Following last month's exploration into bridging solutions between satellite DTH and terrestrial DVB-T2, this second piece of our two-part article provides an overview of new and innovative solutions for delivering local content using advanced multiplexing technology to bridge the satellite and digital terrestrial broadcasting worlds.
Multiple PLPs
DVB-T2 networks are inherently flexible with regard to adding regional content. DVB-T2 may be operated in single Physical Layer Pipe (PLP) mode or multiple-PLP mode.
Single PLP operation means that the total capacity is used for a single transport stream (TS), and in this case, deterministic processes would be the preferred approach for local adaption. (See Figure 1 on page 54.)
Multiple PLPs may be used to carry content for multiple regions or content for multiple device types such as mobile phones. When using multiple PLPs to deliver regional TV services, it is normal to distribute a T2 MI signal with one shared PLP containing the national content, and one dummy PLP that later will be replaced by a regional PLP at the transmitter sites. For SFN operation, the timing of the T2 MI signals needs to be synchronized by means of the T2 time stamps.
This approach may be useful in some situations, but using multi-PLPs introduces limitations and operational drawbacks. The most serious limitation is that a T2-MI stream over satellite cannot be used to combine DTH broadcasting with the feeding of terrestrial transmitters.
T2-MI stream contains a national PLP carrying the services going to all regions plus a dummy PLP. The multi-PLP signals may be distributed over satellite. In this particular case, the dummy PLP carries the content for region 1 and thus is not required to modify the T2-MI stream in that region. At the transmitters in the other regions, the T2 bridge will replace the dummy PLP with a regional PLP, while a correct T2-MI signal will be delivered to the T2 exciters. The T2-MI signal cannot be used to feed DTH subscribers as no set-top decoders are able to parse the T2-MI signal. In this particular example, the regional T2-MI stream is distributed over IP to the transmitters. By using S2 multi-stream, it is possible to transport multiple regional T2-MI signals over the same channel.
The broadcast signal is distributed over satellite and is a TS containing the video and audio content. Also, the multiplexer includes TMP packets used for controlling the re-multiplexing process as well as reconstructing timing. The satellite signal may also be used to feed DTH subscribers since the signal is compatible with a normal S2 set-top box. At the transmitter site, the T2 bridge will drop all but the national services and relevant regional service, as well as PSI/SI information for this region. These components are combined into a correct T2-MI signal and delivered to T2 exciters. (See Figure 2.) The deterministic processing ensures multiple transmitters in the same region generate identical T2 MI signal as required for SFN operation.
Comparing alternatives
For single PLPs, the entire capacity is available within one entity. This offers greater flexibility in using the capacity than if it is split into two transport streams. Think of splitting a three-lane highway into two highways — one with two lanes and the other with one. As the ability to switch lanes is lost, the flexibility and capacity of 2+1 lanes is restricted compared to the full three.
With a multi-PLP approach, copies of all service information (SI) tables in every PLP are necessary in order to display a complete EPG for the viewer. With the single PLP alternative, only one copy of the SI is needed to build a complete EPG, meaning less overhead when running a single PLP.
With a single PLP, bit rates can be allocated per service, whereas in multi-PLP, bit rates are fixed per pipe. For different regions, it may be necessary to insert differing numbers of local channels. Therefore, a degree of flexibility is required when dimensioning the bit-rate allocations per region. Using a single-PLP allows for more flexible composition of national and regional capacity. The multi-PLP is inflexible in terms of bit-rate allocation.
Freedom to build a DVB-T2 network using multiple sets of modulation schemes results in different bit-rate capacities and enables operators to best cater for different challenges like high-rise urban and rural environments.
SI tables
SI tables are used to generate an overview of channels and programs and present it in the form of an Electronic Program Guide (EPG). This contains information about current and future programs. Operational experience shows that viewers appreciate a correct and complete EPG that covers both the national and local TV and radio programs. When generating local multiplexes, it is critical to cross-carry SI from both the national multiplex and the local channels.
An accurate and complete EPG is essential for the programming and correct operation of PVRs (i.e. start recording and stop recording).
Multiplexing functionality
The techniques described in this article, although new, are either already deployed or are technically feasible, and will be deployed in the coming year. These techniques are now being implemented in new IP and ASI transport stream remultiplexers that have been specifically designed for DVB-T2 regional adaption.
These new remultiplexers are capable of individual bit-rate policing of services in the transport stream, ensuring that if one service exceeds the allocated bandwidth, only that service is affected instead of the entire multiplex. A maximum bit rate can be assigned to a group of components or to a single service. These remultiplexers ensure graceful degradation of non-conforming services by discarding the least important components first. This frees up considerable bandwidth previously allocated to allow overflow, enhances QoS and creates zero impact on conforming services.
Some broadcasters already employ some of today's most advanced remultiplexing solutions for more efficient bandwidth utilization. In the UK, one broadcaster deploys an SI Manager — in effect a specialized remultiplexer — for the efficient insertion of local EPG data. Among the benefits reported: bandwidth gains from the removal of null-packets; improved user satisfaction through a faster EPG; and the deployment of additional new services. Other benefits include: improved reliability with a secondary satellite backup feed; simpler management; improved day-to-day operation; and minimized cost of operator training thanks to enhanced ease-of-use. The company also uses a remultiplexing solution for bit-rate policing in order to improve uplink QoS for all customers by isolating errors related to bandwidth overflow.
Future DVB-T2 roll-outs will see increasingly advanced multiplexing and remultiplexing solutions improve operational efficiency and control.
Summary
By bridging the satellite, IP and terrestrial broadcast worlds, deterministic processes give operators much greater choice in how best to design and deploy their new DVB-T2 distribution network. Delivering HDTV and local content over an infrastructure that is prepared for future multi-screen TV services, in a state-of-the-art, terrestrial network, is good news for consumers and operators.
Dr. Ing Helge Stephansen is CTO at T-VIPS. B. Tommy Jensen is senior engineer at T-VIPS.
