DTV Set-top Decoder Boxes Promise Better Reception

By Greg Tarr On Jul 23 2001 - 6:00am




Even before the great COFDM vs. 8-VSB modulation system debate momentarily clouded the television transition, integrated circuit developers and consumer electronics companies were hard at work on more robust tuning and demodulation technologies for DTV receivers.

The next fruits of those efforts are now starting to appear in new DTV set-top decoder boxes, fully integrated HDTV sets and DTV PC-tuner cards from a handful of manufacturers.

Most of these companies assure that their next-generation devices will allow the use of indoor (rather than rooftop) antennas in more areas as well as in so-called problem areas that lie both close to and miles from transmission sources.

The two major problems in receiving DTV over-the-air indoors is weak signal strength (centered on the transmission end) and a condition called signal multipath (sometimes correctable in receiver designs).

Signal multipath arises typically in urban areas with many large buildings or moving objects, such as cars and planes. Here, incoming signals bounce off these objects and cause the signal to send out reflections or echoes of itself in multiple directions. These reflections (also called ghosts) arrive at the receiver in a series of waves either just before or after the real signal arrives. Similar conditions arise inside the home when an indoor antenna is used, as signals reflect off walls, floors and ceilings.

As these multiple ghosts and signals are received, some early digital tuner/ decoders in certain conditions were known to become overwhelmed, causing pictures to lock up, break up into multiple digitized cubes or disappear intermittently throughout a broadcast.

Using new generation tuning and integrated demodulation designs, which also allow for lower price points, manufacturers are hopeful the penetration of DTV tuner/decoders will begin to catch up to sales of digital-tunerless DTV monitors and take the digital transition to the next level.

Zenith, which is credited with inventing the vestigial side band modulation scheme (or 8-VSB) used in the Advanced Television Systems Committee DTV standard, is preparing to deliver its third-generation DTV set-top tuner (DTV1080, $799.95) and a fully-integrated HDTV set, both of which incorporate a "third-generation" demodulation chipset.

Richard Lewis, Zenith senior VP, said the new receivers are designed to better handle both longer ghosts and fast-changing ghosts.

Longer ghosts, those reflected from more distant objects such as mountain ridges, are addressed with the use of a longer post equalizer (44 micro sec. vs. 20 micro sec. in the second-generation).

"We found (in certain conditions) a significant number of ghosts occurred in a timeframe longer than the equalizer in the previous generation could handle," Lewis said.

To address fast-changing ghosts (those bouncing off moving objects such as cars and planes) the system adds a technology called "predictive slice," which "looks at the signal path and decides in an intelligent manner how to handle it and start up the equalizer," said Lewis.

Already in the works is a fourth-generation receiver design that will appear in a Zenith integrated direct-view HDTV set due in early 2002.

"The fourth-generation will go from a two-chip set analog demodulation system in the third generation to one-chip including digital demodulation and all those activities," Lewis said.

The fourth-generation design will be optimized to look at "very close-in ghosts," such as those bouncing off walls and ceilings inside a room, using an extended pre-equalizer to cancel out ghosts arriving before the main signal. This will allow even greater flexibility when receiving signals on indoor antennas.

"Moving from two chips to one will make it more cost effective to manufacture [DTV decoders], which will bring cost benefits to the consumer," said Lewis, "and it will also have improved indoor reception."

Mitsubishi, too, has announced a third-generation DTV tuner design that will appear in the company's first lineup of fully integrated HDTV sets due this fall.

Although field tests have not been completed yet, Marty Zanfino, Mitsubishi product development manager, said the sets have been designed in the lab for "better reception, meaning better sensitivity and interference rejection" than previous designs.

The new Mitsubishi sets will have "a larger reception radius, because they can handle a weaker signal," he said. "In addition we know we can handle a signal with more multipath interference, which is a problem in some cities."

One of the end benefits of the design is less sensitivity in positioning outdoor antennas for reception in distant areas and the ability to use indoor antennas in urban environments and suburban environments in many situations, he said.

The sets use a third-party demodulation chipset, which Zanfino declined to name, but a significant portion of the improvement comes from a re-engineered "front-end and discriminator (detector)," developed with substantial input from Mitsubishi.

"DTV reception is more than the design of the demodulation chip," Zanfino said. "It's a combination of that with the front-end and the discriminator chip that on balance creates the best overall system."

He said the bulk of Mitsubishi's improvements were made to the front-end and discriminator chip.

The improved "front-end," which is the tuner section of radio frequency (RF) receivers, uses a combination of broad filters and amplification to tune in RF broadcast signals. Mitsubishi's enhancements in this area should help to better tune weaker signals received both inside structures with high attenuation from aluminum siding, metal screens, etc., and in more distant locations from the origination source, Zanfino said.

Chip makers, too, are making bold claims about the enhanced reception capabilities of their new 8-VSB demodulation chipset designs, some of which are used in DTV tuner cards for PCs and are expected in next generation DTV receivers slated for arrival next year.

One IC maker, NxtWave, supplies demod chips to a variety of clients including tuner component manufacturer Alps, Korean set-top box developer Macro Image Technology, and U.S. DBS provider EchoStar, which uses the company's NXT-2000 chipset in its ATSC tuner plug-in module for an HDTV-capable integrated receiver descrambler.

Additionally, DTV PC-tuner card suppliers accessDTV and Hauppauge use the chip in their DTV computer products.

The NXT-2000 was announced as a breakthrough in DTV reception when it was first introduced a year and a half ago.

"The NXT-2000 played an important role in giving the broadcast industry and the FCC confidence that VSB really did work," said Matt Miller, NxtWave CEO. "Truthfully, we are disappointed that we haven't sold more of them but the market was somewhat stalled" by the controversy raised by certain broadcasters looking for the FCC to adopt the COFDM modulation scheme as a better solution than 8-VSB.

The chip was hailed as a breakthrough in its ability to deal with dynamic multipath and to process both 64 and 256 Quadrature Amplitude Modulation (QAM), the digital modulation schemes used to carry digital signals over cable television systems.

At January CES, NxtWave introduced the second-generation chip called the NXT-2002, which Miller said "builds upon the NXT-2000 using the knowledge gained from having broadcast signals on air."

Among the improvements in the NXT-2002 is the ability to deal with a wider range of multipath problems, including a wider range of leading and trailing ghosts and the ability to cancel out a "zero db ghost," which is a ghost that is nearly as large as the main signal. The IC includes an increased forward equalizer span to deal with a greater range of pre-ghosts, and the dynamic multipath characteristics have been enhanced to deal with rapidly varying ghosts.

Later this year, the company will announce an IC that builds upon compatibility with digital cable systems. In addition to VSB and QAM capability found in the first two chipsets, the new components will enable the use of conditional access (scrambled signals) based on the OpenCable Interface standard.

Other DTV demodulation chip suppliers including Broadcom, Oren and Motorola are also at work evolving the technology for next generation tuning devices.

Motorola, whose receiver clients include Konka and Changhong, offers a variety of ATSC tuner solutions including MCT5100 M-DTV module, billed as a complete receive/decoder solution.

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