Отчет мсэ-r bt. 2140-1 (05/2009)

Canada 4.1 National planning strategies and policy considerations

Размер2.7 Mb.
1   ...   41   42   43   44   45   46   47   48   ...   55

4 Canada

4.1 National planning strategies and policy considerations

4.1.1 Introduction

For almost 25 years Canada has carried-out, research, demonstrations, put in place a Task Force, Working Groups, Industry Associations, Regulatory initiatives with minimal government involvement and with a policy firmly based on the market place for the transition to digital terrestrial television. Although the core of all of this work has focussed on terrestrial television transition, there have been some notable diversions along the way including the Advanced Broadcasting Systems of Canada (ABSOC), which dealt with video compression issues for standard digital terrestrial television, cable and satellite.

ABSOC recommended that a digital Task Force look at all the issues surrounding the implementation of Digital Television (DTV) in Canada and the Government set one up in late 1995. It included all industry segments and completed its work in late 1997 with a report presented to the Ministers of Canadian Heritage and Industry Canada.

Following the Task Force report Industry Canada responded by accepting the recommendation to adopt the American Television Systems Committee (ATSC) transmission standard for terrestrial DTV services and made spectrum available to all licensed terrestrial television broadcasters for digital services. The broadcasters, distributors and manufacturers set up an industry association to manage and facilitate the transition realizing another recommendation, Canadian Digital Television (CDTV).

Over the next eight years CDTV working with the industry and the relevant interest groups and government departments, provided a platform for testing the technology, educating both the industry and the consumer, demonstrating HDTV services, and encouraging the production and distribution of HDTV programs and services. Over this period, the Canadian Radio-television and Telecommunications Commission (CRTC) also provided a regulatory framework for terrestrial television broadcasters and pay and specialty services to make the transition to digital High Definition service. The important point to note is that the emphasis of all of these initiatives was not just the introduction of DTV service but that service providing HDTV programs. The benefit for the citizen/consumer was defined both informally and formally as improved video and audio as characterized by HDTV.

In 1999, the industry defined Canada’s DTV transition strategy as a fast follow by two years of the US roll out of DTV services. This strategy was consistent with the market place approach and ensured that the high-end costs associated with early adoption of new technology were avoided for both broadcasters and consumers.

A lot has changed in the broadcast environment since the beginnings of HDTV in the eighties. Broadcasters have lost market share to viewing in both in real terms to pay and specialty services as well as viewers receiving their service directly from the transmitter in favour of distributed cable and satellite. More than 30% of all viewing was from terrestrial transmitters in the eighties where today that figure hovers around 10% or even lower in some markets. Consequently, broadcasters have been reluctant to build digital transmission infrastructure noting that there simply is not a business case to do so. There are currently

12 DTV transmitters on the air concentrated in Toronto, Vancouver and Montreal, even though more than
40 temporary licenses have been granted.

Over this time, progress was made in creating digital HD infrastructure in network operations of the major networks and the production community is just now beginning to embrace HD production. However, for the most part the Canadian terrestrial television broadcast system remains a standard definition one (as do the pay and specialty services) and in many regional centres an analogue throwback.

It is against this background that the CRTC is conducting a television policy review and the Minister of Heritage requested an examination of the impact of new technology on the Canadian Broadcasting System. A lot has changed since the Task Force reported 9 years ago. Internet delivery, Video on Demand, mobile television and consumer empowering personal video recorders and devices have and will have an increasing impact on the traditional broadcast model and in fact on the fundamentals of the Canadian Broadcasting system as Canadians have historically understood it. Decisions made by the CTRC, Government and the interests of the Broadcasting system over the coming 12 months will have a profound impact on the future of broadcasting generally and the roll out of conventional terrestrial broadcast services in particular.

The remainder of the paper will look more closely at the history, present circumstances and future options.

4.2 DTV/HDTV History

4.2.1 The Early Years

Canadian engagement with digital television is rooted in the industry’s early interest in High Definition Television (HDTV) as far back as 1982. In that year, the Canadian Broadcasting Corporation (CBC) and the Department of Communications and its research centre organized a Colloquium in Ottawa that drew delegates from all over the world to discuss HDTV and how to develop it as a future service. For almost a decade, there were follow up conferences, demonstrations and debate.

