WHAT IS DIGITAL TELEVISION?
Digital terrestrial television broadcasting (DTTB) is a new type of broadcasting technology that provides a more effective way of transmitting television services.DTTB can:
- provide for better reception of television services than the current analog systems, including the PAL system used in Australia;
- deliver higher quality picture and sound than the PAL system;
- provide high definition television (HDTV) programs or a number of standard definition television (SDTV) programs within the standard broadcast channel;
- reconfigure services (ie change from a single HDTV program to a number of SDTV programs) at any time; and
- carry a range if multi-media services in the form of audio, images, data and text.
How does it work?
DTTB systems use advanced digital techniques to convert an analog signal to a digital signal which is then compressed, along with other signals, before being broadcast from a transmitter. With digital transmission, sound and pictures are processed electronically and converted into binary digits (bits)-a series of zeros and ones. This code is then transmitted as a bit stream and the receiver converts the digital transmissions back to graphics and sound (or text etc). Video compression allows the transmitter to send only the data needed to pass on the difference between each picture frame, rather than the whole picture, therefore removing repetitive information and enabling several digital services to be transmitted within the same frequency.
A multiplex brings together all the television programs (and associated data and sound services) on a single frequency channel.
What is High Definition Television?
HDTV offers approximately twice the vertical and horizontal resolution of a PAL signal, providiing a picture quality approaching 35mm film and a sound quality approaching that of a compact disc. HDTV is particularly suited to large screen television display. It has an aspect ratio of 16:9 and at least 1,000 lines making up the picture-existing conventional television pictures use a 4:3 aspect ratio and 625 lines. SDTV is a digital television system in which the picture quality is approximately equivalent to the current PAL television system.
DTTB brings to an end the direct relationship between one television program and one frequency. DTTB is capable of carrying either one HDTV program, or up to six services using SDTV, or as many as ten services with lesser definition formatting. As with computer technology, it is possible to trade off the bit rate, the channel width and picture quality.
A DTTB channel can be reconfigured at any time to allow for either HDTV or multi-program services. The number of services that is provided depends upon the picture format quality desired at the time. For example, fast-moving live sports could utilise the full HDTV capacity, but for a 'talking head' newsreader SDTV or a lower quality could be used.
Essentially, the type of picture determines how much of the channel's capacity is needed for transmission. A DTTB channel can carry up to 20Mbits/sec of data. HDTV services would use most if not all this capacity, but an SDTV service would use considerably less depending on the nature of the service, eg fast moving sport could require up to 10Mbits/sec data rate and hence possibly only two of these services could be delivered at the one time. By comparison, a 'talking head' picture would utilise about 5Mbits/sec of data.
What bandwidth is required?
DTTB systems can accommodate 6, 7 and 8MHz channel spacings with minimal or no apparent cost disadvantage. Australia uses 7MHz channel spacing for analog services, the USA uses 6MHz and Europe commonly 8MHz although there is also some 7MHz use.
What spectrum will DTTB use?
DTTB can be accommodated within the existing broadcasting frequency bands, generally in UHF but also in VHF bands, using vacant channels adjacent to analog services.
These channels often cannot be used for additional analog services, because of technical constraints inherent in analog systems, but can be used for DTTB as DTTB receivers are expected to tolerate higher levels of co-channel and adjacent channel interference.
The allocation and planning of specific frequency bands for digital television will be a policy issue for consideration by Government.
What about broadcasting the analog and digital services simultaneously?
Both analog and digital services can be broadcast simultaneously ('simulcasting') until digital services become commonplace so that viewers have a reasonable period to purchase digital television sets and/or set top units.
The establishment of a simulcast period will be a policy issue for consideration by Government. At present it is proposed that the simulcast period will continue until 2008. This is subject to a review by government.
What systems are available?
Three systems have been developed:
the US system developed by the Advanced Television System Committee (ATSC) the European system developed by the Digital Video Broadcasting (DVB) group the Japanese system developed by NHK.
