Comparison and Analysis of Digital TV Standard Technology in China

In September 2006, the State Administration of Radio, Film and Television issued China's national standard for terrestrial digital TV, which is a fusion of the Tsinghua DMB-T program and the Shanghai Jiaotong University ADTB-T solution. The former uses single-carrier modulation and the latter uses multi-carrier modulation. This paper describes the working principle of single-carrier modulation and multi-carrier modulation, as well as the advantages and disadvantages of ADTB-T and DMB-T.

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What is single-carrier modulation and multi-carrier modulation? Shanghai Jiaotong University's ADTB-T solution and Tsinghua's DMB-T solution, the focus of debate between the two sides is that single-carrier modulation performance is superior or multi-carrier modulation performance is superior. Therefore, it is necessary to briefly introduce what is single carrier modulation and multi-carrier modulation. The so-called single-carrier modulation is to modulate the data stream that needs to be transmitted onto a single carrier for transmission, such as 4-QAM (QPSK), 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256. -QAM or 8-VSB, 16-VSB, etc. are all single carrier modulation. Shanghai Jiaotong University's ADTB-T program uses single-carrier modulation. At the 1999 50th anniversary of the trial, Shanghai Jiaotong University's ADTB-T solution used 8-VSB digital modulation, which was later changed to 16-QAM digital modulation. . QAM modulation is also called quadrature amplitude modulation, which is called quadrature amplitude modulation. Because quadrature amplitude modulation has many modulation modes, there are 7 kinds as listed above, generally recorded as n-QAM, and n indicates various modulation mappings to constellations. The modulus on the top. The lower the modulus, the simpler the modulation and demodulation circuit, but the code rate of the transmission is also reduced. For example, the 4-QAM code rate is 2 bits/s, and the 16-QAM code rate is 4 bits/S. In general, the worse the signal transmission conditions, the lower the selected mode. For example, satellite communication can only choose QPSK, while cable TV can choose 64-QAM and 128-QAM, or even 256-QAM. For terrestrial TV broadcasting, signal transmission is generally Select 8-QAM, 16-QAM, 32-QAM, and select up to 64-QAM. Quadrature amplitude modulation is to divide a sequence of digital signals (binary code) that need to be transmitted into two groups, and separately encode the two sets of digital signals into different amplitude modulation signals, namely I signal and Q signal. Then, the two signals with the same frequency but the phase difference are respectively amplitude-modulated by the I signal and the Q signal, and finally the two modulated signals are combined and transmitted. Since the input signal has been amplitude coded prior to modulation, this modulation is also referred to as quadrature digital amplitude modulation. For China's HDTV, if MPGE-2 encoding is used, the highest transmission rate is about 20M bit/S. If 16-QAM modulation mode is selected, the spectrum utilization rate is 4 bits per Hechi, that is, the bit rate is 4bit/S. It can be seen that the carrier's highest frequency is about 6MHz, and it is transmitted by high frequency modulation and then transmitted by the vestigial sideband. The carrier frequency bandwidth is about 7 points and more MHz. The so-called multi-carrier modulation is to divide the channel into several orthogonal sub-channels, convert the high-speed data signals into parallel low-speed sub-data streams, and then modulate them to be transmitted on each sub-channel. For example: n-COFDM, where n is the number of subcarriers. Tsinghua's DMB-T solution uses multi-carrier modulation, and the 3780-COFDM modulation method is adopted in the DMB-T scheme. Multi-carrier modulation is also called coding orthogonal frequency division multiplexing modulation. For each carrier in multi-carrier modulation, the working principle of modulation is basically the same as that of n-QAM single-carrier modulation, except that the data to be transmitted is divided into many groups (here, 3780 groups), and then each group is subdivided. Two groups, after amplitude coding, can generate two sets of I signal and Q signal, and then use 3780 sets of I signal and Q signal to orthogonally modulate 3780 carriers with different frequencies, and finally combine all the modulated signals. Transfer together.

