Tuning In To Digital TV

Nov. 16, 2006
Multiple standards and offerings make for a bumpy road ahead when it comes to across-the-board adoption.

Let's get one thing straight: Digital TV is not synonymous with high-definition digital TV (HDTV). It's much more than that. Digital TV encompasses not only HD, but satellite TV, cable TV, Internet Protocol TV (IPTV), and mobile TV as well.

So, is it the next big thing? Believe it or not, you likely have digital TV already, possibly several of them. And you may not even realize it. In fact, the number of digital TV sources continues to climb, as does the number of products that display it. And expectations are that all of these products will find some kind of consumer success.

ATSC HDTV The Advanced Television Standards Committee (ATSC) set the standard for the U.S. version of digital TV, which includes HDTV, back in the 1990s. HDTV is digital TV with a resolution far greater than current analog TV, which is based on the over-50-year-old National Television Standards Committee (NTSC) format.

NTSC television uses 525 interlaced scan lines (with only about 480 effective lines) to present a picture with an aspect ratio (width to height) of 4:3. HDTV provides up to 720 or 1080 lines with an aspect ratio of 16:9, which better matches the movie screen format. Progressive scan is available as well as interlaced scan. Interlaced scan usually is preferred to minimize flicker at low frame rates. Yet progressive scan lines tend to work best with video compression techniques.

The ATSC system compresses the digitized video using the MPEG-2 standard, treats it to a Reed-Solomon forward-error-correction (FEC) scheme and Trellis coding, and then forms it into 188-bit packets. These packets are transmitted over the air via a modulation scheme known as 8VSB, or eight-level vestigial sideband. With 8VSB, the carrier is suppressed, leaving only a weak pilot carrier. Also, it transmits all of the upper sideband and only a portion (vestige) of the lower sideband.

The signal is designed to fit in a standard, 6-MHz bandwidth TV channel. With eight-level coding, every three bits are translated into one of the eight amplitude symbols. This provides a symbol rate of 10,800 symbols/s. The composite data rate is 32.4 Mbits/s, but without the coding, the raw video (plus audio) rate is 19.3 Mbits/s. When delivered by cable, the HDTV signal is modified by dropping the Trellis coding and going to a 16VSB scheme code with 4 bits per symbol and a data rate of 38.6 Mbits/s.

While HDTV was designed for over-the-air delivery, most of it is delivered by cable. More than 60% of the U.S. population gets TV via cable, including HDTV. Less than 15% still receive TV through the old-fashioned, terrestrial-radio method. The remaining 20% or so get their TV by satellite.

HDTV has stumbled through a slow and painful conversion. The greatly improved picture quality over standard NTSC analog TV hasn't pushed droves of consumers to make the switch. Lack of good HD content is part of the problem. More prevailing, though, is that most consumers seem satisfied with the current TV quality.

But with rapid declines in price for large plasma, LCD, and projection sets, momentum is picking up. Also, the FCC mandated the end of analog TV transmissions by February 17, 2009, meaning it's time to buy a digital or HD set... or read a book. But not all is lost. Analog holdouts will be able to get converter boxes to translate the digital signals into an analog equivalent, though with a degraded image.

The FCC's mandate has to do with attempting to reclaim huge portions of the UHF spectrum previously assigned to TV stations. By pushing TV stations to the lower frequencies, the FCC can reallocate that poorly utilized part of the spectrum to cellular and other wireless services.

Virtually all HDTV content these days is 720-line progressive. There's little in the way of 1080i HDTV content. The average consumer can see a definite improvement in picture quality with 720i, but a large screen is really needed to appreciate it. Also, the 720i format degrades on screens that are more than 42 in. Screens of 42 in. and more are needed to see the full potential of 1080i HD. And don't forget, viewing distance is critical in seeing and appreciating the HD content.

