There are several digital TV standards developed such as: DVB-T (Digital Video Broadcasting – Terrestrial) ATSC (Advanced Television Systems Committee), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial)
Digital Terrestrial Television is a digital technology that refers to the sending and receiving of video and audio information by means of digitally modulated signals. It provides a greater number of channels and better quality of picture and sound in the same amount of frequency bandwidth required by analog television. Digital television also provides different interactive information services like on-air program guide (EPG), TV shopping and on-air games to name a few. There are several digital TV standards developed and deployed in different countries: ATSC in the US, North America and South Korea, ISDB-T in Japan and Brazil, DVB-T in Europe and most countries in Asia, Africa and in Australia and New Zealand. Both ISDB-T and DVB-T are based on COFDM modulation that has several variants: QPSK, 16QAM and 64QAM. ATSC is based on 8VSB modulation, which is similar to the vestigial sideband modulation used in analog TV.
Figure 1: Digital TV standards in the world
All standards use MPEG-2 compression technology for transport of video and audio information in available bandwidth. Demand for more channels and high-definition (HD) content forced the development of better compression techniques like MPEG-4 AVC or H.264.
2. Advanced Television Systems Committee (ATSC)
In the early 1990’s, work was started in the US to develop a digital TV system allowing the highest possible bit rate payload in a 6MHz analog channel. The objective was to be able to transmit HDTV via terrestrial TV frequencies. This was achieved and demonstrated in 1990 and became the basis of the ATSC standard that was launched in 1995. The modulation used in ATSC is 8VSB that is in some ways similar to analog NTSC based on a single carrier. This modulation technique provides 19.39 Mbps data payload in a 6 MHz TV channel. With this, it is possible to transmit one HDTV program or up to four standard definition programs in a single 6MHz channel. The transport stream used is MPEG-2 and audio coding is Dolby Digital AC-3 format (5.1-channel surround sound) providing theater-like quality audio. Various auxiliary datacasting services are also possible.
3. Digital Video Broadcast-Terrestrial (DVB-T)
A similar development happened in Europe in the early 1990s. Broadcasters, equipment manufacturers and regulatory agencies came together to discuss the formation of a group that would oversee the development of digital TV in Europe. A Memorandum of Understanding was then drawn up by the stake holders and was signed in September 1993 giving birth to the DVB project. The objective of the group is to ensure open inter operable standards for the global delivery of digital media services. The modulation used in DVB-T is COFDM that provides data payload up to 19 Mbps (depending on type of modulation used: QPSK, 16QAM and 64QAM). With this, it is possible to transmit one HDTV program or up to four standard definition programs in a 6, 7 or 8MHz channel. COFDM works in this manner: it splits the information stream into several data streams, which then modulates a number of adjacent carrier frequencies within a TV channel (8, 7 or 6 MHz). For DVB-T, there are two modulation schemes in terms of the number of carrier frequencies: 2K (actually 1,705 carriers that are 4 kHz apart) and 8K (actually 6,817 carriers that are 1 kHz apart). DVB-T also employs a "Guard Interval" technique that allows receivers to ignore received data for a short period of time to minimize the effects of multi-path reception. It is this characteristic that gives DVB-T the advantage over ATSC when it comes to multi-path performance making the former more suited for mobile applications. The same Guard Interval feature also allows single-frequency network (SFN) operation for DVB-T, where two or more transmitters carrying the same transport stream operate on the same RF channel within the same geographical area.
4. Integrated Services Digital Broadcast-Terrestrial (ISDB-T)
Digital TV broadcasting using the ISDB-T standard started commercial service in Japan in December 2003 and gone nationwide in December 2006. There are now 30.6 million ISDB-T receivers and 20.5 million mobile TV phone being used in Japan as of December 2007.
Figure 2: ISDB-T
ISDB-T also uses COFDM modulation with almost all the same techniques as DVB-T: QPSK, 16QAM and 64QAM modulation variants; 2K and 8K number of carriers; and use of Guard Interval allowing mobile applications and single-frequency networks (SFN). The Japanese, however, introduced additional improvements to these techniques. What they did is divide the 6MHz TV channel bandwidth into thirteen (13) segments. One of these segments is dedicated to mobile transmission providing mobile TV service alongside HD or SD programs in the same transmission facility. ISDB-T also uses Time Interleaving technique providing more improvements in reception performance especially under noisy, mobile and indoor reception conditions. Another important standard feature they added is Emergency Warning System (EWS) that automatically provides warnings to the population when there are emergencies. In ISDB-T, it is possible to transmit one HDTV program or up to four standard definition programs plus one 1seg program (for cell phones and other portable mobile devices) in a 6MHz TV channel.
5. Features and Deployment Comparison
Figure 3: Technical comparison
Based on the data of figure 3, DVB-T and ISDB-T are similar in some ways because they use the same multiple carrier COFDM modulation technique. This allowed the two standards to provide mobile broadcast services and deploy single-frequency networks (SFN) in same geographical areas. ISDB-T utilized an additional technique called Time Interleaving providing more improvements in noise suppression and better mobile and indoor reception. ATSC uses only a single carrier that gives it advantage when it comes to signal interference. It is suited for high-power, large area transmissions as in the case in the US where single transmitters cover large geographical areas. There are also very exciting developments going on in digital terrestrial television like DVB-T2, a second generation DVB-T technology that will facilitate the broadcast of HD programs and data services, taking advantage of advances in modulation and video compression technologies. ATSC is also evaluating proposals to add mobile TV capabilities to the standard. The two systems under consideration are the Advanced VSB System developed by Samsung and Rohde and Schwarz and the Mobile/Portable/Handheld (MPH) system developed by Harris and LG/Zenith.
Based on article written by Diego Villa