It is probably fair to say that the Department of Communications led a lot of Canada’s participation through the eighties and into the nineties. In 1987, a major public demonstration of the Japanese MUSE system of HDTV was done with the cooperation of government, a number of Canadian industry players and the Japanese. It was successful but not practical for terrestrial display in North America because of the amount of bandwidth needed for broadcast, although the Japanese used the MUSE technology from the late eighties through to today via Satellite DTH. At the same time, the CBC produced the first North American High Definition program series, Chasing Rainbows.

As the eighties drew to a close the Canadian Government was involved in that process testing proponents of five different systems in 1991/92 and then the eventual successful effort in the mid nineties. Canada worked closely with US industry and agencies in this process. At the same time Canadian industry recognized the need to become involved in the digital initiatives became apparent and in 1990 ABSOC was set up to perform that role.

From 1990 through to 1997 ABSOC played an important role of both informing the industry on digital developments and recommending standards and practises for MPEG 2 compression technologies as it effected production and distribution of standard digital television. Representing a cross section of the broadcast and distribution community with government liaison and support ABSOC brought a practicality and application to the new digital technologies as they developed.

As the initiative matured and accepted a new digital transmission technology capable of delivering High Definition signals within MHz of spectrum or multicast digital delivery of standard television, ABSOC came to realize that Canada needed to focus on what this new technology meant for Canadian viewers and the broadcast industry. They recommended a Task Force to examine the elements required to implement digital television in Canada and the government responded by naming a Task Force in November of 1995.

It is important to understand the environment that Canadian broadcasters enjoyed in the mid nineties. Although conventional broadcasters faced increasing market fragmentation, they still enjoyed a transmitted market share of their viewers of over 20%. Although pay and specialty services were growing, they had not fragmented the audience share to the degree that would develop and is seen today. The internet as a delivery mechanism, video on demand and other platforms that define today’s multi platform broadcast world were barely a dream very much on the horizon but in a business sense not a huge blip on anyone’s radar screen. By the end of the nineties, the view of the broadcast world was rapidly disintegrating. What was real was MPEG 2 compression, which made possible digital standard television satellite and cable delivery. Providing for more pay and specialty services with cheaper delivery to Broadcasting Distribution Undertaking (BDU) head ends and production facilities, and the prospect of better quality pictures and sound with HD services very far down the road.

For the newly announced Digital Task Force these problems were all in the future and it focussed on its mandate to recommend the best way to implement digital television for Canada.

Digital Television Task Force

The Task Force was truly representative of all industry interests plus the production and consumer manufacturers’ community. Over ninety people were on the Task Force or committees and many more were consulted throughout the Task Force’s work. It has been noted that Canada does Royal Commissions and Task Forces very well, as they are often vehicles for inaction. However, they also do some remarkable work from time to time and by the time the Task Force reported in late 1997 an industry had been somewhat educated, consulted and had arrived at a consensus; albeit kicking, screaming and probably thinking that many of it’s recommendations were so far down the road that there was nothing really to worry about.

The seventeen recommendations were rooted in the work of four committees who recommended the substance to the Task Force members. The committees included; technology, production, policy and regulation, and economics, consumer services and products. It is interesting to note as Canada moved to an implementation stage those areas of work continue to provide guidance and direction. While it is not useful to review the entire Task Force report and recommendations, it is useful to recognize that much was achieved and many recommendations were acted on:

The ATSC transmission standard, A53, was adopted by Canada and a subsequent allotment plan was adopted providing digital spectrum for all licensed analogue conventional broadcasters. Broadcasters were to make the transition to digital transmission while retaining their analogue spectrum for simulcast until the transition was complete. This was important since it provided a secure business basis for broadcasters to begin the transition.

Many of the policy and regulatory recommendations have found their way into CRTC licensing and carriage frameworks. Again, this was to provide stability during the transition for the industry business models, as they were understood at the time.

A period was suggested for the digital transition with an end date that would be a year to 18 months behind the US. While not acted upon in Canada, virtually every other country in the world has either a notional or a firm target date for analogue shutdown. The Canadian transition has lacked clarity and definition in the absence of such an initiative.

Initiatives concerning the production community for training and HDTV content were never acted upon and regrettably this industry sector has lagged behind many in the global community and Canada has a lack of HD production.

