DTV reception

Many factors contribute to the successful delivery of services to over-the-air digital television viewers. In addition to the different technologies involved, each delivery platform has its own requirements. DTV receivers come in many forms, including set-top boxes, integrated receivers, handheld devices, professional receivers and even PC adapters. Because of the different environments in which these operate, their characteristics are diverse. This month, we'll take a look at a number of the technical and logistical issues that affect DTV reception and display. (See Figure 1.)

Where conversion happens

Integrated receivers are by far the most widely deployed DTV receiving devices. Front-end (tuner and demodulator) performance has evolved quickly in the past 10 years, with some designs having passed five or more generations. As a result, early concerns about multipath performance have largely disappeared. On the video side, because receivers incorporate both decoders and displays, and the displays are of fixed resolution, the user generally will not have a choice as to how video is upconverted (or downconverted) for display. This makes the quality of the final format conversion entirely up to the manufacturer, for better or worse.

The DTV set-top box (STB) offers existing analog viewers a simple way to upgrade to digital reception. While STBs have been available for several years, popularity until recently was rather low, owing to a relatively high cost. Through the efforts of Congress, affordable subsidized coupon-eligible converter boxes are now widely available. At press time, more than 13 million converter box coupons have been redeemed as part of NTIA's TV converter box program, and an uncounted number of nonsubsidized STBs have been purchased as well.

Beyond coupon-eligible functionality, DTV tuners have also found their way into advanced STBs, as well as cable and satellite STBs. With HD output capability, one confusing aspect of this arrangement is that the upconversion of non-HD material can take place either in the STB or in the TV display (acting here simply as a monitor). In fact, the service provider often will perform its own upconversion of SD material, adding yet one more place for reformatting to occur.

The introduction and use of active format description (AFD) and bar data signaling in the transmitted bit stream can help this situation, so that HD 16:9 originated content is rendered correctly on SD 4:3 displays. It is important that this information is handled appropriately, as well as the cable and satellite reinsertion of closed captions, Nielsen data, TV Guide data and V-Chip program rating information.

For cable headends, DTV converters are required for retransmission of ATSC signals. Often set up to convert many channels at once, the signals can be taken to analog or digital baseband or transconverted from VSB to QAM. In addition to the emergence of digital-to-analog STB converters, a new type of customer-premises device has also become available, the wideband cable receiver. Capable of simultaneously tuning and demodulating up to 16 6MHz channels, these units allow the consumer to operate multiple TVs all from the same STB or home-server device.

Mobile DTV

Various mobile DTV standards have emerged that enable DTV reception on handheld devices. Power management is a critical issue, as conventional DTV silicon can draw more than 2W of power. To address this, mobile standards such as DVB-H and MediaFLO use time-division multiplex (TDM) and frame slicing to allow the receiver to turn on, download data and then go to sleep. ATSC-M/H (a candidate standard at press time) is also expected to use this or a similar method for power efficiency. The A-VSB proposal (one of the contributing elements to ATSC-M/H) also defined a virtual reference signal, which provides a way for the receiver's equalizer to lock on to the signal quickly, additionally preserving receiver power.

DVB-T (and therefore DVB-H) and MediaFLO are capable of hierarchical modulation, where the signal constellation can be segmented into high- and low-priority streams. This would allow high- and low-resolution versions of programs to be sent with different levels of transmission robustness. (ATSC-M/H and ISDB-T, while not using constellation-segmented hierarchical modulation, can nonetheless also be configured to support similar multiple-priority operation.)

For the PC, DTV reception adapters come in two flavors: USB and PCI tuners. PC adapters use software to decode the ATSC (MPEG-2) video, so the performance of the host PC will usually determine the quality of the video decoding. One consequence of this can be dropped frames if there is not enough CPU horsepower to decode and render the video. The adapters often house both a VSB and QAM tuner, and some include an analog NTSC tuner as well. The digital tuners transfer the MPEG-2 transport stream over a USB-2 or PCI interface to the PC, where it is demultiplexed, decoded and delivered to the display. Those offering NTSC reception must also include an internal analog-to-digital converter to deliver the video to the PC, which often adds an external video capture function.

Antennas revisited

Antenna design has staged a comeback thanks to the DTV transition. Conventional antennas are experiencing a resurgence in sales, and small-footprint indoor yagis and smart antennas are gaining attention as aids to problematic reception conditions.

Viewers in fringe areas will benefit from high-gain antennas and/or mast-mounted preamplifiers. Here, low-noise, high-gain preamplifiers will offer the best performance, but at a price sometimes exceeding that of a converter box, especially a subsidized one. A preamp that covers both VHF and UHF bands, when needed, with separate amplification in each band, along with a switchable FM trap, will lessen the likelihood of overload from strong signals.

Viewers in rental properties may not fully appreciate their rights to install outdoor antennas. As directed by Congress in the Telecommunications Act of 1996, the FCC adopted the over-the-air reception devices (OTARD) rule concerning governmental and nongovernmental restrictions on viewers' ability to receive video programming signals from DBS, broadband radio service providers and television broadcast stations. The rule prohibits restrictions that impair the installation, maintenance or use of antennas to receive video programming, and applies to video antennas that are less than about 3ft in diameter (or of any size in Alaska) and TV antennas. In 1999, the commission amended the rule so that it also applies to rental properties where the renter has an exclusive use area, such as a balcony or patio. In 2000, the rule was further amended so that it applies to customer-end antennas that receive and transmit fixed wireless signals.

The consumer's first reference for reception issues is usually the point of purchase of their DTV product. However, many of the elements affecting the delivery of high-quality over-the-air video to the viewer's home are beyond the consumer's and retailer's grasp. Broadcasters should pay close attention to these numerous factors as a system — and not just when the equipment is installed. A successful DTV transition means handling all of the post-transition service issues with the same care that all customer service requires.

Aldo Cugnini is a consultant in the digital television industry.


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