As digitisation of television becomes more widespread, the focus is now shifting on achieving impeccable transmission. We look at some of the prerequisites for delivering quality and choice to the viewer in a cost-effective way It is common knowledge in the broadcast industry that all television transmission will eventually move from analogue to digital, and […]
As digitisation of television becomes more widespread, the focus is now shifting on achieving impeccable transmission. We look at some of the prerequisites for delivering quality and choice to the viewer in a cost-effective way
It is common knowledge in the broadcast industry that all television transmission will eventually move from analogue to digital, and in many parts of the world this transition has already been completed. The technologies available to achieve this are maturing, so we should now be considering how we can deliver the best viewer experience in digital transmission.
We can define three fundamental requirements for any transmission system.
The first and foremost requirement is quality. Audiences perceive digital transmission to be the route to consistently good images and audio. Wherever possible these should be available in high definition, accompanied by surround sound. After 60 years of analogue television, audiences also expect digital to be seamless and flowing smoothly, not beset by freezes and judders.
The second requirement, which is often used as a justification for analogue switch-off, is increased choice. While maintaining our first goal of quality, we want to pack as many channels into the available bandwidth as we can.
The third most important requisite is that we should do both these things cost effectively. Budgets are not expanding, so we have to find ways of delivering choice and quality without increasing operational costs.
We should also remember that various delivery platforms are now in competition. Ease of use and added functionality like the ability to record on PVRs, extra data and red button services are also important. One critical element of usability is to include the full service information (SI) on all multiplexes. Known as cross-SI, this enables viewers to browse the electronic programme guides (EPG) on other multiplexes, while still watching their choice of programme, for example, in a picture in picture window.
What follows are some suggestions of how these requirements can be achieved by optimising the implementation of DVB-T2.
Terrestrial transmission is still hugely popular because it is relatively simple for consumers to receive through a rooftop aerial. Apart from extreme topographical areas deep narrow valleys, for instance it is practical to reach close to 100% of the population. Extra television receivers in the home can be served with no additional equipment.
Typical transmission networks will have distributed hardware. Individual channels will be delivered from broadcasters to a national headend. There may be regional headends to allow for the insertion of localised content. A regional headend may feed a group of transmitters, or it could be located at a single transmitter.
Delivering the best quality signal to the headend allows the processor to be more aggressive with the delivery compression without degrading the viewer experience. We recommend using JPEG2000 compression for the circuits from broadcaster to headend to get the best quality for the bitrate. It gives you the option to squeeze harder in the headend, and thereby push through more content.
Multiplex
At the headend, a number of channels are multiplexed together into a transport stream, along with other data, such as EPG information, for example. Critical to operational efficiency is to use as much of the capacity as possible, without any content packets being squeezed out.
The traditional technique was to allow a significant part of the stream for stuffing packets to ensure sufficient headroom whatever happened. The result was the transmission of a large number of null packets: packets containing no data. In some systems this could result in as much as 1 Mb/s of null packets, which in a transport stream of 40 Mb/s is a big loss in efficiency and waste of available resources.
We developed a technique, originally for broadcasters in the UK, which intelligently manages EPG data in the form of SI packets in the headend. It interprets the data and calculates, according to defined rules, when it must be sent and how frequently it needs repeating. EPG information for the next 24 hours will be repeated more often than information for seven days ahead, for example.
An opportunistic data inserter then feeds SI information into the multiplex as soon as null packets appear. The result is a typical bitrate saving of 3 to 5% thanks to having virtually no null packets.
For the JPEG2000 signal from broadcaster to headend, and for the DVB-T2 stream from the headend to the transmitters, it is now standard practice to use IP networks. This is a significant operational cost saving over dedicated video lines and a major boost to efficiency.
IP networks are now extremely reliable, but the infrastructure must be carefully designed to overcome any problems which do occur. The edge devices in the DTT delivery platform must be protected against packet loss, burst loss, jitter and latency variations. The first step to overcoming these issues is through good forward error correction.
SMPTE already provides guidance in FEC for transmission networks, through its 2022-1 standard. DVB-T2, which is susceptible to packet loss in the distribution network, will work well given a good implementation. To support the most challenging cases, Nevion offers an extended version of the SMPTE FEC matrix so that the terrestrial signal can safely reach the transmitter sites.
For even higher resilience, some networks will consider redundant paths from the headend to the transmitter, with the receiving device selecting packets in real time from the stronger input. Broadcasters routinely use redundant paths for critical content like contribution circuits from major sporting events.
In some territories, the cost of building out a distribution network to a relatively large number of transmitters can be significant. The alternative is to use a satellite to distribute the national signal to transmitters or to regional headends. The national multiplexes can be streamed over DVB-S2 and received at any number of transmitter locations.
Single frequency network
Digital transmission has brought about a new way of improving coverage, the single frequency network (SFN). Traditionally, neighbouring transmitters, even if broadcasting the same content, needed to be on different frequencies to eliminate interference. Where countries border each other, there needs to be international agreements to ensure that frequency allocations are clear and one countrys signals do not affect reception in another.
The single frequency network, as its name suggests, puts all the transmitters in a region or a country on a single set of frequencies. It makes better use of the spectrum, and it allows the network to use many cost-effective lower power transmitters rather than a few high power transmitters. That improves coverage while reducing operational costs and releasing spectrum for more television and other services.
The challenge for SFNs is that each transmitter must carry an identical signal and be precisely synchronised. The lack of interference comes from the fact that each transmitter is radiating exactly the same information at exactly the same instant. As long as the distance to a receiver from the SFN transmitters are within the guard interval, the beams add up and become a stronger signal.
The other challenge is that broadcasters will want to insert local content into national channels. This may be regionalised advertising, or local news, or area-specific programming. Because the SFN expects the transport stream to be identical, the usual solution to local content is to combine the national and local content at a local headend.
To combine this with satellite distribution is clearly a poor use of bandwidth as the common services will need to be sent multiple times. A better option is to use a deterministic multiplexing solution which can allow local insertion. This uses a time stamp in the satellite feed and a set of rules for the multiplexing process.
This allows multiplexers in each region to rebuild identical streams as required for SFN operations. Each multiplexer at each transmitter site will build, packet for packet, an identical combination of national and local services. Deterministic re-multiplexing allows local content to be added to national signals without disturbing the SFN integrity of the transmission.
These technologies, along with other functionality and intelligent network monitoring and management, combine to maximise the effectiveness and efficiency of a digital terrestrial transmission network. Using the best of todays technology, broadcasters and network operators can offer their audiences a broad choice of channels, in excellent technical quality and high availability, while controlling the costs of the distribution and transmission network.