The recommendation to set up an industry organization to help manage, facilitate and advise government on the transition was put into place and will be discussed later in this paper.

Some recommendations like that calling for a universal box which would work for terrestrial television and distributed BDU services were not realized and probably too idealistic.

One recommendation calling for universally available terrestrial services is worth noting:

Recommendation Fourteen”

Basic terrestrial broadcast television services that are freely and universally available are central to achieving the objectives of the Canadian broadcasting system. This must continue in future digital terrestrial distribution packages.

Freely available broadcast television services are the foundation of the Canadian broadcasting system. This universality of access must be preserved in the emerging digital system.”

This was fundamental to the system in 1997 but in today’s environment terrestrial broadcasters are not committed to this principle given the change in how viewers receive there television services. In fact, the costs associated with this recommendation and the lack of any kind of business case will characterize the discussions of future policy hearings. This issue has also characterized the industry reluctance to move ahead with the digital transition in a timely way.

In looking back, the Task Force got many things right as evidenced by the overwhelming number of recommendations implemented. It set the agenda for the transition for terrestrial services and coincidently the pay and specialty services. However, it did not anticipate the rapid change in the broadcast environment; its multi platform distribution opportunities and the availability of the devices, which would empower consumers with both choice and schedule. Combined with a market place approach these factors inhibited a timely transition to digital High definition services.

Implementation 1998 to 2006

Following the Task Force report the broadcasting and distribution industry, along with manufacturers and producers came together to create CDTV, as recommended by the Task Force. In September of 1998 the organization was formally created as a not profit association, with by-laws, a Board of Directors based on industry sectors and a work plan. Relevant Government Departments and the CRTC were welcome to participate and contribute to committee work and observe in Board meetings.

The Board created Working Groups in the technology, policy and regulation, economics and marketing, communication and education and production. This was not very different from the original Task Force committees. These working groups were a part of the association to a greater or lesser extent through the life of the association responding to the approved work plans from the Board and the changing environment

The work of the association was totally funded by the industry with both direct and indirect funding. Industry Canada provided funds to test the frequency allotments at the CDTV test transmitter in Ottawa in 1998/99.

For eight years, CDTV represented the industry in helping manage and facilitate the transition. The early years focussed on testing, education, and understanding the standards. As time passed demonstrations, seminars, policy, regulation and business models dominated the agenda. Over the last few years CDTV focussed on operational implementation, the creation of HDTV programming, consumer education and awareness, and the impact of new technology including; improved compression technology, IPTV and mobile service. Throughout its mandate, CDTV participated with ATSC committees and on the Board, bringing back to the Canadian broadcasters and relevant government departments and agencies changes and improvements to the ATSC family of digital standards and Canadian input to those discussions.

An industry association that tries for consensus on issues, or at the very least an overwhelming majority is not the easiest of vehicles to manage in an environment of competing interests and agendas. The consensus and goodwill, which characterized the Task Force was not always seen as CDTV grappled with some of the business and regulatory issues where the interests of the principals were seen to be on the line. Yet for all of that the achievements were many over the life of the association and in fact defined the steps of the transition to digital terrestrial television to date.

Test transmitters were set up and operated in Ottawa, Toronto and Montreal. These gave the broadcasters and distribution communities the opportunity to work with the new digital transmission standard, understand its properties, coverage areas and delivery to BDU head ends.

The transmitters were used to test the frequency allotments (funding from Industry Canada), coverage reach, receiver strength and signal strength. This work became increasingly important, as improvements were made to off air receiver reception.

Canada was also called upon by consumer electronic manufacturers and the ATSC to test improvements and additions to the ATSC family of transmission standards.

Demonstrations for both the public and the industry of HDTV programming and delivery on the Canadian broadcasting system.

Seminars and workshops were held to explain to and educate the industry on the full range of the issues surrounding the production and distribution of digital High Definition programs.

A great deal of time and effort was spent on attempting to develop business models that digital terrestrial television in terms of program and non program related data and multi channel delivery. It was hoped that these models could lead to additional resources to help fund the transition. While the process certainly educated the industry there was not a consensus on the right model or an agreement between the conventional broadcasters and the distributors over revenue sharing of distributed terrestrial data and services.

