Before going into the details of the platform, it might be useful to focus on the basic concepts and terminology of DVB-T.

This standard (together with other complementary regulations) deals with the coding and transmission of digital television signal. The conventional analog TV signal must be digitized and compressed according to the MPEG2 standard.

The encryption of each audio-video-data component generates a digital stream called Elementary Stream (ES). Each audio-video-data ES of the service is “packaged” by breaking up the ES flow into packages of variable length, identified by a Packet ID (PID). This results into a PES (Packetized Elementary Stream).

The different PES undergo a multiplexing operation that combines them into a single digital flow called Program Stream (PS). The combination of several PS generates the so-called Transport Stream (TS). The TS represents the fundamental logical level of transport. In detail, it is divided into Single Program Transport Stream (SPTS), consisting of different PES belonging to the same service and share the same time base, and Multi Program Transport Stream (MPTS), resulting from the multiplexing of several SPTS. Some PIDs are dedicated to the so-called PSI/SI, i.e. the information that allow to identify the TS and ES inside it. The information encrypted in this way are distributed to the emission sites through conventional telecommunication channels, such as radio links, and finally transmitted through multi-carried COFDM modulation.

The standard that defines the DVB-T transmission standard is ETS 300 744. Tables 17, E.5, E.6 hereto attached clear show how you can make a compromise between transport capacity (bit rate) and signal protection related to modulation parameters (Bandwidth, IFFT size, guard interval, FEC, Constellation).

A practical and common reference is the transmission on a 8 MHz channel in UHF band (a common situation in Europe) and fixed home reception with roof antenna. A typical choice is mode 8K, 64QAM, 2/3, 1/32 which allows to transmit about 24 Mbit/sec information. As the signal is MPEG2 coded, with the same bandwidth allocation of an analog channel and the same perceived quality, we can now transmit a bouquet of at least 4 or 5 audio/video services (the equivalent of current TV programs).

Transmissions are also possible with video and multi-language audio, only audio, only data o interactive DVB-MHP (Multimedia Home Platform) applications, whether associated to classical A/V contents or not. As the TS is a package-based multiplexing structure, described and managed through Program Specific Information and Service Information (PSI/SI) tables, a key feature of DVB is the possibility to build an EPG (Electronic Program Guide) spanning up to 15 days, based on the information contained in the PSI/SI of the TS.

This feature can be fully supported also by simpler and cheaper user terminals or Set Top Boxes (STB).

The Head-End is the first functional unit of the DVB-T chain.

More than other sections of the chain, it is the most important innovation as compared to an analog transmission chain. Including also the section for the broadcasting of interactive applications, it integrates the ADDITIVE part of the technology.

All other chain blocks making up the analog broadcasting system, with more or less significant modifications (for instance, consider the difference between an analog and a DVB-T exciter), already existed in terms of function and their layout has not changed radically.

The Head End simply did not exist, it is the key element for the transition from analog-to-digital, from single to multiple service within each channel, to add new services.

Obviously, this implies costs that are often seen as expensive and additional.

However, you should not overlook the opportunities they provide in terms of improved broadcasting capacity, including

• Quantity (number of programs)
• Quality (new services, service quality)

Obviously, this required that the real value of these investments is carefully evaluated, considering the business model, as well as:

• Platform scalability and flexibility
• Level of experience and technology integrated in the product (manufacturer’s experience, man-years and life of the product)
• Measurable performance
• Compact size of the platform and support provided by the supplier or integrator.

The Head End section packages the television signal (bouquet) as it will be received by the final user. Summing up, the key components are:

• Encoder
• Multiplexer

As already mentioned, a service consists of a video contribution (V), one or more audio contributions (A), and possible associated data (d).

A/V contributions can be analog (PAL) or digital (SDI), but in both cases they must be coded according to the MPEG2 standard. The encoder’s MPEG2 coding quality is essential to deliver a good quality TV signal to the final user. Using low-cost solutions, the signal quality might be deteriorated starting from the studios, so that the transport and transmission network can only worsen it further. These decisions must be evaluated carefully, taking care to check the performance of the initial section of the chain from a both subjective and objective point of view.

Other contents can already be coded, for instance when receiving a transponder via satellite (to do this, the platform must integrate a DVB-S receiver), of which you want to re-transmit some services in DVB-T. Obviously, one encoder is required for each analog or SDI A/V to be sent on air. Each MPEG2-coded component is sent to the multiplexer input, which packages the TS to be transmitted. The multiplexer is allocated with the total bandwidth taken up by the bouquet (say 24 Mbit/s), then you select which services will be included in the bouquet, taking care that the sum of the bit-rates of each contribution does not exceed total available bandwidth. Each service can also be associated to data (MHP) which are sent to the multiplexer input from the INERACTIVITY section, as described later.

This is a simplified description for “educational” purposes and to define an entry-level solution with moderate technical and financial impact.

For the evolution of a Head End, many more functionalities should be taken into consideration. Hereunder we introduce the key functions among those we have tested, that can be reproduced in laboratory.

• Statistical Multiplexing
• Transrating
• Encoding
• Redundancy
• Regional Splicing
• System control and monitoring

FILES

In this section, we will illustrate the equipment that encapsulates the data to be associated to the services sent to the multiplexer input. For clarity’s sake and as this equipment can be used or not used according to the broadcaster’s strategy, we analyze it separately, although it belongs to the Head End architecture, as already mentioned.

