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The worldwide woe of delivering DTV content

Posted: 03 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:deliver DTV content? DTV standards? DVB-S2?

Today, DTV is available through transmission media (such as cable and satellite) and over the air. Hundreds of companies have been involved in defining standards for its reception. With better communications making the world smaller, you might expect that when new standards are defined, the goal would be to make them global. That has not been the case, however, for DTV.

To make DTV available to the consumer, the TV content needs to be encoded. The main purpose of the encoding process is to digitize the information with the lowest possible bit rate at the highest possible quality, and there are different encoding schemes available.

The most common method is MPEG, in which motion vectors determine the moving parts of the picture. By transmitting picture information with motion vectors, the decoder can retrieve the original content. Today, the MPEG-2 standard is the most widely used. With the move toward bigger screens with higher resolution and the ever-increasing demand for more bandwidth, however, improved encoding schemes such as MPEG-4 are required.

Transmission issues
Once the content is digitized, it needs to be transmitted via satellite or cable, or over the air. Since all transmission channels have different limitations, different technologies are used to bring digital content to the consumer's home.

Satellite reception relies on the direct line of sight of the dish receiving the signal. The signal is not impaired by reflections, but it is weak. Thus, the transmission method uses a modulation scheme that is very robust against noise. This scheme is QPSK, which is also used in the DVB-S standard. The digital content is represented by the phase of a complex carrier with an in-phase and quadrature component. Both components of this complex carrier have two phase possibilities, which result in four states (2bits of information per symbol).

The bandwidth used for satellite transmission goes up to 45MHz. Bit rates close to 50Mbps can be achieved. A new standard, DVB-S2, was defined. It doubles the number of constellation points to eight (3bits per symbol). The bandwidth used for satellite transmission also ranges to 45MHz, and bit rates close to 50Mbps can also be achieved.

In a cable environment, the problem is not the weak signal level. Thus, modulation schemes with a higher resolution can be used. Besides information about the phase of the carrier, amplitude is used to represent the digital information. This modulation scheme is QAM. Currently, up to 256 phase and amplitude levels are being used, which adds up to 8bits of information per symbol. The bandwidth of a digital cable signal using the VHF and UHF frequencies goes up to 8MHz. As with satellite, a bit rate of up to 50Mbps can be achieved.

The limiting factor for cable is the quality of the network; bad connections will result in reflections. The receiver system can handle only a limited amount of reflection. The higher the resolution of the QAM, the more difficult it is to distinguish among the various phase and amplitude combinations.

Hundreds of companies have been involved in defining standards for DTV reception. Click to view full image.

Ground setting
In terrestrial environments, signals can be very weak (when far away from the transmitter) or very strong. In either case, reflections play a key role. Generally, there is no direct line of sight to the transmitter. The signal will be distorted and bounce because buildings or other objects are in the way. Hence, the modulation schemes used for terrestrial transmissions need to enable reception in high-dynamic and strong-interference environments. Both single-carrier (similar to cable and satellite) and multicarrier systems are used. Typically, bit rates of up to 20- to 25Mbps are achieved in one VHF or UHF channel.

Politics aside, you might wonder why different standards have been used to define terrestrial TV in different regions, especially with "air" as transmission channel. And air, after the level of pollution has been subtracted, is the same as everywhere. The answer lies in two areas: network planning and targeted applications.

Consider final app
When defining a standard, it is important to consider the final application. A rooftop aerial or indoor aerial can be used to target fixed reception. An indoor aerial can also be used to target portable or even mobile reception.

The signal conditions will be different for each application. For fixed reception, the ability to work with weak input signals and slowly moving reflections is important. For mobile reception, you need to be able to handle fast changes and Doppler effects. When looking at the TV-on-a-cellphone trend, there is an additional requirement for low-power operation.

Since transmissions began in the late 1990s, the DVB-T standard defined by the European Digital Video Broadcasting Group has been adopted throughout Europe and by many countries around the world. In the past few years, consumer applications that support DVB-T reception have seen strong growth. The standard uses OFDM.

Different standards
The Japanese standard for digital terrestrial TV is designed with both HDTV and handheld reception in mind. It uses OFDM and has characteristics similar to DVB-T, but the used channel is divided into 13 segments.

In contrast to DVB-T and Japan's ISDB-T, the North-American-Advanced Television Systems Committee (ATSC) standard is a single-carrier system. It is designed to have a high bit rate in a Gaussian noise environment.

The modulation mode used is eight-level trellis-coded vestigial sideband (8VSB). While the equalizer in an ATSC receiver is capable of echo correction, ATSC is more susceptible to multipath interference than OFDM-based systems. Thus, it is unsuitable for SFNs.

While mobile reception is practically impossible under the current spec, discussions are underway to come up with an improved version of ATSC to take the standard mobile.

China's new standardChina Digital Multimedia Broadcast-Terrestrial/Handheld (CDMB-T/H)is a combination of two earlier proposals: ADTB-T (a single-carrier system like ATSC) and DMB-T (an OFDM-based system with around 3,900 carriers). Both substandards are supported as different modes of CDMB-T/H. ADTB-T yields good overall performance with high bit rates, making it suitable for fixed HDTV reception. But unlike ATSC, because of improved equalizer training sequences, extended coding and longer interleavers, ADTB-T is also more suitable for mobile reception.

Unlike DMB-T, DVB-T uses time-domain synchronous OFDM to reduce overhead. Instead of containing the cyclic prefix, a pseudonoise sequence to perform synchronization and channel estimation is inserted in the guard interval.

Many standards are being used to bring DTV content to the consumer. Even now, new standards such as DVB-S2, DVB-T2 and AVSB have been or are being defined to increase bandwidth or provide better mobile capabilities. The challenge is to overcome the technical hurdles of each so that they can be implemented properly.

- Wim Renirie
General Manager

- Hans Stoorvogel
Senior System Engineer

DRX Business Unit, Micronas GmbH

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