Costs for the transition were also carefully calculated and included transmission, master controls, editing and production all in high definition. Suggestions for upgrading as equipment became obsolete were made available so that the capital costs of conversion would not be an overnight hit and distort budgets. Again, the identification and process were helpful but no overall industry plan was adopted.

Very early in the transition the Board of Directors of CDTV created the policy of a two-year lag behind the US in Canada’s transition to digital television. This built on a recommendation in the Task Force report that suggested a year to 18 months. Given the Government’s view that Canada’s transition to digital high definition broadcasting should be driven by the market this two-year lag policy was sensible and virtually adopted by all parties. It was successful in saving the industry and consumers a great deal of the costs associated with the early adoption of new technology.

Education and consumer awareness was a major focus of the transition work. This work involved not only the broadcast and distribution industry but the consumer electronic manufacturers and the retail sector as well. Several editions of pamphlets aimed first at the retailers and then directly at the consumers were prepared and delivered through retail outlets and reprinted in consumer electronic magazines. They explained digital television and all the choices and variables in services, programs and consumer equipment. This work was recognized as an effective tool in education and adopted by other countries as part of their transition work.

From the work done on consumer education it was decided that a web based information source of information would be a useful tool. CDTV resourced and created a bilingual consumer section open to everyone on its website. Since its creation a couple of years ago hundreds of thousands of Canadians have used it to gather more information about HDTV. In addition a 15-minute infomercial and several 30 s promos were produced and aired to both provide HD information and push people to the website. Similar efforts will be required in the future, as analogue shutdown becomes a reality in Canada.

The education, training and development of the independent production on HD production were the final major projects taken on by CDTV to aid the transition. Again, a bilingual website was created that contained information and practical experience about, equipment, facilities, production and editing of HD material. Originally conceived as a series of training modules that may be adapted to workshop environments, the website has proven a valuable tool for Canada’s content creators. It is sad to note that additional funding could not be achieved to run workshops in all regions of the country to work with the production and broadcast community to create a better understanding of the challenges associated with HD production and how to meet these practically and efficiently. The production of HD content is still very modest in Canada but this is beginning to change and it should be encouraged.

While the core mission was on terrestrial broadcasting a great deal of time and effort was spent on assisting pay and specialty services to make the transition and supporting their needs for effective policies and regulation, facilities and capacity, and education.

During this period CDTV became the principle source of HD information in Canada for both trade press and general media. In the late nineties and in the early part of the two thousands the interest tended to be more industry related but today the Canadian consumer is engaged and very hungry for relevant information. Importantly, it is not about digital television that engages the consumer but it is High Definition, which is capturing their interest.

It is probably fair to ask if a transition association like CDTV was working so well, why it ceased its work a few months ago. Probably for two basic reasons:

The environment in 1998 was very different than it is today. There was less concentration in the broadcast industry and generally more reliance on associations to represent the industry sectors in designated areas. Emerging platforms and new technologies like IPTV and mobile applications were not a huge market factor in 1998, yet they are increasingly dominating discussions today.

At the core broadcasters, who were to make the transition from analogue to digital transmission platforms, drove CDTV. As markets fragmented and viewing reception for transmitter received services declined, the consensus achieved by the Task Force to transit to digital transmitted services began to break down and eventually eroded the support for an association whose mandate was to see the transition through.

With the above in mind, the industry members felt the association had gone as far as it could and its mandate was complete from their perspective given the new environmental realities. Many elements of this 8-year phase of Canada’s DTV transition were done well and made substantial contributions to the process. Issues of timeliness, a focus on what the Canadian broadcast system should be when the transition is complete, and an end date for analogue needs to be urgently answered before the transition may proceed.

The Present

The Current Players and the Issues

Canadian broadcasters have demonstrated reluctance to build transmission infrastructure and thus there are only transmitters in Toronto, Montreal and Vancouver as noted earlier in this text.

Conventional broadcasters have invested in considerable digital HD equipment in their network centres but very little in regional locations across the country. To date they have depended on cable and satellite delivery of their HD signal to locations across the country. In some cases because of cable and satellite bandwidth constraints and the strict application of the carriage rules, this national coverage is not as good as the broadcasters would like.

