1998-11-10, BG9808,
Technical Backgrounder from Philips Semiconductors

Despite the eventual shift to fully integrated digital TV (DTV) sets, most people's first experience of digital media broadcasting (DMB) will be via a  set-top box (STB) that allows them to view digitally broadcast TV channels on an existing TV set. For DMB service providers, the supply of these boxes at heavily subsidised prices will be the only way to attract subscribers in sufficiently large numbers to ensure long-term profitability. However, because digital set-top boxes are considerably more difficult to design and more expensive to produce than their existing analog counterparts, unit cost becomes an extremely critical issue. Set-top box manufacturers also face another problem. Although the broadcast standards for satellite, cable and terrestrial DMB systems are already well defined, the interactive user services that these systems will deliver are likely to remain in a state of flux for several years to come.

Compared to the era of analog TV in which three different TV standards (PAL, SECAM and NTSC) were used in different parts of the world, the DTV era will have one distinct advantage. MPEG2 has been universally adopted as the preferred method of digitally encoding the video and audio channels. Hence the differences between set-top-boxes destined for different parts of the world, or between set-top boxes for satellite, cable or terrestrial broadcast reception, lie almost entirely in their front-ends - in the carrier frequencies used by the broadcast transmitters and in the modulation schemes used to impress the digital information onto these carriers.

Many people think that because DTV is digital, broadcast transmitters send a simple stream of 0s and 1s to the receiver. This is not true. Because there is insufficient bandwidth in a TV channel to achieve the required data rate using a simple bit stream, a specified number of 0s and 1s are grouped together and sent as a unique 'symbol' that corresponds to their bit pattern. These symbols are modulated onto the carrier by the transmitter and must be demodulated in the receiver. Depending on the broadcast type, it is these modulation methods that differ. Hence for satellite broadcasts QPSK (Quadrature Phase Shift Keying) or BPSK (Binary Phase Shift Keying) are typically used, whereas for cable TV channels QAM (Quadrature Amplitude Modulation) is typically used. However, once the symbols are demodulated from the carrier and converted back into their corresponding bit patterns, the result is always an MPEG2 Transport Stream containing several video, audio and data channels.

It is because of the world-wide acceptance of MPEG2 as the standard for digital video encoding that there has been the near instant appearance of universal silicon for video signal processing in digital set-top boxes. This is in stark contrast to the world of analog TV where it has taken some time, and a good deal of digital intervention, to arrive at universal PAL/SECAM/NTSC solutions such as Philips Semiconductors' highly successful Global TV solutions.

With MPEG2 functionality so well defined and universally used, silicon solutions for set-top boxes will be differentiated almost exclusively on price and programmability. Lower pricing will be necessary not only to minimise the equipment subsidy that service providers have to offer in order to attract new subscribers, but also to bring a new generation of universal set-top boxes, capable of receiving digital cable, satellite and terrestrial broadcasts, down to a price level that will be attractive to consumers. Ever increasing functional integration into fewer and fewer ICs is the key to low silicon cost, which is why Philips Semiconductors has reduced its 5-chip MPEG source decoder solution down to the two 2-chip solution being demonstrated at  Electronica '98, and why it will ultimately integrate these two new ICs into a single-chip solution.

Given that the silicon cost must be low, programmability then becomes the critical issue for set-top box designers because it is the key to hitting very narrow windows of opportunity in the set-top-box market. With rapid changes in the user services offered by DMB broadcasters, it is predicted that life-times for set-top boxes may be as short as 6-months, and the lead times required to develop them even shorter. Constrained by such short development times, set-top box manufacturers must therefore choose fully tested platform solutions which free design engineers to concentrate on service related application issues rather than designing hardware from the ground up or writing low level programs to control individual ICs.

Philips Semiconductors' particular expertise in RF technology, digital demodulation, MPEG2 decompression and TV signal processing, together with its wide ranging experience in other areas of consumer electronics such as smartcard technology, multimedia systems and digital audio processing, put it in a unique position to supply platform solutions for digital set-top boxes. The broad range of silicon process technologies at its disposal - bipolar and BiCMOS for RF functions, high density CMOS for digital processing and mixed signal (analog/digital) processes for audio and video output - allow it to produce ICs for every part of the system. Its comprehensive system-level know-how in DMB receiver design means that these ICs fit together without the need for additional glue logic and that they achieve optimum overall system performance. To eliminate virtually all the hardware design problems, Philips Semiconductor has produced a complete set-top box reference design that includes fully populated printed circuit boards for the front-end tuner/demodulator and MPEG2 source decoder functions. Customers can either incorporate these directly into their set-top boxes or use them as the basis for their own pc board designs.

In addition to solving hardware problems, Philips Semiconductors' digital set-top box platform also relieves designers of a vast amount of software effort. Because the reference design is in itself a complete DMB receiver, with standard interfaces to peripherals such as a card reader or cable/telephone modem, Philips Semiconductors is able to supply it with a comprehensive suite of fully tested software modules. In addition to the low-level drivers required to control individual ICs, these software modules provide system-level control functions such as satellite/cable/terrestrial channel tuning, channel switching, conditional access, PSI (Program Specific Information) table management, audio and video decoding and graphics/teletext handling. The software also includes an API (Application Program Interface) on which programmers can develop advanced user services. Application-level functions supplied with the reference design include STB remote control, user preference menus, simple e-mail delivery, electronic programming guides, impulse pay-per-view services and programme tagging. Service provider applications include software for STB testing and firmware downloading. In anticipation of some of the more advanced user services that will be provided by DMB broadcasters, Philips Semiconductors reference design software also supports interpreters for Open-TV, MHEG-5 hypermedia and MHEG-6 Java-based scripts.

To protect its customers' IP (Intellectual Property) investment in set-top boxes based on the reference design, Philips Semiconductors will provide a smooth migration path from its current levels of IC integration to its third generation STB solutions. For example, software partitioning on a function-by-function basis means that existing software will continue to run even when functions currently accommodated in different ICs are integrated into a single chip solution. Upwards compatibility will also be maintained when the 32-bit MIPS PR3001 RISC microprocessor core integrated into the  SAA7214 Transport MPEG Source Decoder is upgraded to a 64-bit MIPS core that can handle advanced interactivity with provider services.

The DMB revolution that is about to take place will change the way we use our televisions more radically than any previous development in TV broadcasting. Only an open, easily programmed, set-top box platform such as the one being demonstrated today by Philips Semiconductors at Electronica '98, will enable designers to keep pace with rapidly changing market requirements and ever decreasing time-to-market deadlines.

Philips Semiconductors, a division of Royal Philips Electronics, headquartered in Eindhoven, The Netherlands, is the ninth largest semiconductor supplier in the world and the third largest supplier of discretes in the world. Philips Semiconductors' innovations in digital audio, video, and mobile technology position the company as a leader in the consumer, multimedia and wireless communications markets. Sales offices are located in all major markets around the world and are supported by systems labs.