Advantages and Disadvantages of ADTB-T and DMB-T Above we briefly introduce the working principle of single-carrier modulation and multi-carrier modulation. Below we further analyze the advantages and disadvantages of single-carrier modulation and multi-carrier modulation. According to the above analysis, 16-QAM single-carrier modulation is used, and the highest code rate is 24 Mbit/s, and the carrier frequency is 6 MHz. If multi-carrier modulation is used, 3780-OFDM multi-carrier is used when the code rate is also 24 Mbit/s. Modulation, for 3780 carriers average, the average transmission rate of each carrier is only about 6.3Kbit/S, so even if each carrier uses QPSK modulation, the highest frequency of the carrier can be selected very low; if 16 -QAM or 64-QAM modulation, the highest frequency of its carrier can be further reduced. But this is an ideal situation without considering the time required for decoding and image signal processing, which is not the case. On the one hand, in digital TV sets, the carrier frequency chosen should not be too low, because the speed of digital signal transmission must be greater than the speed of image signal processing, so that the final output signal will not be interrupted. For example, the line scan frequency of China's HDTV is about 32KHz. If the time required for the input signal processing by the MPEG decoding circuit and the image signal processing circuit is not considered, then the lowest frequency of the multi-carrier cannot be lower than 32KHz. Otherwise, the line scan circuit There will be no signal to scan, and the image will appear intermittent. Therefore, the MPEG decoding circuit and the image signal processing circuit have the same requirements for the digital signal transmission speed. On the other hand, when multi-carrier demodulation performs batch processing on digital signals, each time it is necessary to wait for all the data transmitted by 3780 carriers to arrive, the data can be processed at one time, that is, the signals need to be converted and converted. Processing; therefore, its demodulation process consumes relatively long time, and its lowest frequency cannot be achieved very low. In addition, multi-carrier modulation generally does not use vestigial sideband transmission, so the modulated bandwidth is approximately twice as wide as the vestigial sideband transmission. Based on the above factors, on average, the average frequency of multiple carriers can be relatively low, but the highest frequency of the carrier is not significantly different from the frequency of the single carrier. The biggest benefit of low carrier frequency is that it can reduce the multi-reflection interference (ie, image ghosting effect) during signal transmission. Below we analyze in three cases: for example, the single carrier frequency is 6MHz, the period is 0.17uS, the two orthogonal carriers are 1/4 cycle (90 o), which is 0.0425uS; because the speed of electromagnetic wave is about equal to the speed of light, That is, every subtle is 300 meters, then the carrier with a frequency of 6MHz corresponds to a period of 51 meters, that is, the wavelength is 51 meters; the half period is 25.5 meters (half wavelength), and the quarter period is 12.75. Meter (1/4 wavelength). If the path of the reflector is exactly 12.75 meters (or an odd multiple of 1/4 wavelength) from the TV receiving point, that is, the television receiver is equivalent to receiving two signals at the same time, one is the main signal and the other is the reflected signal. The phases of the two modulated carrier signals are exactly 90 o (1/4 wavelength); thus, after the two modulated carrier signals are superimposed on each other, not only will the phase of the original signal be changed, but also the amplitude of the signal will be changed. The result is equivalent to I and Q two signals crosstalk each other, and when the phase difference between the two signals is closer to 90 o, that is, the stronger the reflected signal, the more serious the interference; in this case, the modulation and demodulation circuit of the digital television receiver It may not be able to decode properly. If the path of the reflector is exactly half the wavelength (25.5 meters, or an odd multiple of half a wavelength) from the TV receiving point, the phase of the two modulated carrier signals is exactly 180 o (half wavelength) due to quadrature modulation. The two signals (I and Q) are amplitude-modulated for the half-wave of the carrier. Therefore, after the two-way modulated signal is synthesized, it is equivalent to modulating the quarter-wave of the carrier; when the original signal is positive or negative When the half cycle is symmetrical, the result of superimposing the two signals on each other will weaken the received signal, which is equivalent to the decrease of the receiving sensitivity of the television receiver. When the positive and negative half cycles of the original signal are not symmetrical, the result of superimposing the two signals on each other will cause reception. The signal distortion is equivalent to the crosstalk of the two signals (I and Q), which is not much different from the phase of the two modulated carrier signals by 90 o.

If the path of the reflector is exactly one wavelength (51 meters, or an integer multiple of 1 wavelength) from the TV receiving point, the phase of the two signals is exactly 360 o (1 wavelength); when the original signal is positive or negative When the half cycle is symmetrical, the result of superimposing the two signals on each other will enhance the received signal, which is equivalent to the improvement of the receiving sensitivity of the television receiver; when the positive and negative half cycles of the original signal are not symmetrical, the result of superimposing the two signals on each other will cause reception. The signal distortion is equivalent to the crosstalk of the two signals (I and Q), which is not much different from the phase of the two modulated carrier signals by 90 o. It can be seen that as long as the phase angles of the received two signals (the main signal and the reflected signal) are exactly an integer multiple of 90 o (1/4 wavelength), the interference to the single-carrier quadrature modulation signal is the most serious; The phase difference also produces interference, but the degree of interference is relatively less than 90 o. Assume that the maximum frequency of multi-carrier is only 3MHz, which is one-half of the single-carrier frequency. Relatively speaking, the wavelength of multi-carrier is relatively long. According to the theorem that the electromagnetic field strength between two points is inversely proportional to the square of the distance, it can be obtained. At the 1/4 wavelength, the most severe interference is the difference between the electromagnetic field strengths of the two. For all carrier averaging, even a few hundred times difference, that is, multi-carrier modulation multi-interference is relatively lighter than a single carrier. However, the process of multi-carrier modulation and demodulation is very cumbersome. Multiple carriers are modulated and combined together, and it is very difficult to separate them during demodulation; because, under high bit rate transmission, it cannot be filtered. The circuit selects each modulated carrier signal and can only use the synchronous separation method. Therefore, it has strict requirements on the phase of the synchronization signal, so the requirement of phase noise of multiple carriers is much higher than that of a single carrier. If the carrier signals are not cleanly separated when the multi-carrier modulated signal is demodulated, it is equivalent to interference between the multiple carriers, and the signal-to-noise ratio (S/N) is lowered. Generally, the multi-carrier demodulation circuit requires that the signal-to-noise ratio (S/N) threshold be several db higher than the single-carrier demodulation circuit (according to the report, 3-4 dB). At present, there are many ways to reduce the single-carrier multi-reflection interference. For example, digital delay equalization technology is adopted, that is, a part of the signal is taken out from the signal and delayed by one phase and then superimposed with the original signal. Now this technology can be controlled by software. To achieve this, all digital signal receivers can adopt this technology in the future.