SATELLITE TV Many of you have satellite TV because it is one of the greatest broadband TV delivery bargains around. Also known as Direct Broadcast Satellite (DBS), its major vendors are DirecTV and DISHnetwork (EchoStar). For years, each has stolen many customers from cable. Satellite TV has likely reached its peak potential, since the receive-only (RO) format limits its usefulness. Dial-up modems can provide a two-way connection back to the provider, but that's unsuitable with most two-way applications, such as Internet access.

Nevertheless, satellite TV is digital. The digital signal is beamed down to the 18-in. dish in the Ku microwave band. Typical downlink frequencies are in the 10.95- to 12.75-GHz or 12.2- to 12.7-GHz bands. Uplink to the satellite is commonly in the 14- to 14.5-GHz range. The channel bandwidth is about 24 MHz with a 27-Mbit/s data rate.

Modulation is typically quadrature phase-shift keying (QPSK). Digital packets are 147 bytes long and include 127 bytes of data plus 17 bytes of FEC. Also, MPEG2 compression is used. A low-noise block (LNB) converter on the dish down-converts the Ku band signal to a lower frequency in the L band between 950 and 1450 MHz. That signal is more compatible with the coax going to the receiver.

CABLE TV Cable TV used to be all analog. Today, 30% to just over 50% of all cable companies have converted to digital, depending on the location. This means some signals are transmitted digitally, and a converter box at the subscriber's home demodulates the digital programming and converts it to an analog format for display.

Most cable providers use a hybrid-fiber-cable (HFC) system with a fast fiber backbone that serves coax cable drops to homes and businesses. Cable-TV systems still transmit analog TV to subscribers who won't sign up for the digital feature. The system uses some channels to broadcast multiplexed digital TV to subscribers with the digital set-top box (STB). The digital systems supports video on demand (VOD), which provides movies and other content when desired.

Digital cable-TV systems use the Data Over Cable Service Interface Specification (DOCSIS), a protocol developed by Cable Labs. Supported by the cable companies, this organization researches how to deliver cable, voice, data, and video over cable systems. The protocol is standardized by the Society of Cable and Telecommunications Engineers (SCTE) and the ITU.

DOCSIS now permits digital TV, especially HDTV, to be distributed to home subscribers along with Voice over Internet Protocol (VoIP) services and high-speed Internet access—the so-called triple play. Basically, DOCSIS uses 64QAM (quadrature amplitude modulation) or 256QAM to transmit downstream over 6.4-MHz bandwidths. Upstream is usually accomplished with QPSK or 16QAM, although newer systems may use 32QAM, 64QAM, or 128QAM.

Data rates typically reach up to 40 Mbits/s downstream. The most recent version of DOCSIS 3.0 supports a data rate to 160 Mbits/s downstream and 120 Mbits/s upstream. It uses IPv6 and channel bonding that combines channels to boost bandwidth and data rate. Cable TV systems could also carry Internet Protocol TV (IPTV) using DOCSIS. Though not expected to happen, that along with a conversion from DOCSIS to IPTV is possible in the future.

DIGITAL VIDEO BROADCAST-TERRESTRIAL DVB-T is Europe's digital TV system. It's gradually replacing Europe's analog TV systems—PAL in the U.K. and the rest of Europe, and SECAM in France. Both of these analog systems have better resolution than the NTSC analog system (e.g., 625 lines versus 525 lines and greater bandwidth). The phaseout of analog to digital is under way in this region, too. It's expected to take longer than the U.S.—by 2010 in the U.K. and 2012 for the rest of Europe.

DVB is a standard-definition digital standard (480 lines), but it also supports an HD format. Select VHF or UHF channels are used, depending on the European country involved. Modulation is either QPSK, 16QAM, or 64QAM in combination with coded orthogonal frequency-division multiplexing (OFDM) and MPEG2 or the newer, more efficient H.264 compression. Channels may be 6, 7, or 8 MHz wide, again depending on the country. Packets are 188 bytes long.

INTERNET AND IPTV Internet and Internet Protocol Television (IPTV) are forms of digital video and audio delivered over the Internet. They aren't the same thing. Internet TV or video is the video you can get now on your PC—short clips, ads, humor, and the stuff you might find on YouTube or get as an e-mail attachment. It's low resolution and a bit jerky at times, but it has been around for years thanks to Windows Media Player, Apple's QuickTime, and Adobe's Marcomedia Flash.

