NHK Science and Technical Research Laboratories (1993), High Definition Television: Hi-Vision Technology (New York: Van Nostrand Reinhold).

ATSC A/53, Digital Television

Standard.

analog service. (In early deployment of HDTV, program material was simulcast on a conventional analog transmitter.)

Japan

HDTV broadcasting based on 1035i30.00 scanning and MUSE compression was deployed in Japan in the early 1990s; the system is called Hi-Vision. MUSE is a hybrid analog/digital system optimized for direct broadcast from satellite (DBS); it is documented in the book from NHK Labs. MUSE predates the MPEG standards; nowadays, it is generally agreed that Japan adopted analog HDTV transmission standards prematurely.

In 2003, MUSE was superseded by ISDB-T, a terrestrial broadcasting system based upon MPEG-2 video coding and OFDM transmission.

United States

HDTV developers in the United States planned for broadcasters to gradually replace analog SDTV transmission with digital HDTV transmission. Partway through the development of HDTV, it became clear that the same compression and transmission technology that would allow one HDTV channel to be coded and transmitted at about 20 Mb/s would be equally suited to allow five SDTV channels to be coded, multiplexed, and transmitted at 4 Mb/s each! So, what began as highdefinition television evolved into digital television (DTV), which encompasses both SDTV and HDTV. Compression is in accordance with MPEG-2 MP@ML for SD and MP@HL for HD, with restrictions specified in ATSC A/53.

DTV standards in the United States were developed by the Advanced Television Systems Committee (ATSC). Those standards were adopted by the Federal Communications Commission (FCC), with one significant change: The FCC rejected the set of 18 formats documented in Table 3 of ATSC Standard A/53 (presented as my Table 15.1, on page 143). Bowing to pressure from the computer industry, the FCC deleted that table, but left the rest of the ATSC standards intact. The fact that Table 3 is absent from FCC standards has virtually no practical import: In practice, consumer receivers are

MPEG-2 specifies upper bounds for picture size at various levels. Surprisingly, ATSC A/53 specifies exact values. MPEG-2 MP@ML allows 720 image columns, but ATSC A/53 does not.

EIA-708-B, Digital Television (DTV)

Closed Captioning.

4.5684 ≈ 10.762

286

obliged to decode the Table 3 formats; they cannot be depended upon to decode anything else.

The FCC’s deletion of Table 3 supposedly left the choice of raster standards to the marketplace: In principle, any format compliant with MPEG-2 MP@HL could be used. In practice, no U.S. broadcaster has chosen, and no consumer equipment is guaranteed to implement, any format outside ATSC’s Table 3. In practice, DTV decoders conform to MPEG standards, and additionally conform to the restrictions imposed by the ATSC standards.

In the United States, DTV audio is standardized with Dolby Digital audio coding (also known as AC-3),

a coding scheme not specified in the MPEG-2 standard. Dolby Digital is capable of “5.1” channels: left and right (stereo) channels, a front center channel, left and right surround channels, and a low-frequency effects (LFE) channel (the “.1” in the notation) intended for connection to a “subwoofer.” ATSC audio consumes a maximum of 512 kb/s.

EIA-708-B standardizes a method for conveying closed caption data (DTVCC).

One or more MPEG-2 program streams, the associated audio streams, ancillary data, and other data is multiplexed into a transport stream with a bit rate of about 19.28 Mb/s. The transport stream is augmented by Reed-Solomon forward error correction: Each 188-byte transport packet is augmented by 16 FEC bytes, resulting in 204-byte packets that are presented to the modulator.

ATSC modulation

The ATSC standardized 8-level digital vestigial sideband (8-VSB) modulation, transmitting about

10.762 million 3-bit symbols per second. To enable the receiver to overcome errors introduced in transmission, two forward error-correction (FEC) schemes are concatenated: The outer code is Reed-Solomon (R-S), and the inner code is a simple 3⁄2 trellis code. Between the R-S and trellis coding stages, data is interleaved. Synchronization information comprising segment and field syncs is added after interleaving and coding. A low-level pilot carrier is inserted 310 kHz above the lower band edge

I and Q refer to in-phase and quadrature; these are the same concepts as in NTSC chroma modulation, but applied to completely different ends.

