Digital Television

22.11.1998

Satu Virtanen sevirtan@cc.hut.fi
Kosti Rytkönen Kosti.Rytkonen@hut.fi

Helsinki University of Technology

Abstract

Digital television has been developed during this decade from draft to a usable product. Two main standards have been formed; one U.S. and one European. The quality of the digital television varies from resolution of analog television to several degrees higher. Digital television offers surround sound and multimedia services. MPEG-2 standard is the base of the compression and transport of digital television signal. Digital television is already in use in several locations all over the world, and it is well on its way in replacing the old fashioned analogue television systems.


1 Standars

There are two different fundamental digital television standards: European Digital Video Broadcast (DVB) project, as standardised by the European Telecommunications Standards Institute (ETSI), and U.S. digital television standard documented by The Advanced Television Systems Committee (ATSC) and standardised by FCC. This essay is concentrating mainly on U.S. ATSC standard; the European DVB standard has only slight differences to ATSC standard, thus no separate coverage is necessary. [1, 8]

ATSC DTV standard consists of the following standards [1]:

2 Overview

There are two types of digital television in the American standard: High Definition Television (HDTV) and Standard Definition Television (SDTV). SDTV has the same resolution as broadcast quality television uses nowadays, but as a digital medium it has a perfect quality, thanks to the error-free transmission. HDTV has a resolution 10 times better than VHS (Video Home System). It uses the wide screen mode (screen proportions of 16 x 9). Both types have surround sound and some bandwidth reserved for ancillary data. Even though transmission media will be digital, old analogue televisions can be used to receive the transmission with the help of set-top boxes that convert digital picture to analogue. [2, 6]

DTV standard describes reliable transfer rates for channel of 6 MHz bandwidth of about 19Mbps in a terrestrial broadcasting channel and about 38 Mbps in cable television channel. This requires compression rates of 50:1 or greater. [6]

3 Components

The standard consists of three subsystems; the first subsystem, source coding and compression, defines MPEG-2 for video compression and Digital Audio Compression (AC-3) standard for coding audio. European standard uses MPEG-2 also for audio (Musicam). Service multiplex and transport subsystem, the second subsystem, uses MPEG-2 transport stream syntax for multiplexing video, audio and ancillary data into a single data stream. In RF/Transmission subsystem (the third), two separate modulation methods are used in transmitting the signal: a terrestrial broadcast mode (8 VSB), and a high data rate mode (16 VSB). In addition to cable and terrestrial broadcast European DVB standard defines standard for satellite broadcast.[2, 6, 8]

3.1 Source coding and compression

3.1.1 Video

6 MHz channel allows about 20 Mbps transfer rate. High compression is required to stuff 1 Gbps HDTV into such channel. DTV uses MPEG-2 compression standard for video compression. Initially HDTV was supposed to use MPEG-3 compression, but the idea was dropped because development of MEG-3 was terminated and MPEG-2 was sufficient for HDTV anyway. [6]

Different systems require different characteristics of a compression algorithm. Therefore MPEG-2 is divided into profiles and levels. DTV uses Main Profile at High Level (MP@HL). High Level provides resolution of 1920 x 1152. Main Profile limits compressed data rate to 80 Mbps maximum. [3, 6]

MPEG Main Profile stream consists of three different picture frames: Intra (I), Predicted (P) and Bi-directional Interpolated (B). Intra frames are entry points for random access providing only moderate compression. Predicted frames reference to previous frame receiving a fairly high amount of compression. Bi-directional frames require reference to both previous and next frame but provide the best compression ratio.

Encoder divides pictures in 16 x 16 pixel squares called macro blocks and searches similar blocks from adjacent frames. When match is found only the first block need to be transmitted. Vector movement code is also provided for moving picture elements. Furthermore compression ratio is enhanced through discrete cosine transform (DCT).

Picture sequences can vary in spatial resolution (480 lines, 720 lines or 1080 lines), in temporal resolution (60 fps, 30 fps or 24 fps) and in scanning format (2:1 interlaced or progressive scan). In interlaced scan even and odd lines are scanned by turns reducing information to half. In progressive scan each line is scanned. [3, 5]

3.1.2 Audio

Audio system in DTV is based on the Digital Audio Compression (AC-3). System uses six audio channels per audio program bit stream: Left, Centre, Right, Left Surround, Right Surround and Low Frequency Enhancement (LFE). Bandwidth of the LFE is 120 Hz while other channels have bandwidth of 20 kHz. Audio system has frequency of 48 kHz and supports resolutions up to 24-bit.

3.1.3 Auxiliary data

DTV system offers digital data services in addition to normal video and audio. MPEG-2 transport layer allows new services flexibly. Basic services like Program Subtitles, Emergency Messages and Program Guide are possible.

3.2 Service multiplex and transport systems

DTV system uses MPEG-2 transport stream syntax for multiplexing video, audio and auxiliary data into a single data stream. This syntax was designed for efficient transmissions over channel of limited bandwidth. It is well suitable for digital television transmissions and offers good compatibility with media and standards. Broadcasted bit stream contains redundant data for error correction.

