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By Dewight Warren, Microtune, Inc.

The transition to digital requires a new design for set-tops and upping performance specs for the RF front-end of the box.

The traditional cable television set-top box is undergoing a significant transformation from a passive entertainment receiver to an interactive home gateway that enables upstream and downstream multimedia. Today's advanced set-tops may support applications such as Web surfing, telephony, picture-in-picture video, personal video recorders (PVRs), video-on-demand (VOD), e-commerce and others. This variety of applications means set-top designers need components with exacting output power, linearity, harmonic distortion and noise performance.

The radio frequency (RF) front-end portion of the set-top is responsible for most of these functions, and manufacturers of the tuners and amplifiers that make up this front-end are being asked to raise the bar on performance. In the United States, their efforts are being guided by specifications from the Federal Communications Commission (FCC) and CableLabs, and in Europe, they are being driven by the Digital Audio Video Council (DAVIC), the International Telecommunications Union (ITU) and the European version of Data Over Cable Service Interface Specification (EuroDOCSIS).

Cable operators traditionally have provided set-tops in a unidirectional, single-channel format, receiving and demodulating signals that came from the cable headend. When cable TV channel capacity increased beyond 12 channels, most televisions were not "cable ready," so the set-top would demodulate the cable signal, then remodulate it onto channel 3 or 4, similar to how a VCR works (see Figure 1). Even when a TV was cable-ready, multiple system operators (MSOs) were concerned about ensuring picture quality and generating revenue for premium channel access, so they continued the practice of deploying set-tops to their customers.

The transition from analog to digital cable plants requires that the traditional set-top evolve into a more advanced architecture dramatically different from its predecessor (see Figure 2).

Changing to a digital cable system has advantages for both customers and service providers. Consumers tend to appreciate the crisper picture and advanced features, such as electronic program guides (EPGs). MSOs like being able to provide revenue-generating interactive functions, such as e-mail or home shopping. In addition, the MSO may make better use of bandwidth, transmitting three to four times as many channels in the same bandwidth by using Moving Picture Experts Group (MPEG) compression techniques.

But all of these attractive new features require more components and materials. Perhaps the one thing that has slowed the market in the United States is the increased cost of digital set-tops. This disparity is not the case in markets where analog systems are not incumbent. For instance, in many European and Asian countries without analog set-top systems already installed, the new systems are digital.

Performance

Because it offers multiple features, such as video reception, picture-in-picture, video recording and a cable modem, a digital set-top requires a more sophisticated RF front-end (see Figure 2). Each new feature requires the signal to be split and passed through a digital cable tuner and an intermediate frequency (IF) amplifier.

When an RF signal is split among multiple paths, the signal is degraded. Because advanced set-tops require signal-splitting, they need to be designed carefully to minimize the overall noise figure. (Noise figure is the measure of noise added to a signal as it passes through a component.) Too much noise in a signal may cause white noise on analog systems or cause the signal to freeze or drop out in digital systems.

One technique to minimize the noise figure is to add a low-noise broadband amplifier (with a noise figure of about 3 decibels, or dB) early in the receive chain. In an RF system, the first component has the greatest effect on noise figure, so it is important to select it carefully. A low-noise amplifier boosts the signal before it is split, reducing performance degradation further down the receive chain.

In addition to multiple receive functions, most advanced set-tops also have the capability to send information back to the cable plant, typically with an embedded cable modem (see Figure 2). This return capability may be used to enable pay-per-view, e-mail or e-commerce features, or the set-top could have an Ethernet or universal serial bus (USB) jack that connects to a personal computer (PC). These designs require an upstream amplifier to boost the signal after it comes from the cable modem's modulator.

The most important performance parameter for this upstream amplifier is its linearity, because it is necessary to minimize any distortion to the upstream signal. Minimizing distortion is important because the upstream bandwidth is 5 MHz to 42 MHz (in North America), and this frequency range is more susceptible to electromagnetic interference (EMI) and system noise than the downstream bandwidth.

In addition, this upstream amplifier must be capable of meeting a minimum level of output power, measured after it passes through all of the devices in the signal path and exits the set-top. For example, DOCSIS requires output power of up to +58 decibel millivolts (dBmV) after the signal has left the F connector on the outside of the set-top. Because it needs to overcome some power loss from passive components (such as the diplexer and splitter in Figure 2), the upstream amplifier needs to offer output power of up to +61 dBmV or better.

Refining standards

In the United States in 1996, the FCC ruled that set-tops be made available in the retail market by June 2000, spawning the CableLabs OpenCable initiative. Even though market forces precluded meeting this deadline, digital set-tops have been coming online across the country. In general, MSOs are interested in set-tops that conform to interoperability standards, similar to the DOCSIS standards for cable modems. Standards-based set-tops have the ability to spur innovation and reduce system costs (because of competition).

Any set-top standard must take into consideration all the possible cable environments to which the set-top could be exposed. As a result, the U.S. OpenCable initiative requires a high level of performance from the set-top. Currently, a few manufacturers control the majority of the U.S. set-top market, but major consumer original equipment manufacturers (OEMs), such as Pioneer, Sony, and LG, also are interested in this market, and a standards-based set-top--such as what OpenCable proposes--opens the market to all interested manufacturers.

As standards are refined and new vendors enter the market, the pressure on set-top designers to control costs and improve functionality will continue, and for that they will need to focus on high-quality, cost-effective designs. Major alignments are beginning to occur between semiconductor manufacturers and systems integrators to provide cost-effective, high-performance designs. These designs will require a high-performance RF front-end to support the vast array of features that advanced set-tops will deliver.

Dewight Warren is technical marketing manager for Microtune. Reach him at .

 Back to June 2001 Issue

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