ATSC defines a cable mode using 16-VSB without trellis coding, but this mode hasn’t been deployed. Digital cable television is detailed in the book by Ciciora and colleagues cited at the end of this chapter.

to aid in carrier recovery. Analog techniques are used to upconvert to the UHF broadcast channel.

At a receiver, analog techniques downconvert the UHF broadcast to intermediate frequency (IF), typically 44 MHz. Demodulation is then accomplished digitally. Typically, an analog frequency and phase-locked loop (FPLL) recovers the carrier frequency based upon the pilot carrier. A quadrature demodulator then recovers I and Q components. The I component is converted from analog to digital at 10.76 MHz to recover the bitstream; the Q component is processed to effect phase control. The bitstream is then subject to trellis decoding, deinterleaving, R-S decoding, and MPEG-2 demultiplexing.

It is a challenge to design a demodulator that is immune to transmission impairments such as multipath distortion and co-channel interference from NTSC transmitters. An interference rejection filter – a variation of a comb filter – is built into the demodulator; it attenuates the video, chroma, and audio carriers of a potentially interfering NTSC signal. An adaptive equalizer built into the demodulator alleviates the effects of multipath distortion; the field sync component of the signal serves as its reference signal. An adaptive equalizer is typically implemented as an FIR filter whose coefficients are updated dynamically as a function of estimated channel parameters.

Cable television has very different channel characteristics than terrestrial broadcast. DTV over cable typically does not use 8-VSB modulation: Quadrature amplitude modulation (QAM) is used instead, with either 64 or 256 levels (64-QAM or 256-QAM).

For DBS, quadrature phase-shift keying (QPSK) is generally used.

Consumer receivers in the United States must accept the diversity of frame rates and raster standards in ATSC’s Table 3 (my Table 15.1 on page 143). Although multiscan displays are ubiquitous in computing, both price and performance suffer when a display has to accommodate multiple rates. Most consumer HDTV receivers are designed with displays that operate over a limited range of scanning standards; the wide range of ATSC Table 3 is accommodated by digital resampling. In early deployment of HDTV, many receivers used

DVB standards are promulgated by ETSI [www.etsi.org].

1366× 768 displays, upconverting 720p or downconverting 1080i to that format. Today, most consumer HDTV receivers are 1080i-native, and convert other formats to 1080i.

Europe

In Europe, huge efforts were made in the 1980s and 1990s to develop an HDTV broadcasting system using 1250/50 scanning and a transmission system built upon the MAC transmission technology originally designed for 576i. MAC failed in the marketplace. HD-MAC failed also; this was partially a consequence of the commercial failure of MAC, partially because of technical weaknesses of HD-MAC, and partially because HD-MAC did not address worldwide markets.

HDTV broadcasting in Europe was late, but digital broadcasting of SD is deployed based upon MPEG-2 MP@ML, with 720× 576 image structure. The Digital Video Broadcasting (DVB) organization has created a comprehensive set of standards for cable (DVB-C),

satellite (DVB-S), and terrestrial (DVB-T) broadcasting. DVB audio conforms to MPEG-2 audio. These standards are promulgated by ETSI.

The RF modulation system chosen for DVB-T is coded orthogonal frequency division multiplexing (COFDM). COFDM uses a large number of subcarriers to spread the information content of a signal evenly across a channel. The subcarriers of COFDM are individually modulated, typically using QPSK or QAM. COFDM exhibits greatly improved immunity to multipath distortion compared to 8-VSB. Also, COFDM accommodates transmission using single-frequency networks (SFNs) where the same bitstream is transmitted from multiple transmitters at different locations but operating on the same frequency.

Further reading

Adams, Michael, (2000), Opencable Architecture (Indianapolis, Indiana, U.S.A.: Cisco Press).

ATSC A/54A, (2006), Recommended Practice: Guide to the Use of the ATSC Digital Television Standard, including Corrigendum No. 1 (Dec.)

Ciciora, Walter, James Farmer, David Large, and Michael Adams (2004), Modern Cable Television Technology, Second Edition (San Francisco: Morgan Kaufmann).