April 9, 1998

SET-TOP-BOX EVOLVE FOR DIGITAL TV CHIP SETS

STEPHEN KEMPAINEN, TECHNICAL EDITOR

This year, over-the-air digital TV (DTV) is set to begin broadcasting in the United States and Europe. DTV standards allow new set-top and converter boxes to take advantage of better pictures and sound and of two-way data. As a result, it's no surprise that vendors have responded by offering flexible chip sets and reference designs, so that innovative designers can customize TV peripheral products for the big wave in DTV consumerism.

US television broadcasters have committed to using digital-television (DTV) broadcasting in the top-10 US markets by November. In exchange for extra free bandwidth from the FCC, broadcasters must include some high-definition TV (HDTV) along with standard DTV. Beyond part-time HDTV, it is anyone's guess what types of DTV formats will evolve as this technology becomes mainstream. Consumer response will determine how broadcasters will use the multiple high-quality video and audio formats, multichannel broadcasts, and multimedia services specified by the Advanced Television Systems Committee (ATSC).

However, what is certain is that the DTV broadcasts will accelerate digital video and audio programming content in satellite and cable transmissions. Consequently, consumers will need flexible set-top boxes (STBs)--almost any peripheral device that enhances TV--for these services. Chip vendors are leading the charge with versatile STB chip sets for regional niche markets evolving worldwide.

In Europe, over-the-air DTV broadcasts begin this year in the United Kingdom and Sweden. The European Digital Video Broadcasting (DVB) DTV standard currently in use is the same standard that Direct Broadcast Satellite (DBS) systems already use in France, Japan, the United States, and various other countries. The DVB standard is similar to the one that the ATSC adopted in that both guidelines use the MPEG-2 video-compression and transport standard.

But DVB differs from ATSC in some ways. The modulation for over-the-air broadcast is coded orthogonal frequency-division multiplexing (COFDM) instead of the vestigial sideband (VSB) that the ATSC uses. Another difference is that audio coding in the DVB standard is MPEG-2 instead of the Dolby AC-3 that ATSC uses. In addition, although HDTV has been a cornerstone from which to build the ATSC standard, the DVB Standard did not initially include it. Interest is now growing in Europe for HDTV, however, and the DVB group is working to add it to the standard.

When you throw cable TV into the mix, the confusion over STBs grows ever greater. On the standards track, the CableLabs OpenCable initiative is developing a standardized digital STB for the cable industry. The specification maintains an open architecture that is independent of operating systems and µPs. The OpenCable digital STB will include two-way data capability for Internet access over cable modems and will accommodate advanced DTV formats. However, the FCC continues to debate whether cable TV must carry the broadcaster's full-resolution HDTV or allow them to down-convert to lower resolutions. Lower resolution would reduce memory and processing power requirements in cable STBs.

Along with niche markets, the DTV chicken-and-egg problem tends to baffle industry prognosticators. DTV broadcasters will not benefit without an audience equipped with digital receivers--or, for that matter, consumers willing to buy receivers without compelling digital broadcasts. Broadcasters are slow to embrace DTV because they have yet to figure out how to make money from it. In addition, broadcasters are unsure whether consumers will buy digital equipment whether it takes the form of converter STBs or integrated-receiver-decoder (IRD) TV sets. Consumers, on the other hand, may not buy new digital equipment until they see and hear the entertainment value DTV offers. The first year of service will be full of experimenting with broadcast formats and data features while broadcasters attempt to ascertain what's valuable to consumers.

For all this uncertainty, the question is when the future of television will be wholly digital. Broadcasters are scrambling to install the infrastructure required for DTV signals (Reference 1), because this new technology will change the way broadcasters store, process, and transmit the signals. For decades, the industry has relied on either NTSC or PAL formats and a single frame rate. Now, the industry has to move to the versatile formats and frame rates of the ATSC and DVB standards. In addition, broadcasters must now compress video and audio signals and use packets to transport the compressed signals and data. Consumers receive the resulting bit stream from terrestrial, satellite, or cable broadcasts. The transition to digital production and transmission results in a better product delivered to consumers, who will then update equipment to take advantage of new services and entertainment values.

