载波“ COFDM（Coded Orthogonal Frequency Divi－sion Multiplexing编码正交频分复用）”调制为当今的信息利用提供最大的稳定和灵活的传输结构。

     在密集的城市环境中或在陡峭的山区或水下接收数字电视信号可能是困难的。

     COFDM特别适合有同频道干扰和多重反射的环境。具有多个载波意味着当你的信号遇到障碍时有相当大的抗图象损伤的“保险”。

     即使在美国，也对COFDM相当感兴趣，尤其是希望在密集的市区服务于廉价的接收设备的广播公司。当芯片降价时…

     两个部分地兼容的数字电视标准的出现对制造业中的一个集团即MPEG-2芯片制造者是个好消息。

     现在适用于美国的HDTV接收机的相同的芯片组同样可用于DVB。用 MPEG-2 HDTV解码芯片替换DVB接收机中的MPEG-2 SDTV解码芯片，你将拥有DVB HDTV。

     与任何其他数字地面标准相比，有更多厂商的更多的DVB-T适用 的COFDM芯片可以利用，消费者类 型的DVB-T接收机已经进入世界 市场。

     在欧洲，一些国家的观众将在年 底以前欣赏到数字地面电视。---------------------------

     作者：不详

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