The United States established its national standard for terrestrial broadcasts of high-definition digital television, known as ATSC (for Advanced Television Systems Committee), in 1996. The European Union settled on its standard, Digital Video Broadcast-Terrestrial, or DVB-T, in 1997. Japan developed its Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) in the 1980s and adopted it in 2003. But China just finalized its digital television standard in late 2006, beginning transmission with last summer’s Beijing Olympics.
Being late in this particular game is not necessarily a bad thing. It allowed China to take advantage of advances in information-coding technologies that make digital television in China—unlike that in the rest of the world—work well even in bad weather. These technologies mean that China’s digital television can be viewed on the go; it won’t break up even at 200 kilometers per hour—you can watch a broadcast on a cellphone while sitting on a high-speed train. (The United States is only now trying to retrofit its digital-television standard for mobile reception.)
Development of what is formally called GB20600-2006 (GB stands for guo biao , ”national standard” in Chinese) began in 2000 at China’s DTV Technology Research Center, in Beijing, established by my company, Legend Silicon Corp., of Fremont, Calif., and Tsinghua University, also in Beijing. The research team studied the digital-television standards of various countries around the world—including DVB-T and ATSC—looking to adopt the best features of each and improve upon them whenever possible. Their goal: to deliver free-to-air HDTV to the Chinese people in time for the Beijing Olympics.
Digital-television standards differ in two important aspects. One is their capacity in terms of bandwidth, which limits just how much information broadcasters can send. Second, some standards allow not only televisions sitting in living rooms to decode the signal but also ones that are moving fast, such as those on an automobile or a train. And, of course, each standard uses modulation schemes and encoding technologies that reflect the state-of-the-art technology at the time it was developed.
The U.S. ATSC standard uses the 8-VSB, for Vestigial Sideband Modulation Scheme, which means the payload bits are mapped to eight different levels in the vestigial sideband. This scheme meets the needs of a large-screen digital television that’s in a fixed location. In 1996, developers of the standard considered it more important to accommodate a high-definition full motion-picture broadcast (equivalent to a payload bandwidth of about 19 megabits per second) to a stationary digital television screen rather than support mobile television reception. Given that developers were working in an era before smartphones and built-in rear-seat car entertainment systems, this mobile reception capacity didn't seem relevant.
The DVB standard of Europe, on the other hand, uses COFDM (Coded Orthogonal Frequency-Division Multiplexing), which compromises on the bandwidth available to send picture data. This results in a lower digital-television resolution, but it can also support mobile reception.
China’s GB20600-2006 standard can fully support both fixed and mobile systems and send high resolution images, thanks to newer modulation schemes and information-encoding technologies. The standard supports maximum payloads of nearly 24 Mb/s compared with 19 Mb/s in the United States, and mobile reception at speeds greater than 200 km/h compared with similar speeds supported by DVB.
The core technology behind China’s standard is Time Domain Synchronous-OFDM, or TDS-OFDM, invented by Legend Silicon and developed jointly with Tsinghua University.
Any digital-television broadcast signal, from the moment it leaves the transmission tower, is bashed around as it propagates in the atmosphere. Tall structures such as buildings will obstruct, deflect, and weaken this signal; trees and foliage in the signal path will dissipate its strength. If the signal encounters a body of water, the ripples on the surface will interfere with the transmission. The ultimate result is a weak and unpredictable television signal, varying in direction as well as strength. And this is just for a fixed television receiver. If the television unit itself is moving at rapid speeds, then the reception situation gets worse, with severe fade-ins and fade-outs.
The challenge of any digital-television standard—and hence any digital-television receiver that supports such a standard—is to embed in it the technology to smoothly put the bashed-up television signal back together and turn it into a high-quality picture. That would be easy given an infinite amount of money to put into technology at both the transmitter and receiver end, but the commercial success of any standard depends on the affordability of the technology.
To understand how OFDM technology works, imagine a large, high-bandwidth digital payload—for example, a live football-game feed—embedded in the digital-television transmitter signal as it passes through the atmosphere. If this payload is packaged and transmitted as is, without any preemptive corrective mechanisms as it moves over large areas, it will lose strength, the amount of loss increasing as the distance between the transmitter and receiver grows. For mobile reception, it gets even more complicated because the distance continually varies.
Next, the atmosphere through which the signal travels is in a constant state of flux, with both slow and rapid electromagnetic changes that directly affect the progress of a digital-television signal through the air. At the same time, the movement of large and small vehicles on the roads and freeways contributes to atmospheric variations. All these phenomena stretch and twist the payload, as if it were a piece of cloth.
Finally, the multiple reflections from tall structures, trees, and foliage spread the signal over time, bouncing it back and forth and varying the arrival time at its destination. This can create an effect like that of artificially speeding up or slowing down the video/audio playback.
OFDM technology minimizes such debilitating effects on the television signal, rearranging the transmitter signal before it is transmitted in anticipation of the atmospheric effects. When the signal reaches the television receiver, a matched OFDM circuit in the television set can recover the payload—in this case, the live football game—with high fidelity.
In this rearranging, the system organizes the large digital payload: Rather than transmitting the entire payload in a single carrier of one frequency, as is done in FM radio and ATSC systems, it breaks up the payload and distributes it into several smaller carriers of different frequencies, hence the name Frequency-Division Multiplexing. In such multicarrier transmission, each individual carrier is smaller in bandwidth, so the impact of the atmospheric degradation on each individual carrier is also smaller.