Clear entertainment value

DTV's services and entertainment capabilities are impressive. The biggest change is the 18 ATSC formats for picture quality (Table 1). One of the 18 video formats is a suitable match for all programming requirements--from home shopping to action sports and movies. Formats range from progressive-scan standard resolution at 24 frames/sec, to interlaced scan at 60 frames/sec and 16-to-9--letter-box--aspect ratio. The ATSC recommends receivers that decode and display all 18 formats in the format available. Add to that CD-quality audio and surround sound, and you have TV like you've never seen (or heard) before. The advantages of DTV don't end there, because two-way data and multiple standard-definition TV (SDTV) channels per signal open more possibilities for features such as Web-surfing TV.

Even wary consumers unwilling to make the wholesale leap into DTV can benefit from DTV broadcasts: They can buy a low-cost converter box to receive and display DTV on older analog sets. DTV improves on NTSC or PAL broadcasts because only clear images appear within the broadcast coverage area, ensuring the elimination of snowy images. With this system, the signal is either excellent or absent, because DTV eliminates color-tint errors and chrominance artifacts, which show up as false or artificial colors.

Upgrading homes for DTV is the biggest change to the broadcast industry since the introduction of color television, but don't expect a succinct description of new TV equipment. TV peripherals could deliver all or a subset of the ATSC formats and data-enhanced features over the next few years. The first TVs with IRD will be large projection screens. For example, sports bars will install these big screens to draw crowds to the HDTV pictures and sound. These sets, costing approximately $3500 to $9000, will prime the market for home sets. The first home IRD TVs should be available for this year's holiday season and likely will cost more than $2500. At this price, the home sets probably will serve SDTV and display both conventional 4-to-3 and the new 16-to-9 "wide-screen" aspect ratios. The first sets will include no interactivity or data reception and will offer only limited conditional-access capability.

The converter STB will be the fast facilitator to the DTV era. Although many consumers expect a few more years of service from their analog TVs, they also want to take advantage of the crystal-clear sound and pictures that DTV offers. Filling this requirement is the STB with digital reception and decoding capability that downconverts the signal to display on an analog TV. You can even add features such as conditional access for subscription services, surround sound, and picture-in-picture to the old TV set. Vendors will try to price low-end converter boxes at approximately $150. A combination converter and STB adds features such as DVD playback, multiple reception networks such as satellite and cable, and Internet surfing, along with two-way data. With such features, this combination converter and STB will garner a price greater than $500.

Another possibility for DTV-capable equipment is the specialized component model that has worked for the audio industry. Instead of consumers buying a single unit TV, they have to buy specialized components. For example, your home entertainment center will have components for the TV receiver and decoder, display unit, speakers, a DVD player, and recorder--all connected with an IEEE 1394 cable. This trend is already beginning with the introduction of stand-alone plasma and flat-panel displays that have 16-to-9 aspect ratios and measure 100 to 200 mm thick but do not include a receiver. Components such as multinetwork--satellite, terrestrial, or cable--digital receivers and combined ATSC DVD decoders allow consumers to mix, match, and upgrade components as new services and features become popular. Other potential component products include PC-based receivers and decoders as well as set-top computers.

The dizzying array of new features from DTV and two-way data packets will lead to experimentation and fast product cycles in the early years. Consumers will eventually decide which features will become mainstream. In the interim, designers do not want to limit their feature options. Hence, by providing programmability, chip-set vendors are building flexibility into their offerings. By offering flexibility, the chip sets are trading off cost. But, trading off cost for flexibility is a temporary solution in consumer electronics: When services and features gain mainstream acceptance, chip-set and software optimizations lead to lower cost implementations.