There are no French language networks, which are providing digitally transmitted HD or SD services aside from SRC. Most of the transition developments have been within English services. While there have been more than 40 temporary licenses issued there have been relatively few actually act upon. Most of these are English services. With some 12 transmitters on the air and broadcasters reluctant to build out their digital transmission infrastructure the future of conventional terrestrial television, has we have historically understood it, seems to be poised for a change.

Digital HDTV set penetration is projected to be over 3 million by year-end in Canada and most of the sets now coming to market have built in tuners.

Hook ups to HD services from a BDU are still modest in Canada with numbers approaching 600K by year-end in Canada. This figure is expected to dramatically increase over the next few years.

It is difficult to asses IPTV, mobile, and multi platform delivery and their impact on the terrestrial digital transition. All industry sectors are coping with these challenging issues and they are increasingly becoming central issues in developing future business models. However, it is a difficult to suggest that conventional broadcasters have not made the transition to transmitted digital services because of these emerging technologies. At this stage, they are just too peripheral to the core business. The only apparent reason is the declining viewing to terrestrial services directly from the transmitter and the costs of duplicating the existing analogue system with digital transmitters for a decreasing audience return. In simple terms, there is no business case.

Although this paper focuses on terrestrial television it is important to understand the steps taken by the BDU industry to increase capacity that provides both more choice and HDTV capacity. Cable has worked to upgrade its capacity in recent years and has migrated its customer base to digital delivery with demonstrable success. The end of analogue conventional television would ease the bandwidth crunch that is clearly apparent in a transitional environment. Measures to speed up this process would benefit both the consumer and the industry interests. By necessity, these measures must be part of an agreed overall transition plan with a firm analogue shut off date.

Satellite DTH providers are already all digital but face similar capacity issues in this transitional phase which must be addressed. Likewise, Satellite carriers will face increasing demand and capacity issues as more services move to digital HDTV demanding more bandwidth in a finite satellite universe. Delivery to BDU head end, collection and backhaul in a HD environment puts tremendous pressure on the carrier and cost for the service provider whether conventional or pay and specialty. New compression technology and new Satellites may well be part of the solution for DTH providers and Carriers but a definable end to the digital transition would provide some certainty in the market place for all the players.

The above discussion provides some of the background that the recently held Television Review and the Canadian Government Directive concerning the impact of new technology on the future of broadcasting has considered. The reports and decisions, which arrive from it, will be very important to the future digital transition of the industry.

In reviewing the many submissions for consideration in this process, it was clear that most conventional broadcasters do not want duplicate their entire analogue transmitter structure and many see little or no future in transmitted services at all. The difficulty of these submissions is there seems to be no clear alternative or plan for what a new conventional broadcast system would look like in a non-analogue world.

Virtually every country in the world, which has embarked on a Digital Transition plan for terrestrial services, whether it includes HDTV or not, has a definable plan including scope and timeframe. The Canadian situation has suffered from this lack of definition and this now needs to be addressed.

Action Required

In order to expedite the transition of Digital Television, the regulatory process would have to address the following issues:

A policy decision about the future of terrestrial television.

If transmitted terrestrial services are to remain in the digital world do they mirror the current analogue coverage, a part of that coverage or not at all?

If there are Canadians disenfranchised by a decision to reduce transmitter coverage how do they receive their basic service?

Coincidental with this decision an analogue shut off date needs to be established with definable and measurable milestones.

A plan for informing the public and ensuring that all Canadians can receive a television signal with analogue shut off needs to be established.

The digital benefit for consumers needs to be defined (HDTV and/or enhanced choice) and realized by conventional and pay and specialty broadcasters.

Attention needs to focus on the new technologies; how they can both challenge and enhance the core conventional services in a multi platform environment.

Capacity needs to be assessed in the distribution system to ensure that all services that need to transit to digital HDTV can do so in a timely cost efficient manner. There will be a capacity crunch and it cannot be a barrier to transition.

A plan for regional and local participation in the digital transition needs to be addressed, including local HD production and services.

A plan for the creation of Canadian HDTV content in all program genres to service Canadian HD services that now rely largely on foreign produced HD product.

It is worth repeating that a great deal of good work has been accomplished in the last decade and it is important to see these suggestions in light of that work and building upon it. At the same time, the current transition to digital HDTV is in crisis and needs to be firmly put on track, particularly for conventional terrestrial broadcasters. Canada has gained a lot of first hand experience and knowledge of other countries and their challenges and triumphs. It is now time to take that experience and knowledge and resolve the future of the Canadian Broadcasting System in the digital HD world.

