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Advancing the set-top box

Posted: 22 Aug 2005 ?? ?Print Version ?Bookmark and Share

Keywords:set-top box? digital video broadcast? dvb? dvb-c?

By Carey Ritchey and Dewight Warren
Microtune Inc.

Residential communications of the future have the potential to dramatically change the way the people of the world interact. The roadmap for cable system services contains multiple functionalities, including digital and analog television, cable modems, personal video recording (PVR), cable telephony and wireless connectivity, that will help drive these changes.

Designers of advanced set-top boxes or residential gateways that support these services must consider multiple issues. The first are the international protocols, including the cable digital video broadcast (DVB) variant, DVB-C, and the North American OpenCable standards. The second are the technological challenges involved in implementing multiple functions in the same hardware, including optimized routing and splitting, supporting multiple tuners, power dissipation and footprint.

Down the road, these advanced set-top boxes must overcome some additional integration issues to realize true economies of scale, and vendors will need to find marketable scenarios for transporting these new services around the residence.

Serving multiple masters: protocols
Developing a worldwide set-top box requires adherence to multiple protocols and standards, including input power levels, intermediate frequency (IF) ranges, phase-noise requirements, and different requirements for all-analog, all-digital, or digital/analog systems. Fortunately, digital video broadcasting-cable (DVB-C), the dominant protocol in Europe, and the OpenCable protocol from CableLabs in North America use the same modulation scheme, quadrature amplitude modulation (QAM), making it possible to consider combined functionality in one advanced set-top box.

Some of the major design considerations and challenges involved in increased functionality in a set-top box include power dissipation, footprint, and routing/splitting of the signal. Additionally, each new service feature, such as picture-in-picture, interactive television, high-speed Internet access, and PVR, requires its own dedicated tuner. Consequently, advanced set-top box designers need to consider the impact of putting multiple tuners in a single box. The capability to stream data and video throughout the house wirelessly requires additional silicon technologysuch as 802.11x, Bluetooth, or other solutions.

Power, size crunch
As with nearly any electronic design, as more components are squeezed into a smaller space, power dissipation becomes an issue. The physical nature of electronics creates thermal management issues with a tighter, more functional design. This is complicated in a small consumer device because market forces prohibit the use of costly, large, and noisy heat management devices such as fans.

When using semiconductors, power dissipation becomes an important issue because if the devices heat up too much, they lose performance, reliability, and lifespan. The actual threshold depends upon the process, but a typical figure of merit for a maximum junction temperature is 100 degrees C. Staying below this figure requires careful design and maximum exploitation of radiation and/or conduction to cool the integrated circuits within the set-top box.

In conjunction with crunching down power consumption figures, system designers also need to find ways to squeeze down the size of their system designs. Achieving a small footprint is desirable because the smaller the device, the less expensive it is likely to be in terms of material, PCB requirements, storage, and manufacturing.

Handling multi-tuner designs
In addition to reduced power and size, integrating multiple tuners is one of the clear trends in the development of next-generation set-top boxes and residential gateways (Figure 1). In a traditional single-tuner set-top box, routing the signal is fairly straightforward. However, when more than one tuner is deployed in an advanced set-top box, it is important to carefully consider how to split the signal between the tuners. The major parameter of concern here is noise figure, which is a measure of how much noise is added to the signal during its path through the set-top box.


Figure 1: An advanced STB block diagram showing functionality for analog TV, digital TV, cable modem and PVR

For instance, if a signal is passively split between two tuners, the noise figure (NF) will be immediately degraded by 3.5dB to 4 dB, which is no trivial amount. The way to solve this problem is to use a low noise, high linearity amplifier at the front end before splitting the signal.

For example, if a single tuner has a noise figure of 10dB and the signal is split using a passive splitter with loss of 4dB, the cascaded noise figure would be 14dB. If a front-end low noise amplifier (LNA) were used instead with a noise figure of 3.5dB, the cascaded noise figure would be 10.8dB, which is a 3.2dB improvement.

The reason that the LNA must feature low noise and high linearity is that it cannot compromise what is downstream in the signal chain. Linearity is characterized by the following performance parameters: composite second order (CSO), composite triple beat (CTB) and cross modulation (XMOD). These parameters typically must be -53/-53/-48dBc, respectively, for digital applications and -60/-63/-57 dBc, respectively, for analog/digital cable (Table 1). Analog/Digital cable systems, such as the ones in North America, require higher performance than digital-only systems, such as DVB-C systems.

Many North American cable systems have a large number of analog channels in the cable plants. In fact, some plants have more than 100 analog channels. All of those channels add up to a very large signal. Therefore, it is important for set-top boxes used in these applications to have a very linear tuner and upfront amplifier in order to ensure good dynamic range and a distortion free picture.