Common functional blocks

Even with the various features and formats possible with DTV, every STB has a common set of functions constituting the receiver-to-display bit-stream path. Five blocks can describe the basic components (Figure 1). STB chip sets partition and integrate these functions differently, but every multimedia bit stream needs to flow through these functions. The multimedia stream path begins in the tuner or network-interface block. The stream moves to the formatting and CPU block, then to the MPEG-2 video and audio decode, onto the graphics feature block, and finally to the output block. The reason for having so much commonality is that ATSC, DVB, and DVD all use MPEG-2 as the video-compression and transport technology.

Chip-set vendors supply these functions in their STB reference designs and software-programming platforms. Most STB chip-set vendors understand the importance of reference designs to develop differentiating features and to shorten product-development cycles. The reference designs include hardware and software to customize a vendor's chip set to accommodate your required feature set. For example, VLSI offers the Horizon application-development board, a complete STB application lab, and software JumpStart ARM tools for fast STB-feature development.

By following the multimedia stream through the STB, you can investigate the functions and examine chip-set vendors' approaches to providing the functionality. The "network interface" refers to the receive tuner in which the modulated analog-signal stream enters the box. The signal can originate from a satellite, a cable, over-the-air broadcasts, or a DVD optoelectronics module. A/D conversion and forward error correction also occur in the block. The output is the digital MPEG-2 packets, the "transport stream." Satellite-transmission systems typically use the quadrature-phase-shift-keying (QPSK) modulation (Reference 2), whereas cable-transmission systems rely on quadrature amplitude modulation (QAM) (Reference 3). Over-the-air broadcast uses VSB in ATSC systems or CODFM for DVB-compliant systems.

Most chip sets offer at least one network interface, and some offer a range of network interfaces (Table 2). For example, Lucent-Mitsubishi and Motorola-Sarnoff chip sets both offer the VSB demodulator. Sony's chip set offers the QPSK demodulator for satellite reception. For a complete network interface offering, LSI Logic's Integra chip set has three devices for the channel demodulator (Figure 2).

The network interface may also be interactive by including a modem for an upstream connection. In a cable system, the upstream modem uses the QPSK transceiver. Stanford Telecom offers an integrated modulator and demodulator for interactive cable reception. But terrestrial-broadcast and satellite systems use an analog modem such as V.34 or a V.PCM (56-kbps) modem for the upstream link.

Format the multimedia stream

Now, you should focus on tracing the multimedia stream through the functional blocks. After the signal enters the box through the network interface, it moves to the formatting and CPU block. The block accepts the MPEG-2 transport stream from the network interface in 188-byte packets (defined by the MPEG-2 standard). These transport packets allow multiplexing many programs into a single bit stream, and each program's packet contains audio, video, data, and control information. Along with synchronization and other controls, the information carries conditional access data that interacts with a user device, such as a smart card, to authorize program access. The formatting function includes descrambling and demultiplexing audio, video, and data streams and forwards them to the suitable decoding circuitry. The CPU assists in transport functions and controls the I/O circuits for the STB, such as keypad, IR, and smart-card interfaces. The CPU also works with a graphics controller for functions such as an on-screen display (OSD).

The conditional-access system (CAS) is an important part of the formatting and CPU block. CAS is important to service providers who use it to control theft and to bill for programming. ATSC has yet to describe a security system, whereas DVB is further along in the standards process. CAS is probably the biggest unanswered design question for new digital STBs. Thus, you should expect the earliest digital STBs to omit CAS because the standards still need to evolve. And, you should plan for CAS upgrades because of the system's importance to service providers. (Check the ATSC and DVB Web sites for up-to-date status reports on CAS.)

The formatting and CPU block offers product differentiation through programming, which is critical for conditional-access algorithms, transport functions, upstream modem communications, and user interfaces. The trend is toward more powerful processors, allowing more STB functions to be programmable. A PC-based receiver carries this trend to the extreme.

But STB programming need not be as complicated as programming a PC, for example. STB programming simply ensures flexibility to modify and upgrade designs as DTV "uncertainties" evolve. Programming is also useful for adding multifunctional capabilities to a design. For example, even though OpenCable, ATSC, and DVB all use the MPEG-2 transport functions, they define different system-information table formats for the streams. Having different table formats means that each standard handles the MPEG-2 control information differently. Consequently, chips with hardware-transport support that target DVB may not work with ATSC streams. But if all the transport functions are programmable, a single chip set can handle cable, ATSC, and the DVB protocols. The C-Cube Avia chip set offers multistandard levels of programmable support for MPEG-2 transport functions.

