The alliance is designed to forge a DTV transmission standard for China that combines spread-spectrum and orthogonal frequency-division multiplex technologies. With an eye to future mobile applications, the standard, if successful, could leapfrog dated US and European specs, some observers said.

The digital transmission technology is being explored by researchers at Tsinghua University, based in Beijing, together with Legend Silicon. The goal of the group is to develop "a robust and yet very flexible, future-proof modulation scheme," said Lin Yang, Legend Silicon’s chairman and president.

Borrowing heavily from telecommunications schemes, the new DTV transmission technology is designed to allow China to use its DTV spectrum not only for standard- and high-definition TV broadcasting but also for future data services and even cellular phone applications.

"If China wants to jump start its information industry, we need a DTV standard that uses its spectrum efficiently," Yang said. "We regard this as a serious natural-resource issue."

Seventy-five percent of China's available spectrum under 1 GHz is set aside for TV applications. Of the remainder, 10 to 15 percent is controlled by the military and the rest is available for existing cell phone applications. Hence, some engineering executives here said China needs a DTV standard that allows the sharing of spectrum reserved for broadcasting with emerging information delivery services.

Third Way

As a result, Chinese government officials and industry executives, along with their US-based partners, appear committed to developing a third way to launch digital TV broadcasts, based neither on US nor European digital TV specs. Both US and European camps have waged lobbying efforts to persuade the Chinese government to adopt their differing approaches to digital broadcasting. But beyond trials, neither has received a firm commitment from Beijing.

China has been carefully monitoring the slow deployment of digital TV in the United States and Europe. Persistent DTV transmission problems, particularly with the US vestigial sideband 8-VSB modulation scheme, have "worried us somewhat," Yang said, "but it also convinced us that there is an opportunity" for China to create its own DTV standard.

Observers in China and in the United States agree that China will go its own way.

"Whatever they adopt will be called a Chinese standard," said Robert Graves, chairman of the US Advanced Television Systems Committee. "They do seem quite intent on putting their own stamp on whatever standard they pick."

A Chinese DTV standard that incorporates a combination of broadcast and spread-spectrum technologies "is going to happen," asserted Ya-Qin Zhang, managing director of Microsoft Research China, here, and a former video engineer with US HDTV Grand Alliance member Sarnoff Corp.

Responding to the Chinese government's plan to roll out digital TV broadcasts in 2003, three different groups are expected to submit unique terrestrial DTV transmission technologies to the Standards Institute of the State Administration of Radio, Film and TV（Sarft）. In May, lab testing, followed by field tests, is scheduled in Beijing, Shanghai and Shenzhen. The tests will provide side-by-side comparisons of the homegrown systems with the US Advanced Television Systems Committee spec, Europe's Digital Video Broadcast standard and Japan's Terrestrial Integrated Services Digital Broadcasting（ISDB-T）. The deadline for submitting DTV technology proposals is April 30.

Technology Teams

Besides the Tsinghua University group, the competing DTV technology development teams are the HDTV Technical Expert Executive Group（TEEG）, headed by Wenjun Zhang, and the Academy of Broadcasting Science(ABS)team, led by Baichuan Du.

Zhang said his test system has a mobile telecommunications feature that uses coded orthogonal frequency-division modulation (COFDM). "We own the intellectual property rights of the technologies," he said. "They are based on QAM（quadrature amplitude modulation）."

TEEG, backed by China's Ministry of Science and Technology as well as China's State Planning and Development Committee, has already developed China's first two prototype DTV systems, based on 8-VSB and the COFDM multiplex scheme. Some observers said TEEG may be furthest along in implementing 8-VSB and COFDM.

Meanwhile, the Academy of Broadcasting Science group is working on the modified version of QAM technology for terrestrial DTV transmission. The group's goal is to use 64 QAM both for cable and terrestrial DTV modulation.

The ABS group is reportedly using a VSB/QAM chip supplied by Broadcom Corp. It is leveraging a large number of filter taps built into the Broadcom chip’s equalizer, originally designed for better VSB reception, to minimize delays associated with QAM-based terrestrial DTV transmission.

"It is not so clear which technology will be China’s final choice," said Du, vice president of ABS. "But it’s obvious that China needs a technology of its own.

Up to now, the Tsinghua University group has been secretive about its DTV work. Of the three efforts, industry sources said, the Tsinghua team appears to have the most novel and perhaps the most ambitious solution.

The project is overseen by the State Key Lab for Microwave and Digital Communication and is led by Tsinghua professor Ke Gong.

Legend Silicon, co-founded by three Tsinghua University graduates transplanted to Silicon Valley, is a member of the National Key Lab's Digital TV Transmission Technology Development Center. Legend Silicon executives said they expect to receive transmitter and receiver chips based on the company's designs from an unnamed fab outside China by the end of March.

Responding to doubts about China's chip-design capabilities, Legend Silicon's Yang, also a Tsinghua professor, said, "We've done solid computer simulation designs and have a very good design flow." After completing the design work, "we got it working last July," Yang said. "We had no problems in signing off ASIC designs either."

The National Key Lab is also equipped with Cadence Design tools and serves as a design-training center. Yang is a former director of the wireless group at Cadence’s Design Services Group.

Leapfrog Approach

If China succeeds in developing its own DTV standard, some experts said it might be able to leapfrog US and European DTV standards. Political disputes dogging those standards have made technology upgrades difficult.

Whereas the underlying technologies of the US standard were developed a decade ago, the Chinese DTV effort aims to respond to the future needs of the converging communication, TV and Internet industries. Indeed, when the US industry was developing its DTV spec, there was no requirement for either mobile or Internet applications.

"China today is working on a homework assignment” very different from the one given earlier to US developers, Yang said.

The core DTV transmission technology, designed by Tsinghua's Key Lab, is called time-domain synchronous OFDM(TDS-OFDM). While maintaining data rates as high as 32 Mbits/second to cater to multimedia services, TDS-OFDM is designed for better synchronization of mobile and burst data broadcasting.

Using TDS-OFDM, transmission signals are separated into two parts: Synchronization signals, used primarily for channel selection, and signals that carry actual programs. "We use spread-spectrum technology to send synchronization data, while we depend on OFDM to send continuous TV broadcast programs," Yang said. Spread spectrum is used for the synchronized signals, Yang said, because they need to remain "robust and easy to detect in a very noisy environment."

Signals Saved

Illustrating the importance of synchronization, Yang said the biggest DTV problem is the so-called cliff effect, wherein a digital receiver goes dark if signal reception is poor. By integrating control signals in the synchronized data, sent separately with digital broadcasts using spread-spectrum technology, consumers could, for example, use such signals to adjust an antenna to receive pictures.

Tsinghua University has also developed a DTV protocol, Digital Multimedia Broadcast-Terrestrial(DMB-T), that could allow an 8-MHz DTV channel to be reused for cellular network applications.

"We have 10 million people living in Beijing alone,” Yang said. "If all these people wanted data services and video-on-demand services at the same time, we’d have a problem. We need a technology that supports multiple RF, signal RF and cellular networks."

Developing its own intellectual property is another goal of China's DTV effort. Yang described Tsinghua’s DMB-T approach as "a lot of public domain technologies combined together." But Key Lab has filed for a patent covering the entire system. Seven others have been filed for individual transmission technologies.

To drive standards, Key Lab also plans to submit its technology to the International Telecommunication Union in Geneva.

(China Daily 01/31/2001)