A newer form of Internet TV is IPTV, which includes movie downloads from suppliers like Apple and Amazon. Both sell movies that you download to your hard drive and play on your PC or iPod. The number of films available is limited today, but it's expected to multiply as interest develops.

A standard-length movie can take anywhere from an hour to over two hours to download on a typical DSL connection, making this mode of digital TV annoying at best. And there's no appreciable cost savings. While movies play in a 640-by-480 format on a PC, the picture quality is a bit less than watching the same movie from a DVD. If you don't have a nearby video store or Netflix and you like watching TV on your PC, this is the service for you.

IPTV has yet to be implemented in any higher-quality form on a grand scale. Nonetheless, it's coming fast in the form of Video on Demand (VOD), the ability to stream video, and even record it, at any time.

Market-research firm iSuppli predicts the VOD market to grow to nearly $13 billion by 2010 (Fig. 1). Both Apple and Amazon are getting ready to launch major services to serve this market. While IPTV will gain a growing share of the VOD market, cable and satellite providers are already making money and will also continue their growth.

Moreover, IPTV will be part of the triple play offered by telecommunications carriers who offer VoIP, Internet access, and video over their broadband connections. Just rolled out in San Antonio, Texas, AT&T's U-verse TV service provides as many channels of video as cable, including VOD, using the company's Lightspeed network. Verizon has a similar program utilizing its FiOS fiber optic network and cable-TV delivery technology, but the company expects to switch to an IP format in the future given its great broadband fiber network.

The driving force behind IPTV is the telcos' declining revenue, which has been affected by voice service saturation, declining average revenue per user (ARPU), high customer churn, and government deregulation. This deregulation has put cable-TV companies in competition with the telcos, thanks to VoIP. Thus, the telcos are fighting back with IPTV to garner a new revenue stream.

Another potential delivery source for IPTV is wireless via the forthcoming WiMAX-based broadband wireless broadcasters. Adequate bandwidth will be available, so the potential is there. Because the whole IPTV business is in its very early stages, the definitive model has yet to be formulated. The problem with IPTV is that you have to invest in the whole system, then deploy it to see if it works.

MOBILE TV Mobile TV, meaning digital TV for cell phones and other portable devices, is just now beginning. Short clips of music, news, weather, and entertainment are available now, delivered over the current cell-phone system. Resolution is low and screen sizes are small, but many people believe it will be a business of its own once there's a successful business model.

Movies aren't expected to be a factor because of the screen limitations. More importantly, the cell-phone infrastructure doesn't offer sufficient bandwidth to support many subscribers watching video at the same time. Most carriers indicate that too many viewers will crash the network, and they would rather allocate their bandwidth to Internet access.

Yet a newer form of cell-phone digital TV has been developed. It offers broadcast TV via digital methods to a separate TV receiver in the cell phone. With spectrum now available, several companies are beginning to set up stations nationwide using Europe's DVB-H standard and Qualcomm's MediaFLO. It will offer good quality digital video for movies or any other content (see "The Cell Phone: Now That's Entertainment," May 11, 2006, p. 42, ED Online 12436).

DIGITAL TV DESIGNS Like most electronic products today, LCD TVs have one or more embedded processors at their core to perform most of the signal processing (Fig. 2). They're surrounded by support chips that deal with the inputs and outputs. Signals come into the set via one or more tuners. Though some of the older can tuners are still in use, they're increasingly being replaced by single-chip IC tuners like the Xceive XC3028/L.

These devices not only down-convert over-the-air signals or cable inputs to an IF, they also incorporate the analog-to-digital converter (ADC) and demodulation circuits, as well as digital-to-analog converters (DACs), to deliver both video and sound outputs to the signal-processing circuits. Many tuners lack this capability, so ADCs and demodulators are required before signal processing.