Programming is common for the MPEG transport functions, and almost all vendors employ embedded processors in their chip sets. The ARM Thumb CPU is a popular choice in many STB applications. Both the AV7000 from TI and the Vista from VLSI use an embedded ARM Thumb processor. Another popular choice is the PowerPC embedded in both Motorola and IBM chip sets. However, stand-alone processors also work in STBs. For example, the Integrated Device Technology R4640, which powers appliances such as TV Internet browsers, is a typical processor that works well in STB.

Because of the increasing importance of programmability, the embedded OS becomes a critical component in designing a STB system. The OS needs to have excellent RTOS capability, and it must be compact and efficient. One of the most popular OSs is pSOS from Integrated Systems. pSOS is a compact RTOS that performs all of the functions an STB requires. In addition, the pSOS development environment favors embedded systems, such as an STB. Development kits from TI and Philips provide pSOS. Coincidentally, the ARM7 is a good processor for running pSOS, and ample program development--such as TCP/IP stacks and Internet browsers--already exists.

STBs can also use other OSs. The OS-9 RTOS with a digital audio/video interactive decoder (DAVID) from Micro-ware and NanoOS from Sony are also popular with STB developers. DAVID provides an open standard for developers to author applications and send them across broadband networks. NanoOS is Sony's embedded OS, which the company first used in the CXD 1930 video decoder to control real-time task switching of the proprietary RISC processor. NanoOS provides a consistent application-programming interface for developer customization.

Developments in cable-STB OSs recently garnered news coverage because of the Tele-Communications, Inc (www.tci.com) manipulated duel between Microsoft (www.microsoft.com) Windows CE and Sun (www.sun.com) PersonalJava. Even though the stated goal of the OpenCable initiative is open-architecture (read OS-independent) STBs, Windows CE, and PersonalJava are trying to stake a claim in cable STBs. Most developers are waiting until the OpenCable standard settles to see how these OSs will affect the next generation of STB design.

Now, examine the multimedia stream flow. The next block for the received bit stream is the decoding processor. The processor decompresses and reconstructs the video and audio streams that the demultiplexer forwards. Most chip sets simply decode the MPEG-2 video and audio for the near-typical analog resolutions that DVB and DBS use. On the other hand, the ATSC HDTV standard uses high-level MPEG video coding and Digital Dolby AC-3 audio coding, which both require more processing power and memory than DVB and DBS.

The decoding-processor block is the "main event at the main value" to borrow from MPEG terminology, because it delivers DTV value to consumers. The MPEG-2 standard specifies encoding levels and profiles to help identify encoder and decoder requirements (Table 3). The digital STB display-output quality depends on the MPEG-2 stream decoder. Each of the formats available in DTV combines resolution and frame rates that require different bit rates. As the bit rates increase, so does the processing power required to decode the MPEG stream. The typical analog television signal closely resembles the main-level-at-main-profile (ML@MP) coding complexity. HL@MP (high-level@MP), used in HDTV applications, typically requires two to eight times more processing power than ML@MP broadcast applications, such as DBS.

The MPEG-2 decoders in Table 2 perform at least the ML@MP. Because the current DVB and DBS standards require only ML@MP performance, most chip sets supply only this level of perform-ance. However, encoder enhancements provide critical differences among vendor offerings. For example, the C-Cube Avia family offers an enhanced decoder that features Dolby AC-3 decoding along with the MPEG-2 audio decoder. Another offering with enhanced decoder features is the LSI Logic L64105 MPEG-2 decoder with 8-bit graphics overlay (Figure 2).

