DVB-T传输实施指南：数字化地面广播标准详解

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本文档是关于欧洲广播联盟（EBU）与欧洲电信标准研究所（ETSI）联合发布的《DVB-T实施指南》（ETSITR101190V1.2.1, 2004年11月），针对数字视频广播（DVB）地面服务的传输方面提供详细的实施建议。DVB-T，全称为Digital Video Broadcasting Terrestrial，是DVB标准体系中的一个重要组成部分，主要用于电视信号的数字广播和接收。本指南的核心关注点在于确保DVB-T系统的高效、可靠和兼容性。它涵盖了以下几个关键知识点： 1. **DVB标准背景**：DVB是国际上广泛采用的数字电视广播技术标准，致力于为全球范围内的广播服务提供高质量的数字电视信号传输。DVB-T标准针对的是地面广播，即通过地面天线接收电视信号，与卫星和有线电视系统相辅相成。 2. **MPEG编码技术**：DVB-T标准基于MPEG-2编码，这是一种高效的数据压缩标准，能够提供高质量的图像和音频编码，适合长距离无线传输，同时保持良好的比特率效率。 3. **信号传输技术**：指南详细阐述了DVB-T系统的信号发射和接收过程，包括调制方式（如QPSK、8PSK等）、频率规划、信道编码（Turbo编码或卷积编码）、以及频道保护和错误检测措施。 4. **频谱管理**：由于DVB-T使用的是地面广播频段，因此文档强调了如何在有限的频谱资源内实现多频道广播，保证不同服务之间的有效共享和互不干扰。 5. **设备兼容性**：为了确保用户能够顺利接收信号，指南规定了DVB-T设备的设计和制造规范，包括解调器、接收机和调制器等，旨在确保所有符合标准的产品可以相互协作。 6. **服务质量（QoS）保障**：对于不同的应用场景，如移动接收、室内和室外覆盖，指南提供了相应的QoS参数设置和优化策略，确保不同环境下的信号质量。 7. **测试和验证**：文档还涉及了测试方法和验证流程，确保DVB-T系统的性能达到标准要求，并提供了故障排除和维护的指导。 8. **版权和知识产权**：最后，文中强调了版权和知识产权的管理，以及如何遵循相关法律法规，确保合法的数字内容分发。 ETSITR101190V1.2.1是DVB-T领域的重要参考资料，对于电视台、广播运营商、设备制造商以及技术研究人员来说，它是实施和优化DVB-T服务不可或缺的技术指南。

ETSI

TSI TR 101 190 V1.2.1 (2004

A longer guard interval could compensate longer echoes:

• lengthening the guard interval without changing the absolute duration of the useful interval would accordingly

decrease the channel capacity, thus reducing the deliverable bitrate;

• alternatively, lengthening both the guard interval and the useful interval would not bring any penalty to the

channel capacity, but would make the signal processing more difficult because of the higher number of carriers

that would result from the larger symbol duration.

Table 1 summarizes the possible lengths of the guard interval specified in the DVB-T (see EN 300 744 [5])

specification depending on the chosen FFT length.

Table 1: Specified lengths of the guard interval

Length of the guard interval Proportion to the

length of the useful interval

8k-mode 2k-mode

1/4 224 µs 56 µs

1/8 112 µs 28 µs

1/16 56 µs 14 µs

1/32 28 µs 7 µs

The longer guard intervals are suitable for networks with longer distances between the particular transmitter station, as

for example with national single frequency networks. The shorter intervals are suitable for regional or local broadcast

transmissions.

According to table 1, there are two different modes regarding to the number of carriers. The length of the useful

interval is 896 µs for the 8k-mode and 224 µs for the 2k-mode. Due to the orthogonality of the system, this corresponds

to a carrier distance of 1 116 Hz and 4 464 Hz, respectively.