The set may also get video inputs from DVD players and other devices via NTSC, CVBS (composite video blanking sync), S-video, RGB, DVI, HDMI, 1394, or YPrPb formats. A video switch, decoder, and multiplexer provide compatibility with all common formats. A DSP chip handles the MPEG2 decompression, FEC, and other video functions.

The DSP outputs the decompressed video to the DACs for visual display on the LCD or to outputs for other devices. A separate audio codec delivers stereo and surround outputs to the amps and speakers. Today, virtually all of the video and audio functions are combined into one or two chips. Chip companies like Broadcom, NXP (Nexperia), and Texas Instruments (DaVinci) have complete solutions and reference designs.

In a typical STB design for IPTV, the most common input is an Ethernet cable from the home broadband router or gateway from the cable-TV or DSL provider (Fig. 3). The MPEG4/H.264 compressed video in the IP packets is passed to a DSP that performs its magic decompression and FEC decoding.

Separate video and audio outputs drive their respective DACs. Other video inputs can come from a tuner, NTSC, or other source. Note that HDMI and DVI interfaces are provided for DVDs and other devices. IR, UART, modem, and power-line-modem (PLM) interfaces are provided as needed.

The compression and decompression process lies at the heart of all these designs. The Motion Picture Experts Group (MPEG) standards are widely used: the older MPEG2 for ATSC HDTV and satellite TV, and the newer and more efficient MPEG4 and H.264 ITU standard for IPTV and mobile TV (see "Video Processing Brings New Meaning To Motion," Sept. 1, 2006, p. 48, ED Online 13291).

The key problem with IPTV is getting the broadband connection to the STB. The cable or DSL modem usually resides at the PC location well away from the TV sets. One solution, the digital media adapter (DMA), transfers audio and video signals to the TV sets. This device implements a major home network solution and may well provide a hard drive for audio and video downloads.

Many homes already have a home network, mostly wireless, so multiple PCs can share the high-speed Internet connection. Homes adopting IPTV will need a change or upgrade. Most current wireless networks aren't fast enough to support IPTV. And since the incoming broadband line comes into a router or gateway connected to the PC, you'll need networking to get the TV signals to the TV sets.

Recent surveys indicate that the average home has 4.3 TV sets. A better than average home network is the only hope for IPTV to be successful. Wireless in the form of 802.11a/b/g isn't fast enough and has limited range or spotty coverage, especially as it goes through walls.

Ultra-Wideband (UWB) is supposed to fulfill this need for speed with its 480-Mbit/s rate, but only over very short distances. Anything longer than 10 m will be a challenge, especially through walls. Wi-Fi 802.11n multiple-input/ multiple-output (MIMO) upgrades to your Wi-Fi network may be a better choice, but it's yet to be proved. Apple's iTV STB contains an 802.11 wireless network that lays early claim to solving the problem of streaming video to TV sets.

Two newer technologies may emerge as the home network of choice for broadband and DTV. MoCA (Multimedia over Coax Alliance) uses the cable-TV wiring available in most homes today. HomePNA 3 combines both installed cable-TV coax and twisted-pair telephone wiring.

HomePNA 3, which can connect virtually any device (PC, TV, VoIP phone, etc.) using existing wiring, can deliver up to 240 MHz of bandwidth. Developed under the sponsorship of the Home Phoneline Networking Alliance, this system was recently adopted by AT&T's Uverse IPTV system for home networking. Coppergate Communications makes a chip set for implementing the HomePNA 3 networking solution in set-top boxes for the AT&T U-verse system.

For more, see "Digital TV: Issues And Impacts" at www.electronicdesign.com, Drill Deeper 14016.

Broadcom Corp.
Coppergate Communications Ltd.
Digital Living Network Alliance
Federal Communications Commission
Home Phoneline Networking Alliance
www.homepna.orgiSuppli Corp.
Multimedia Over Coax Alliance
NXP (formerly Philips Semiconductor)
Tektronix Inc.
Texas Instruments Inc.
Xceive Corp.
About the Author

Louis E. Frenzel

Click here to find more of Lou's articles on Electronic Design. 

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