The Syndicated

The Evolution of High-Definition Digital Television

By Tam Do, Altera Corporation

Digital television (DTV) deployment began about a decade ago when direct satellite broadcast providers DirecTV and Dish Network introduced MPEG2 digital video transmission. As DTV services continue to expand into cable and satellite networks, DTV technology is advancing from standard definition (SD) to high definition (HD). While many different standards and modulation schemes are used to achieve digital video bandwidth efficiency, the encoding/compression standard and modulation method are critical for next-generation DTV deployment. FPGAs offer the ideal solution for addressing the current and future technical challenges and requirements for DTV transmitter and receiver design.

Video Compression and Encoding

Before video data can be transmitted, video compression, most commonly MPEG2, is used to reduce the bandwidth for raw video. The typical MPEG2 data rate is 20 Mbps for SD in satellite broadcast. MPEG-2 compression is based on the principle that temporal and spatial redundancy in motion pictures make up the majority of the visual information that humans perceive.

As broadcasters produce more HD video content, the MPEG2 format becomes a bottleneck if more channels have to be transported within the stream. Therefore, the next generation of HDTV must utilize a new video compression format to conserve system bandwidth. The emerging H.264/AVC compression standard allows broadcasters to increase both SD and HD programming without having to increase their limited bandwidth spectrum. H.264/AVC typically outperforms all existing standards by a factor of two; with the current industry standard MPEG-2, performance improves by a factor of 2.25 to 2.5.

Compared to MPEG-2, H.264/AVC encoding is at least twice as efficient at all bit rates, offering significantly higher video resolution at the same bit rate and the same video quality at half the bit rate. The primary application of H.264/AVC is in new video services such as mobile applications and IPTV over ADSL, where MPEG-2 is less suitable, especially where bandwidth is limited..

H.264 encoding requirements mandate extreme computational performance and memory bandwidth together with total flexibility, all of which are available in Altera’s high-performance Stratix® III FPGAs. Figure 1 shows the basic block diagram of an Altera® single-chip Stratix III FPGA H.264/AVC encoder.

Figure 1. Single-Chip Stratix III H.264 Encoder

Digital Modulation

Different modulation methods are used for digital video transmission via satellite, cable, terrestrial, mobile, or IP. Table 1 shows the various modulation schemes that are planned or currently in use.

Table 1. Modulation Standard

Digital Satellite Transmission

For over a decade, DVB-S has been the standard method for digital satellite transmission with MPEG-2 SD video. As video moves to HD format, DVB-S is the transmission bottleneck for bandwidth allocation. DVB-S2 will pave the way for effective HD video transmission. It can achieve the best performance, quantifiable with a roughly 30% capacity gain over DVB-S. DVB-S2 also benefits from more recent developments in channel coding. Combined with a variety of modulation formats (QPSK, 8PSK, 16APSK, and 32APSK), this new satellite technology will enable the effective deployment of high data-rate applications such as HDTV, broadband Internet, and interactive services, as well as a significant increase in the number of standard channels that can be broadcast in a conventional transponder.

Terrestrial Transmission

The classical method of analog TV transmission will soon become history. Many equipment manufacturers are already gearing up as worldwide government mandates call for eventual total DTV terrestrial transmission. Unfortunately, due to governmental politics, many different standards are used around the world. DVB-T is by far the most common modulation method in Europe and some parts of Asia, while North America has adopted ATSC, Japan uses its own unique ISDB-T method. In late 2006, China released their DTV standard, called digital multimedia broadcast-terrestrial/handheld, or DMB-T/H (GB 20600-2006). DMB-T/H is a combination of single- and multi-carrier modulation schemes.

By using a FPGA, designers can create a single DTV transmission platform that can be adapted to various standards by simply programming the system to the correct modulation standard in that particular location. This flexibility allows designers to easily customize their product, even upgrading it in the field to address changing standards. Figure 2 shows the block diagram of the Altera Stratix III single-chip implementation of a DVB-T modulator.

Figure 2. Stratix III Single-Chip DVB-T Modulator

Altera and Synplicity® Accelerate Design

Altera and Synplicity have been working together to accelerate the design and verification process for digital video applications. Synplicity offers a powerful DSP synthesis tool, Synplify DSP, that enables rapid algorithm modeling and implementation into Altera FPGAs. Engineers can quickly create high level models of video and wireless algorithms with the Synplify DSP library which includes functions for convolutional encoding, inner and outer interleaving, puncturing, Viterbi decoding, FFT/IFFT transforms, and advanced multi-rate filtering and resampling operations. This design flow also support Altera IP blocks such as Reed-Solomon and Viterbi cores and other IP support by Altera partners.

Once an algorithm is designed an verified, Synplify DSP's synthesis engine can automatically create an optimized RTL architecture for Altera devices. By providing contraints for area and speed, engineers can quickly explore parallel or serial architectures of their digital TV algorithm and choose the best tradeoff for their particular application. Using Synplify Pro software for logic synthesis enables comprehensive support for Altera's advanced DSP technologies including the latest Stratix-III devices.

Conclusion

Compared to the current industry standard, MPEG-2, H.264/AVC encoding is at least twice as efficient at all bit rates. System designers using H.264/AVC will achieve significantly higher video resolution than can be achieved with MPEG-2. However, they still face an abundance of standards currently in use around the globe, including H.264/AVC, that they must consider when designing for DTV transmitters and receivers. The latest FPGA family with increased DSP blocks, internal memories and high speed IO interface will provide an ideal design platform with extreme computational performance, memory bandwidth, and total flexibility, capabilities that are necessary to meet the plethora of transmission encoding requirements.

Information on FPGA-based broadcast solutions is available on the Altera web site (http://www.altera.com/end-markets/broadcast/delivery/bro-del-index.html); information on the Synplicity libraries and reference designs is available at http://www.synplicity.com/products/synplifydsp/index.html.