One basic requirement for the DVB-T system was the bandwidth constraint in order to match an 8 MHz channel

spacing. From this requirement one can derive the number of possible carriers. 6 817 carriers per OFDM symbol for the

8k-mode (6 048 useful, the others for synchronization and signalling) and 1 705 carriers per OFDM symbol for the

2k-mode (1 512 useful carriers) are specified in the DVB-T system. The OFDM symbols can be calculated by the

Inverse Discrete Fourier Transform (IDFT). Virtual carriers are inserted in such a way that the total number of carriers

becomes a power of two, so that the faster algorithm of the Inverse Fast Fourier Transform (IFFT) can be used. At the

receiving side, the corresponding signals can be easily recovered using the respective 2k-FFT or 8k-FFT.

In order to ensure robust transmission of the OFDM signal, an error protection code is applied. In addition to the fixed

algorithm of energy dispersal, block coding, outer and inner interleaving, a Rate Compatible Punctured

Convolutional (RCPC) code has been defined as in the DVB Satellite standard. The mother code has a constraint length

of 7 bits and works with a code rate of 1/2. The two generator polynomials of the convolutional encoder are 171 and

133 in octal notation.

To adapt the error protection to the actual transmitting conditions, several code rates can be chosen. The following code

rates are specified in the DVB-T (see EN 300 744 [5]) (and DVB Satellite (DVB-S) (see EN 300 421 [3])) system:

1/2, 2/3, 3/4, 5/6, 7/8

The code rate 1/2 has the highest redundancy, but the highest transmission safety. This mode should be applied to

strongly disturbed channels. On the other hand a code rate of 7/8 has a low redundancy but a very weak error protection.

Therefore, it should be used for channels with only low interference.

As mentioned above, every carrier is modulated by a modulation symbol. QPSK, 16-QAM and 64-QAM are used as

modulation methods, e.g. 2, 4 or 6 bits per modulation symbol. The bits are assigned to the particular points in the phase

space according to the so called Gray-code mapping. The advantage of this mapping is the fact that closest constellation

points differ only in one bit. The constellation diagrams for each modulation method are illustrated in figure 1.

ETSI

TSI TR 101 190 V1.2.1 (2004

10 00

1000 1010 0010 0000

1001 1011 0011 0001

1101 1111 0111 0101

1100 1110 0110 0100

100000 100010 101010 101000 001000 001010 000010 000000

100001 100011 101011 101001 001001 001011 000011 000001

100101 100111 101111 101101 001101 001111 000111 000101

100100 100110 101110 101100 001100 001110 000110 000100

110100 110110 111110 111100 011100 011110 010110 010100

110101 110111 111111 111101 011101 011111 010111 010101

110001 110011 111011 111001 011001 011011 010011 010001

110000 110010 111010 111000 011000 011010 010010 010000

QPSK

16-QAM

64-QAM

Figure 1: Constellation diagram for the modulation methods specified for DVB-T

A further feature defined in the DVB-T (see EN 300 744 [5]) specification is hierarchical modulation. While the audio

and video quality of the analogue TV decreases gradually, digital transmission techniques preserve their reception

quality up to a certain point but then suddenly show total signal disruption, as the transmission conditions become

progressively poorer.

To overcome this problem, the data to be transmitted can be split into two parts. The first part provides the basic TV

service with a relatively low data rate and a high error protection. The second part could be used for additional services

with higher data rates and weaker error protection. In general, there are two possibilities for using this second data part.

On the one hand, additional programmes can be transmitted, on the other the higher data rate can be used to increase the

quality of the basic service.

The level of error protection can be adjusted by choosing different code rates of the inner convolutional encoder. Both

data streams are modulated simultaneously. Each carrier is modulated by two data symbols with different error

protection. The symbol with the higher protection is modulated using the more resilient modulation method. It carries

the information about the quadrant of the constellation point in the phase space. The other symbol gives the information

about the location of that constellation point within each quadrant.

The need to completely separate signal processing of each data stream is a disadvantage of the method described above.

igure 2 illustrates the constellation diagrams for the hierarchical 16-QAM and 64-QAM.

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