4.HDTV in China
4.1Digital Television Terrestrial Broadcasting Standard of China
A new DTTB (digital television terrestrial broadcasting）Standard named “Frame structure, channel coding and modulation for digital television terrestrial broadcasting system” was published in China in August, 2006. ADTB-T technology, which is developed by ShangHaiJiaotongUniversity and DMB-T technology which is developed by Tsinghua Univercity both laid the foundations of the standard.
4.2 ADTB-T of SJTU vs. DMB-T of Tsinghua
It took a very long time before the national standard was published because of the fierce debating between the two universities for which technology is better. The main difference between the two technologies is that ADTB-T adopt single-carrier technology while DMB-T takes the multi-carrier technology. So the bone of the contention is which technology performs better.
Single-carrier means modulate data flow on one carrier, i.e 4-QAM (QPSK),8-QAM,16-QAM,32-QAM,64-QAM,128-QAM,256-QAM,8-VSB,16-VSB. The ADTB-T technology of SJTU take the technology of 8-VSB and latter changed to 16-QAM.
Multi-carrier is the technology to divide the channel into many sub-channels and transform a serial high-speed data flow to low-speed parallel sub-data flow and then modulate each sub-data flow to each sub-channel. For example, n-COFDM (n is the number of the sub carriers). DMB-T technique of Tsinghua use 3780-COFDM modulation.
4.3 Which is better? ADTB-T or DMB-T
4.3.1 The frequency of the carrier
The main advantage of the multi-carrier modulation is that the highest carrier frequency could be lower. The main benefit of a low carrier frequency is that it could depress the multi-path reflection interference.
If we use 16-QAM single carrier modulation, then the highest bit rate would be 24Mbits/s and the carrier frequency 6MHz. If we use the same bit rate in 3780-OFDM modulation, say, 24Mbits/s, the average bit rate of the 3780 sub carriers would be just 6.3Kbits/s, which means that the highest carrier frequency could be quite low even QPSK modulation is used. But this is just the ideal situation when the time spent on decoding and image signal processing was not considered.
For one thing, the carrier frequency could not be too low for the reason that the transmitting speed should be greater than that of processing image signals. If this condition is not met, the output television signal could be discontinuous. For another thing, when digital signals are processing by multi-carrier modulation system, 3780-OFDM system for example, data could be processed only when all 3780 signals arrive, which means a serial-to-parallel convertor is needed. Consequently, the lowest carrier frequency could not be too low because of the comparatively longer time for demodulation.
Otherwise, commonly VSB is not used in multi-carrier modulation, so the bandwidth of this modulation technique is approximately 1 time larger than that of VSB modulation.
In sum, the average frequency of multi-carrier modulation could be a little bit lower than that of single-carrier modulation, but not much.
4.3.2 Signal-to-Noise ratio
Because the procedures of modulating and demodulating of multi-carrier systems are quite complicating, it becomes quite difficult to demodulate a coalescent signal. Only the synchronization separate method could be used and this require strictly in-phase synchronized signal.
If the sub-carriers are not well separated, the interference between sub-carriers is introduced and the SNR depresses. Generally, the multi-carrier system’s threshold of the SNR is a few dB higher than that of single-carrier system. This advantage make the coverage area of ADTB-T much larger.
4.3.2 Other Comparison
ADTB-T technique can well arrange with the transmitting systems of the broadcasting systems we use now. But we have to replace the current broadcasting system to a complete set of digital transmitter. ADTB-T technique could cut 80%~90% of the digital reconstructing cost.
4.3.3 Conclusion
In sum ADTB-T technique generally perform better than DMB-T! |