Some chip sets integrate the MPEG-2 decoder and CPU processor. For example, the Sti5500 OMEGA (one-chip multimedia engine architecture) from SGS-Thomson is a highly integrated STB chip; OMEGA integrates the PAL/NTSC encoder, MPEG-2 transport engine, and MPEG video and audio decoder into one chip. An added benefit of the integration is consolidation of memory requirements into a single 16-Mbit synchronous DRAM (SDRAM) chip for decoding an MP@ML bit stream.

Chip sets exceeding MPEG-2 performance are increasingly available; such offerings can decode the ATSC HDTV broadcasts that are set to begin this year. However, these MPEG de-coders have heavy memory and processing requirements. For example, the Lucent-Mitsubishi chip set has an MP@HL video decoder. With two decoders and three 16-Mbit SDRAMs working in unison, the chip set can decode all 18 formats. The Mitsubishi display processor, along with a 16-Mbit SDRAM, delivers HDTV to a display unit as either 1080-interlaced or 720-progressive formats.

Another scheme for decoding all 18 ATSC formats uses the Philips TriMedia processor. Philips offers a reference design that includes development tools and an OS for receiving, decoding, and displaying HDTV programs. The initial reference design uses the Trimedia processor, a high-level MPEG-2 de-coder, and a color-key multiplexer to decode HDTV signals. The TriMedia reference design is flexible enough to evolve, which is an important feature given the uncertainties of HDTV terrestrial broadcasts. Philips plans further integration with new generations of TriMedia.

Other vendors are developing MP@HL decoders for use in their consumer-electronics applications. Panasonic and LG Semicon have developed first-generation ATSC-compliant de-coders and display processors, demonstrating them at the Consumer Electronics Show in January. A single chip from Panasonic uses three 500-MHz, 16-Mbit Rambus DRAMs for MP@HL decoding and displaying all 18 ATSC formats. With the Rambus controller integrated on the decoder and display chip, Panasonic reduced package size to a 240-pin device. But it remains unclear whether these decoders will be available as standard catalog products.

Graphics entice consumers

Many consumers have become accustomed to the graphics interface of PCs and, therefore, expect more than the dual color and basic block letters of VCR displays. Therefore, the graphics- processor function is important for customizing your product for consumer awareness and approval. Users see the on-screen display and electronic program guide as the STB's differentiating feature. Because display graphics are often a deciding factor for many consumers purchasing products, vendors have included sophisticated graphics processors with their chip sets. For example, the Motorola-Sarnoff chip set includes the Scorpion graphics processor, which combines video and graphics processing for enhanced graphics in STB applications. In contrast, TeraLogic's graphics processor, designed for DTV STBs, adds accelerated graphics to any chip set, and the processor handles video processing.

Although the previous functional blocks are important, the display function delivers the output format the consumer views. Depending on the desired display options, the output to the display can be analog or digital. If the product is a digital-reception-to- analog-display-converter box, the output must be in NTSC or PAL format. If it is a native digital TV set, both SDTV and HDTV formats are options for display. Most chip sets include an NTSC and a PAL encoder because they are the dominant display outputs for equipment today. NTSC and PAL encoders are also available as standard products from vendors such as Analog Devices, Chrontel, Cirrus Logic, and Harris.

DTV produces clear pictures, but predictions for whether standard STBs, TV peripherals, and peripheral features will become popular remain fuzzy. But this uncertainty opens opportunities for new equipment providers to enter the TV-peripheral market. Experimentation will determine which features consumers want in their entertainment centers. Therefore, STB chip vendors are seeking developers with ideas for using DTV's capabilities. Chip vendors view application developers as partners with whom to work on getting the right mix of features and performance in next-generation chips. For this reason, vendors use the reference designs to help developers become involved in software development for chip sets. The combination of chip vendors and software developers provide the feature-experimentation platform that will drive DTV for consumer products.

References

1. Bhatt, Bhavesh, David Birks, and David Hermreck, "Digital television: making it work," IEEE Spectrum, October 1997, pg 19.

2. Schweber, Bill, "Direct Satellite Broadcast brings downlink directly to your set-top box," EDN, Dec 21, 1995, pg 53.

3. Wright, Maury, "Delivering digital video," EDN, March 14, 1996, pg 39.

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