It uses fixed-length transport stream packetization that allows dynamic capacity allocation: in other words, is flexible in allocation channel capacity among video, audio and auxiliary data. It is also scalable and extensible; larger bandwidths and new elementary bit streams pose no threat or trouble. Error correction and detection are synchronisable to the packet structure. Fixed-length packetization is also MPEG-2 compatible and allows cost effective receivers. [6]

3.3 RF / Transmission systems

DTV uses two modes of vestigial sideband (VSB) multilevel modulation technology for transmission. These are terrestrial broadcast mode (8 VSB), and a high data rate mode (16 VSB) operate in same channel bandwidth (6 MHz), but 16 VSB has twice the transfer rate of 8 VSB. Higher rate is accomplished by trading off robustness and therefore requiring better signal to noise ratio. Both modes use Reed-Solomon (RS) code for forward error correction (FEC). Terrestrial broadcast mode also uses Trellis coding FEC.

ParameterTerrestrial modeHigh data rate mode
Channel bandwidth6 MHz 6 MHz
Excess bandwidth 11.5% 11.5%
Symbol rate 10.76 Msymbols/s 10.76 Msymbols/s
Bits per symbol 3 4
Trellis FEC 2/3 rate None
Reed-Solomon FEC T=10 (207,187) T=10 (207,187)
Segment length 832 symbols 832 symbols
Segment synchronisation 4 symbols per segment 4 symbols per segment
Frame synchronisation 1 per 313 segments 1 per 313 segments
Payload data rate 19.28 Mbps 38.57 Mbps
NTSC co-channel rejection NTSC rejection filter in receiver N/A
Pilot power contribution 0.3 dB 0.3 dB
C/N threshold 14.9 dB 28.3 dB

VSB transmission mode differences [6]

4 Current status and future development

The position of digital television here in Finland will be naturally directed by the European standardisation and research. The DVB system enables four normal channels to be broadcasted in place of one of the current, analogue channels. Limited radio frequency areas will no longer restrain broadcasting as tightly as nowadays. Also other applications than plain television broadcasting can benefit from this deliverance. One scenario, purely in the television world, is that one station could broadcast several SDTV channels during the daytime and HDTV during prime time, thus being able to meet the special needs of the daytime viewers and provide high quality video during the most popular TV time. [2, 4]

The interest for digital television in Europe has been growing fast for the last couple of years; in 1996, at the IBC Conference, a group called DIGITAG was established to "develop and harmonise digital terrestrial television internationally" [8]. The Great Britain began digital television broadcasting in 1997, in Finland this very year is the beginning of it all. Sweden has began building a digital network, and by the year 2005 they plan to have fifty digital TV channels in use. Also the Czech Republic has began the transmissions. Digital television standards have been used in satellite broadcasts for quite a while already.

The forecast for Finland is that in two years we probably have eight digital channels, four of them being copies of the still available analogue channels and four new channels; by the year 2015 the predicted number of digital channels in 24, some of which would not be traditional television, but new multimedia. The digital television and the ever expanding channel variety will change the nature and exactness of commercial broadcasting - television commercial advertisements will become more magazine like, more directed to a certain public, as will the program itself. [4, 7]

5 References

[1] Advanced Television Systems Committee, Standards Documents [accessed 19.11.1998]
< http://www.atsc.org/Standards_sum.html>
[2] Cemacity, What Is The Advanced Television Standard Committee (ATSC) Digital Television Standard? [accessed 16.11.1998]
< http://www.cemacity.org/cemacity/mall/product/video/files/standard.htm>
[3] Gallo, Paul G. & McKernan, Brian, The Guide to Digital Television, 31.12.1997 [accessed 25.9.1998]
< http://www.digitaltelevision.com/toc.htm>
[4] Ilen, Ismo, Digitaalitelevisio räjäyttää ohjelmatarjonnan, 1996 [accessed 22.9.1998]
< http://www.digital.fi/talous.html>
[5] IVidea, Digital Video, 14.8.1998, [accessed 22.9.1998]
< http://www.ividea.com/dvmpeg.htm>
[6] Unknown (server currently unavailable), [accessed 15.11.1998]
< http://www.cgg.cvut.cz/~xhromas/dtv/dtv.htm>
[7] Suomen Lehdistö, Tekniikkapalsta, 4/1998 [accessed 20.11.1998]
< http://www.sanomalehdet.fi/suomenlehdisto/0498T.html >
[8] The UK Digital Television Group (DTG), Tutorial, 23.5.1998 [accessed 25.9.1998]
< http://www.dtg.org.uk/tutorial/tut_frame2.htm>

6 Additional information

Digital Television Broadcasting Standards

Glossary: Satellite and Computer Technology
A small but informative glossary on terminology attached to the digital world of television.

VALIDATE - a virtual laboratory to accelerate the launch of digital terrestrial television
The European VALIDATE project works to verify the DVB standard for digital terrestrial TV broadcasting and to prepare for the launch of services.


This page is maintained by Kosti Rytkönen
Last update November 22nd, 1998
URL: http://www.hut.fi/~krytkone/digitaltv.html