What signal source is dtmb?

dtmb is the signal source of "Digital Terrestrial Multimedia Broadcasting"; the full English name of dtmb is "Digital Terrestrial Multimedia Broadcast", which is the Chinese digital video broadcasting standard. The standards for digital television and mobile digital broadcasting are formulated by the People's Republic of China. .

#The operating environment of this tutorial: Windows 10 system, Dell G3 computer.

What is the signal source of dtmb?

DTMB (GB 20600-2006, the full name is Digital Terrestrial Multimedia Broadcast, that is, terrestrial digital multimedia broadcasting), formerly known as DMB-T/H (Digital Multimedia Broadcast-Terrestrial/Handheld, that is, digital multimedia broadcast-terrestrial /handheld), is China's digital video broadcasting standard, formulated by the People's Republic of China for digital television and mobile digital broadcasting. The format will serve half of China's TV viewers, especially in suburban and rural areas. DTMB is currently used in mainland China, Hong Kong, Macau and Cuba.

DTMB has two modes: multiple subcarriers (referred to as "heavy carriers") and single subcarriers (referred to as "single carriers"); Hong Kong and Macau use the heavy carrier mode, and mainland China has both use. In addition, DTMB only formulated the data transmission standard as MPEG-TS, but did not specify the broadcast stream encoding system. Taking Hong Kong as an example, on October 28, 2012, the former synchronous broadcast channels and new channels used MPEG-2 Part 2 and H.264 as the video encoding for broadcast respectively, but now all digital broadcast channels have used H.264 for broadcast; sound The encoding can be selected from AC3, MP2 and DRA. Mainland China promotes AVS and its upgraded version AVS as the video coding standard, but some also use MPEG-2; the audio standard generally uses DRA, and a few areas use AC3 or other audio decoding standards.

From September to November 2012, RTHK worked with Asia Television and Television Broadcasting Company Limited to test four new technologies, including 3D TV transmission, PN420 transmission, and DTMB-A and E-DTMB.

Main technical features

1 High transmission efficiency or spectrum efficiency

In European DVB-T, pilots used for synchronization and channel estimation The number of carriers accounts for 10% of the total carriers. The PN sequence of DTMB is placed in the OFDM guard interval, which serves as both frame synchronization and OFDM guard interval. European DVB-T C-OFDM uses 10% of the subcarriers to transmit pilot signals for synchronization and channel estimation, etc., and there is a guard interval for the cyclic prefix, while TDS-OFDM uses the time guard interval for transmitting channel estimation signals at the same time. Therefore, the transmission efficiency of the DVB-T system can only reach 90% of the national standard DTMB system. When comparing multi-carrier technology with single-carrier technology, transmission efficiency is considered a weakness of multi-carrier technology. The core technology of DTMB was developed to solve this problem.

2 Strong anti-multipath interference capability

Compared with single-carrier systems, multi-carrier systems have the ability to resist multi-path interference. The size of the resistance to multi-path interference corresponds to its guard interval. length. Since the known (after system synchronization) PN sequence is inserted into the national standard time guard interval, given the channel characteristics, the signal of the PN sequence at the receiving end can be directly calculated and removed. The OFDM signal after removing the PN sequence is equivalent to the OFDM signal with the time guard interval filled with zero values. The performance of the OFDM signal with the time guard interval filled with zero values ​​and the OFDM with the time guard interval periodic extension are equivalent under the same channel. of. Moreover, DTMB can still work when the multipath delay exceeds the time guard interval. TDS-OFDM can jointly process the PN sequences of several OFDM frames, so that the delay length against multipath interference is not limited by the length of the guard interval. However, the traditional OFDM guard interval length design requirement must be greater than the delay length of multipath interference. .

3 Good channel estimation performance

Under the AWGN channel, the channel estimation performance of TDS-OFDM is better than C-OFDM. This is because the PN sequence used for channel estimation by TDS-OFDM has a spreading gain of about 20dB, and at the same time, there is no interpolation error unique to C-OFDM for channel estimation. Although the prototype function of DTMB still needs to be improved, the test results of its AWGN channel are still better than those of domestic and foreign systems based on C-OFDM. For multipath channels, the PN sequence of TDS-OFDM and the interference signal caused by the multipath channel are statistically orthogonal. Although the performance of TDS-OFDM channel estimation cannot be directly compared with C-OFDM in principle, it is comparable to the performance of channel estimation using PN sequences in other transmission systems.

4 Suitable for mobile reception

Mobile reception produces the Doppler effect and occlusion interference, which makes the transmission channel have characteristics that change with time (time-varying characteristics). What needs to be emphasized is that the signal processing of any OFDM system is based on the assumption that the channel transmission characteristics are time-invariant (the basic condition for applying FFT), that is, within the time of one OFDM symbol, it is assumed that the channel is unchanged, and the changes in the channel are It is considered to occur between OFDM symbols. The channel estimation of TDS-OFDM only depends on the current symbol of OFDM, while the channel estimation of C-OFDM requires 4 consecutive OFDM symbols. Therefore, C-OFDM needs to consider the influence of channel changes of 4 OFDM symbols in mobile situations, while TDS-OFDM only needs to consider the influence of channel changes of 1 OFDM symbol. It can be seen that the DTMB system is more suitable for mobile reception than the European DVB-T.

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