The Future

Given the changes to the broadcast environment in the last decade, it is difficult if not foolhardy to try to predict the future. None the less there is some givens that can shape our environment over the next few years.

High Definition programming will become the new norm over the coming years throughout most of the developed world.

All the new emerging technologies and platforms will have a business impact that will benefit and challenge the core conventional broadcast business in a multi platform environment characterized by quality, choice, and consumer empowerment.

Content will need to be created at the highest possible level of quality for shelf life and conversion for multi platform delivery. The 1080 progressive production standard will be the international HDTV program exchange standard. HD delivery will be either 720p or 1080 depending on spectrum availability and the nature of the service distributed

The ATSC family of standards will evolve to an advanced compression codec which will enhance the value of terrestrial television spectrum, this is already happening with the DVB-T standard. Future digital receivers will be capable of receiving both MPEG 2 and MPEG 4 signals (France is currently rolling out these boxes as part of their DTV transition).

Further work on the development of improvements in the ATSC system and receiver sensitivity with emphasis on work which may lead to solutions for wireless services and broadcast services in remote communities. This could be a part of the answer for bringing transmitted digital services to rural Canada.

A plan for analogue shutdown with a responsible agency or group who may be held accountable by the viewer and citizen will be critical to analogue shut off.

The Canadian Broadcasting System will continue to enjoy a balance of cable and satellite delivery along with the internet, and telecommunications services all providing real time, video on demand, and streaming services to the viewer. Consumer devices will enhance the viewer as programmer but for the foreseeable future conventional television will continue to drive the industry in terms of content and national, regional, and local reflection. Wireless delivery of these services has a role to play within this system.


Canadian distribution and collection of programming via satellite led the world in using this new technology to the benefit of broadcasting. Canada built the longest stereo FM network in the world. And Canada’s television production industry has thrived in the most competitive market in the world producing indigenous product for Canadians, while producing and selling for the rest of the world. Not bad! Canada has done so with the right balance of policy, regulation, incentives, creativity and entrepreneurial skill.

Canada is again at another critical point in its broadcast history. The environment has rapidly changed and yet the issue of valued Canadian services for all Canadians in all parts of the country remains as the constant core issue. Decisions made over the coming year will provide the framework that will define Canadian success in completing the digital transition to HD service for conventional broadcasting and in turn the rest of the system. These are important decisions that require a timely response. Not to respond will leave the current system in disarray and less relevant for both the Canadian viewer and the global community in which it has been a player.

ATSC-DTV distributed transmission network


Distributed transmission (DTx) network is a network of transmitters that covers a large service area with a number of synchronized transmitters operating on the same TV channel. DTx offers interesting possibilities for digital TV transmission systems.

As explained in the ATSC Recommended Practice for Design of Synchronized Multiple Transmitter Networks20, DTx networks have a number of benefits over the single central transmitter approach, which has so far been the usual way of covering a large service area with analogue TV transmission. These benefits include:

− More uniform and higher average signal levels throughout the coverage area

− More reliable indoor reception

− Stronger signals at the edges of the service area without increasing interference to neighboring stations

− Less overall effective radiated power (ERP) and/or antenna height resulting in less interference.

DTx networks can also reduce the number of channels used to cover a large service area and can free spectrum for other applications such as interactive TV, multimedia broadcasting, or any other application that may come up in the future.

As a trade-off for these benefits, implementation of a DTx network requires a very careful design when a DTV adjacent channel is operating in the same market area21. A more serious limitation on the DTx operation is that in the possible presence of NTSC adjacent channels operating within the same market area. In such cases, implementation would be very challenging if not impossible. This is due to the higher protection ratios required by NTSC, as opposed to DTV, from an adjacent channel DTV. However, such limitation will not exist after the transition period from NTSC to DTV.

Another important issue affecting the design of a DTx network is the ATSC-DTV receivers’ performance with respect to their multipath handling capabilities. Better receivers, capable of handling stronger pre- and post-multipath distortions (pre- and post-echoes) on a wider range of delays, make DTx network design more flexible and simpler. On the other hand, receivers with weaker multipath handling capabilities put more restrictions on the design and implementation of DTx networks.