Many people believe that only multi-carrier modulation can be used for mobile TV receivers, and single-carrier modulation cannot achieve mobile reception. I think this idea is unreasonable. For example, if the speed of a car is 100 kilometers per hour (28 meters / sec), then the distance it takes to run 1/4 cycle (6MHz) (0.0425uS) is 1.19 × 10-6 meters, which is equivalent The time corresponding to 2.3×10-8 wavelengths; or the distance it travels 1/4 wavelength (12.75 meters) is 0.46 seconds, which is equivalent to 2710,000 cycles. These two results are not comparable in terms of time or distance. Therefore, the phase effect of the car speed on a single carrier is almost equal to zero. The most affected should be, whether the digital delay equalization circuit works normally in 0.46 seconds. For the Doppler effect generated by the high-frequency carrier during the mobile reception process, it only affects the offset of the receiving frequency, and the effect is the same for both single-carrier modulation and multi-carrier modulation. It is particularly noteworthy that since the ADTB-T is a single-carrier technology, it can be easily accessed by the original transmission system of the broadcasting and television. According to the data analysis, in terms of front-end digital transformation, the cost of the R&D program is about 80% to 90% lower than that of the Tsinghua solution. Only an MPEG and ADTB-T modulator can be used to transmit digital signals using the original analog transmitter. The Tsinghua program will need to be replaced with a whole new digital transmitter. This cost is still a huge expense in remote areas. At the same time, when measuring the reception of a digital signal, it is also necessary to consider the reception stability rate of the signal; since the attribute of the digital signal is 0 and 1, it is either received or not received, which highlights the meaning of the threshold; As a result, the reception threshold of DMB-T is lower than that of DVB-T, and the reception threshold of ADTB-T is lower than that of DMB-T, which is why the transmission system using ADTB-T has a coverage ratio higher than that of DMB-T transmission system. The reason for the wider coverage is that because of its low threshold, the sensitivity of the ADTB-T receiver is 3-4 dB higher than that of the DMB-T receiver, so it can receive weak signals over longer distances.

In addition, when the single-carrier system is set up to form a single-frequency network (SFN), the accuracy and stability of the transmitter clock frequency are only E-9. This is three orders of magnitude lower than any multi-carrier system, so its engineering cost is lower. In terms of transmit power, the average power of the transmitter of the large single-carrier system is one-half of that of the multi-carrier system, and its calibration power is one-fifth of the latter. That is, within the same coverage, in order to achieve high data rate fixed reception, the upper crossover scheme requires an average transmission power of 1 kW, and the transmitter's calibration power needs 4 kW because its peak-to-average ratio (PAR) is close to 6 dB. For multi-carrier systems (represented by European standards), if the same area is to be covered, the average transmit power needs to be 2.0-2.5 kW. This is because the carrier-to-noise ratio (C/N) threshold is increased by 3-4 dB compared to a single carrier system for a multi-carrier system. Second, the transmitter's calibration power requires 20-25 kW because its peak-to-average ratio (PAR) is close to 10 dB. In the future, a single-carrier system can also use a tuner and decoder for terrestrial reception and cable reception. Because the modulation methods used in cable TV are basically 64-QAM, the ground transmission generally uses 16-QAM. In the future, 64-QAM can also be used. The above analysis is not necessarily of interest to ordinary people; but the people are interested in how to buy a TV that is both cheap and easy to use. In contrast, the production cost of the digital TV receiver of the Jiaotong University program is about 20% to 30% lower than the production cost of the Tsinghua program. It is very clear that if the people choose their own, they will know what kind of digital TV receiver they should choose.

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