In December 1996, the US Federal Communications Commission adopted the DTTB standard submitted by the ATSC. The standard provides for the transmission of DTTB in a 6MHz channel. It provides for a range of video image formats based on MPEG-2 (Motion Picture Experts Group) video compression. Audio compression is based on the Dolby AC-3 system supporting five surround channels plus a subwoffer channel and the transport subsystem is based on the MPEG-2 transport stream. The RF Modulation is 8 VSB which allows a 6MHz terrestrial broadcast channel to support a digital bit rate of 19.3 Mbits/sec.
The US standard does not mandate requirements with respect to scanning formats, aspect ratios and lines of resolution. This allows for various picture formats (eg HDTV or SDTV) and allows for computer monitors to be adapted to receive DTTB.
The European standard was developed by the DVB group which consists of broadcasters, manufacturers and regulatory bodies and was formed to oversee the development of DTTB in Europe. The DVB group standard comprises a core system which is intended to provide for a family of systems covering all transmission media: satellite broadcasting; cable broadcasting; television distribution; terrestrial broadcasting for 7 to 8MHz bandwidths (known as DVB-T); multi-point distribution broadcasting below 10GHz; and multi-point distribution broadcasting above 10GHz. The DVB-T system uses the common family of picture coding and compression based on MPEG-2, shared with other members of the DVB specifications. DVB-T uses a transmission system based on Orthogonal Frequency Division Multiplexing (OFDM) modulation. DVB-T has been submitted to the European Telecommunications Standards Institute (ETSI) for acceptance as a European standard.
Japan has also been developing a system, called Integrated Services Digital Broadcasting (ISDB), which is intended to provide for the integrated transmission of a wide range of digital audio, video, data, and interactive services. It is based on MPEG-2 video compression, but more complex multiplex and transport schemes are needed to handle the variety of services proposed.
What receivers are needed?
DTTB can offer viewers more choice, new types of interactive services, improved reception, and better picture and sound quality.
Manufacture of receivers is commencing to meet demand in Europe and the USA. The likely options for consumers will include:
- widescreen and HDTV sets which will accommodate the full effects of DTTB;
- standard screen digital set which will still receive HDTV but not with full HDTV effect;
- a set top unit to covert the digital signals for existing PAL sets, which will still receive HDTV but not with the full HDTV effect; and
- computer monitors/television receivers which will still receive HDTV but not with the full HDTV effect.
What is being done in Australia?
The introduction of DTTB in Australia is in the advanced stages of planning.
The Australian Broadcasting Authority (ABA) formed a Specialist Group on DTTB which released its final report in January 1997. The Specialist Group recommended, among other things, that:
- all existing broadcasters should be given access to a full 7MHz bandwidth DTTB channel and be given full control over the use of the delivery capacity; and
- DTTB services should be based on delivery of HDTV services from the outset, but broadcasters should also be permitted to use the multi-programming capabilities.
In July 1997, the ABA made a response to the Specialist Group report supporting the conclusions of the report. Copies of both the Specialist Group report and the ABA's response are available from the ABA.
The Department's Communications Laboratory and the commercial broadcasters have been conducting comparative bench and field trials of the US ATSC and the European DVB transmission systems. The findings of these trials and the recommendation of the selection committee were released in mid 1998. A report on the selection process was also produced by FACTS.
When will Digital TV Commence in Australia?
The Government legislation nominates a launch date of 1st January 2001 for the major capital cities with regional centres being covered by 2004. Digital test transmissions are already operating in some cities.
OVERSEAS DEVELOPMENTS IN DIGITAL TELEVISION
- DTTB commenced Operation in November 1998 using the European DVB system.
- All broadcasters are being converted to DTTB.
- Broadcasters are required to simulcast but will also have capacity to provide new services.
- There are six multiplexes. The BBC has one multiplex. Channel 3 and 4 have joint control of a multiplex. Channel 5 will share a multiplex with S4C in Wales. Licences for the other three multiplexes, which provide new services, have been issued to OnDigital.
- DTTB is due to commence late in 1998 using the ATSC system. (Now Early 1999)
- All existing broadcasters are being converted with access to a full DTTB channel each.
- Broadcasters are expected to but not required to provide HDTV.
- Some simulcast will be required.
- Analog services will end in the year 2006.
- Other countries interested in or commencing DTTB include Japan, Canada, Germany, France, Finland, Denmark, Sweden and New Zealand.
Copyright © Commonwealth of Australia 2000· Last update: 30-Apr-99
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