In addition to providing many guidelines for designing a DTx network and managing its internal and external interference under different conditions, the above mentioned Recommended Practice proposes three methods (or their combinations) for implementing a DTx network.

DTx Methods

The first method is distributed transmitter network, commonly known as single frequency network (SFN), consisting of a central studio that sends baseband signal or video-audio data stream to the SFN transmitters via studio-transmitter-links (STL). STLs can be fiber optics, microwave links, satellite links, etc. The SFNs may be costly to implement and operate. The SFN transmitters in this configuration require subtle (and rather complex) processes for their frequency and time synchronization with each other.

The second method is called distributed translator network in which the transmitters contributing to the SFN, which are some coherent translators all operating on the same channel, translate the frequency of an over-the-air signal received from a main DTV transmitter to a second RF channel. This eliminates the need for a costly Studio to Transmitter Links (STL). On the other hand, frequency and time synchronization for this configuration is quite simpler than the first method. During the translation process to the designated output channel, necessary corrections may also be applied to the signal. In this configuration, however, the main transmitter feeding the coherent translators is operating on another channel and is not part of the SFN. But one may consider this as a sort of frequency diversity in the overlapping coverage area of the main transmitter and the SFN.

The third method consists of digital on-channel repeaters (DOCR) that can differ from each other in the way that they process the signal through the path from their input to their output antennas. The DOCRs contributing to the SFN again pick up their inputs from a main transmitter, eliminating the need for any STL, and transmit on the same channel as they receive. Each DOCR can work on the basis of direct RF operation, conversion to IF or to baseband and up-convert again to the same channel as it receives. In order to form an SFN, however, all the repeaters’ outputs should be synchronized with each other and also with the main transmitter feeding them.

With this approach, two limiting factors exist on the operation of the network. First, the main transmitter signal can create advanced multipath (pre-echo) in the overlapping coverage areas between the main transmitter and the repeaters. For creating pre-echo, the repeater’s signal must be dominant in such overlapping areas. This may be problematic to the ATSC legacy receivers that are vulnerable to pre-echoes. Second, depending on the amount of feedback from DOCR transmitting to receiving antenna, there is a power limitation on the repeaters’ output.

The Communications Research Centre (CRC) of Canada has already studied, by performing various field tests, different applications of direct RF operation OCRs and their performance under different conditions, and has published the results22, 23. The below study focuses on the second configuration of distributed transmission network, which is “distributed translators”.

Setup and Methodology

The distributed transmission network under consideration by the CRC consisted of three coherent translators. The translators received their input signal on channel 67 (788-794 MHz) from a medium power DTV transmitter having a tower height and EHAAT of 209 and 215.4 meters, and located at about 30 km south of Ottawa, Canada. This DTV transmitter covers Ottawa and its surroundings with an average ERP of 30 kW through a horizontally polarized omni-directional antenna system.

The translators converted the received channel 67 to channel 54 (710-716 MHz) through direct RF to RF operation. They were all frequency synchronized and their timing was adjusted to make them transmit with no delay with respect to each other.

The translators were installed on the top of three high-rise buildings in downtown Ottawa. They covered a common rectangular target area of approximately 1.66 by 1.14 km, and their output powers, which were between 15 to 25 W ERP (enough to cover the small rectangular target area), were adjusted to produce equal signal strengths at the centre of the target area. Figure 39 shows the relative locations of the three synchronized translators along with their overlapping target area. Also shown is the direction of transmission of the three translators’ output antennas and their 60º beam width. The main DTV station, which covers the whole Ottawa area including its downtown in which the DTx target area is located, is outside the map in the bottom right direction at a distance of 25 km from the centre of the target area.

Receiving conditions

The receiving conditions for these tests were intentionally selected to make a worst case scenario for the study. A single target area was selected for all three translators (see Fig. 39). In this way, the translators could create a lot of artificial multipaths (active echoes) in the target area. On the other hand, the downtown canyon, in which such target area was located, made the situation worse by creating additional static and dynamic multipath through reflections of each of the translator’s signal from high-rise buildings and moving vehicles (passive echoes).