Maintaining flexibility
One of the major concerns when designing advanced set-top boxes with numerous functionalities for the worldwide market is the flexibility of the components to be used in systems with various input and output requirements.

It is important to select device manufacturers and vendors with the expertise to manage the technical issues of these advanced applications. For instance, traditionally, set-top boxes have used tuner modules, but these have proven to have too large a footprint and too little flexibility to serve well in advanced set-top box applications with multiple tuners.

Instead, set-top box designers are selecting small-footprint, low-power silicon tuners. These devices offer designers cost savings advantages as well as flexibilityboth in determining the on-board position and signal path as well as serving multiple applications.

When selecting a silicon tuner, it is important to partner with a vendor who has expertise with thermal issues (power management). Designers can have a direct impact on system reliability by selecting components with low-power dissipation. This will minimize complications from thermal problems, which have a direct effect on reliability. The tuner vendor should also have experience with less understood issues such as microphonics and surge protection.

Microphonics is a measurement of how well a tuner isolates against the effects of acoustic energy. For instance, if someone taps on the top of a cable modem or set-top box, this could cause enough acoustic energy to frequency modulate the tuner's voltage-controlled oscillators (VCO), which will affect the performance of the set-top box demodulator. As a result, the set-top box could lose the picture or be subjected to visual/audio distortion.

Distortion or a lost signal could also occur if an audio output speaker near the set-top box is set to a very high volume. Or, if the cable modem next to a PC is bumped, it could lose signal lock. Experienced tuner companies know how to ensure against these effects, and their silicon devices are designed to avoid microphonics issues.

Surge protection is another important element that is often overlooked in RF front-end systems for cable modems and set-top boxes. When lightning strikes a nearby location or electrostatic discharge (ESD) causes an overstress or over surge, surge protection circuitry can protect the system. As a result, designers should seriously consider including surge protection circuitry for the RF front-end system in advanced set-top box designs.

Pulling it all together
While the technology exists to bundle a great deal of functionality into a single set-top box, the best implementation approaches are still being determined. Many system vendors are aggressively working on alternative solutions. As a result, there are some promising proposed architectures for deploying set-top boxes with increased functionality.

One reasonable scenario for implementing cable telephony in the residence is to incorporate a Bluetooth radio inside a cordless phone. Then, the controller side of the cordless phone is implemented in the set-top box (Figure 2), and a cable modem is used for telephony over coax cable (PacketCable). In such a scenario, up to seven handsets could access a single set-top box.

In terms of Internet access, a cable modem in the advanced set-top box has the flexibility to be used for interactive or enhanced television (Internet content specific to the program being viewed) or for actual web browsing. One might expect that the PC used for web browsing is in some other location in the residence, so some form of networking would be required, either wired or wireless, to access a cable modem in the set-top box.

Industry experts currently expect that the cable modem functionality in the set-top box will initially be used for enhanced viewing. Regardless of how the modem is used, the set-top box will require good engineering and high-speed microprocessors supporting cable modem functionality in order to ensure that they do not interrupt and corrupt the quality of the digital signal for other applications.

Implementing full-speed World Wide Web access to PCs throughout the home would require a network with a high data rate, such as an 802.11a/b wireless network. Some set-top box manufacturers are experimenting with this approach, and the new class of 802.11 silicon technology is beginning to address the cost issues. In this scenario, the set-top box must also act like a router, handling multiple computers that are accessing the set-top box.

If a wireless local area network (WLAN) is deployed in a residence, this could improve PVR functionality as well. For instance, different television sets within the residence could access the PVR functions in the main set-top box. The lower-cost set-top box at the secondary television could access video using the 802.11system.

Future developments
To realize even greater economies of scale in set-top box that combine functionality such as DVB-C and OpenCable, some changes in regulations and process would likely be required.

For instance, even though international protocols are well known and defined in terms of performance requirements and modulation, it is challenging to implement multiple digital and analog functions into the same physical hardware without degrading the performance of any of the systems. From an RF standpoint, this means that in the preliminary portion of the box, the routing and splitting of the incoming signal cannot degrade in noise figure or linearity or it will affect the distinct functions of the set-top box.

In an all-digital system, the same model of tuners and demodulators can be used, just duplicated for additional functionality. However, if the set-top box needs to receive an analog signal as well (as it would in North America), it would also require a higher performance tuner section for the video portion because analog video requires better tuner performance than digital (Table 1).

About the authors
Carey Ritchey
is the director of system engineering at Microtune. He holds a Diploma of Technology from The British Columbia Institute of Technology and a MS in Advanced Technology from Simon Frazer University. Carey can be reached at carey.Ritchie@microtune.com.

Dewight Warren is a product manager at Microtune focusing on silicon tuners for STB applications. He has a BSEE from the University of Florida and an MBA from the University of Texas at Dallas. Dewight can can be reached at dewight.warren@microtune.com.




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