Figure 39

Ottawa distributed translator network. The rectangular target area is 1.6 × 1.14 km

The measurement points were at the corners of the grids of a lattice covering the target area. A total of 59 points, at distances between 100 to 200 m from each other were measured. For the measurements, which were made on the street sidewalks at about 1.5 m above ground level (AGL), two types of antennas were used, an omni-directional antenna and a low gain directional antenna (usually used for indoor reception) with about 5 dB gain and 60º beam width.

Both antennas were made active by connecting them to a low noise amplifier (LNA) of about 1.2 dB noise figure and 20 dB gain, and also a band pass (BP) filter installed on the same stand as the antennas.

Characteristics of the receivers used for the tests

For these tests, two types of receivers were used, a new prototype, and an older generation receiver. The new prototype receiver, as compared with the older generation, was capable of handling pre-and post-echoes with a much wider delay range.

Figure 40 shows the relative attenuation of a single static echo at different delays, at which the receivers are at the threshold of visibility (TOV). As it is seen, the older generation receiver (Receiver G in the figure) could operate with about –5 dB echo in the range of –3 to +40 μs. The new generation receiver (Receiver V in the figure), on the other hand, could handle pre and post echoes over a wider range. It was capable of handling –10 dB pre- or post-echo with a delay spread of –50 to +50 μs, or –5 dB echo in the range of –25 to +25 μs.


rx characteristics

Performance of the two receivers used for the tests

Test results

In the first phase of the tests, the feasibility of implementation of such a network was verified. In the next phase of the study, measurements were performed in 59 points inside the target area. Table 8 shows the percentage of locations in which successful reception was achieved.


Percentage of reception points with successful reception

DTx (CH-54)

New Prototype Rx.

Older Generation Rx.

Directional Rx. Ant



Omni-directional Rx. Ant.



Main Tx (CH-67)

New Prototype Rx.

Older Generation Rx.

Directional Rx. Ant



Omni-directional Rx. Ant.



Table 8 shows the results for DTx (CH-54) and also for the single distant transmitter (CH-67), using the new prototype and the older generation receivers, and also using directional and omni-directional antennas. As it is seen, the results are somehow better, under all circumstances, with the DTx network as compared to the single transmitter configuration.

Comparison of the results, however, can be made based on the type of the receiver, type of the receiving antenna, or type of coverage. What is quite evident is that under any condition, the reception situation is remarkably improved when the new generation receiver is used instead of the older generation receiver. Another major improvement can also be seen with using directional antenna instead of omni-directional antenna for both DTx and single transmitter. This has probably been due to the attenuation effect of the antenna on signals coming from the directions other than the main signal and acting as multipath.

Another important result that can be highlighted from this table is the fact that the DTx network, as compared to single transmitter configuration, has improved the situation also for the older generation receiver under all conditions (although not significant in all cases). The most significant improvement is when directional receiving-antenna is used. Under this condition, distributed transmission could improve the percentage of points with successful reception from 36% for single transmitter configuration to 54% for DTx network.


For the study in this section, a distributed transmission (DTx) network, consisting of three coherent translators, was used to cover parts of the coverage area of a single transmitter. Two types of receivers and two types of receiving antennas were used and measurements were made in both channels corresponding to the DTx network and the single distant transmitter. The reception conditions were made very tough by choosing overlapping coverage area located in the hostile downtown environment for the DTx network, and also by making the measurements at 1.5 m AGL on the street sidewalks.

The results showed that the DTx network had better reception availability than the single transmitter, especially when omni-directional receiving antenna was used.

The results also showed remarkable improvement in the performance of a new prototype receiver in the SFN environment, as compared to an older generation receiver that was used in the tests. This was because of the major improvement in the multipath handling capabilities of the new prototype receiver, which makes the implementation and operation of ATSC distributed transmission networks possible and reliable.

Another important result was the impact of even small directivity of the receiving antenna on reception. Directional receiving antenna, as compared to the omni-directional one, could provide successful reception for a greater percentage of the measurement points.

The test results also demonstrated reception improvement for the older generation receiver under SFN operation. However, because that receiver was only one generation older than the new prototype one, more tests are required to investigate the performance of the legacy receivers in a distributed transmission environment.

Поделитесь с Вашими друзьями:
1   ...   41   42   43   44   45   46   47   48   ...   55

База данных защищена авторским правом ©vossta.ru 2019
обратиться к администрации

    Главная страница