The DVB-T signal can carry MHP applications that enable interactivity through the TV. We will go into the details of MHP applications in the RECEPTION section, focusing now on the physical aspect of the equipment.

MHP applications (Xlet) are conveyed within a so-called Object Carousel (OC), where the files (objects) are linked through a system of pointers that preserve the application file system, and are transmitted cyclically (carousel) to simplify their acquisition by the Set Top Box (STB). Therefore, a unit is required that receives the MHP applications and can generate the OC and insert it into a TS to feed it to the multiplexer.

Also this process can be fully handled by another GUI specifying the applications to be broadcasted, the bitrate value associated to each Xlet and the parameters needed for the insertion of the OC in the TS. In output from the OC will then be produced new components (ES)to be associated with the contents in the multiplexer.

The test platform available in DMT allows the demonstration of the Java applications management, the OC and bouquet production and its broadcasting.

Furthermore, DMT has the capability to produce and to supply all-inclusive applications, including also the Software and Hardware Back Office support needed for the day-by-day life of the applications (for instance, in the case of updating/publishing system for news to be inserted in Super Teletext).

Another feature of the interactivity and of the new services that can be delivered by DVB-T is the possibility to support also IP traffic.

DMT laboratories are currently developing also a demonstration of IP over DVB-T.

Purpose of this project is demonstrating the functionalities and providing a turn key solution for broadband web surfing through a DTT channel.

The demo platform available in DMT will be fully operating, even though less complex than a typical system used by an ISP. A complete documentation is available to integrate the information for the customers interested in more complex systems.

This is definitely a primary element, as the network operator must guarantee a specific transmission quality defined in contracts where an SLA (Service Level Agreement) is fixed. Therefore, it is necessary to monitor not only the Head End, but also the TV transmission, the transport network, and the associated equipment and systems. For this reason, before being distributed to the emission sites, the bouquet is monitored through a professional decoder that offers a visual feedback of the bouquet quality and contents, as well as objective parameters of the TS construction, to be sure that the user’s STB can decode all transmitted information correctly.

Similarly, the station monitoring can be demonstrated through off-air reception of the DVB-T signal transmitted to check the proper operation of the whole DVB-T chain, consisting of Head End, transport network and broadcasting network.

The control system adopted for the Head End is obviously conceived also to manage the remote monitoring of all stations.

Once the television signal is ready to go on air, it must be transported to the broadcasting sites, which are normally located far from the Head End.

The transmitters for TV DVB-T broadcasting can transmit - with the standard 8 MHz channels - more services within a multiplexing structure known as Transport Stream (TS). The DVB-T (ETS 300 744) standard allows to transmit up to 31.6 Mbit/s (NET, i.e. payload at transmitter input). Transmitted signal protection issues (FEC and Guard Interval) reduce this value within a range of 20 to 27 Mbit/s for common home reception applications (8K, 64QAM).

The approach of standards based on DVB (ETS 300 813, ETS 300 815, ETS 300 815) seems clearly oriented to the maximum exploitation of all possible convergence with the existing Telecom world, which is already implemented and operating for the transport of digital signals.

This approach opens up the network architectures to the collection of external services or transport infrastructures, besides improving the flexibility and value of the assets created with this approach.

The link between the broadcasters (MPEG2 TS typically on ASI) and the telecom radio link is an interface unit, defined by the standards mentioned above, called NETWORK ADAPTER.

In this case, as recommended by DVB, the first Telecom hierarchical flow that can host the TS is the so-called E3 PDH at 34 Mbit/s. The distribution chain consists of the following equipment in cascade:

• Network adapter ASI to E3 (G.703).
• DMT TX radio link.
• DMT RX radio link.
• Network adapter E3 (G.703) to ASI.

Obviously, in a laboratory environment, the “air” section of the radio link is simulated by means of an adjustable fader.

Another transport alternative uses the DVB-S standard.

Basically, it uses DVB-S uplinks and downlinks to convey the Transport Stream to the transmitters.

This is often taken into consideration for quick deployments. The satellite certainly allows to cover all transmitting sites immediately with a signal containing a TS bouquet that can be used to feed the transmitters.

Obviously, there is a fee to pay, not only financial (satellite renting and optimal management of purchased bandwidth), but also technical and legal (problems related to possible conflicts with satellite TV offer and/or the need to encrypt the signal, possible remux or SI restamping requirements).

However, the demonstration platform allows:

• End-to-end simulation using DVB-S modulators-demodulators
• Experience and support to troubleshooting for dish antenna reception. The laboratory is equipped with two antennas, one of which is motor-driven.

At the emission site, the signal must be COFDM modulated (DMT modulator) and frequency-converted from IF to UHF band on the relevant channel (DMT Upconverter). Our laboratory demo does not use the amplification module, which is necessary in emission sites, and the signal is transported via cable up to the RECEPTION section (without any antenna). The full range of complete transmitters can be seen in the manufacturing and testing departments at DMT.

We can also demonstrate the operation of a mini SFN (Single Frequency Network) made up of two transmitters, highlighting the key parameters to manage an SFN network and the problems related to transmitter synchronization.

The receiving section of our demo consists of a common TV set and different STBs (DVB-T).

1) STB Free to air.
2) STB with Conditional Access (CA).
3) STB with MHP middleware.

Conceptually, the STBs for DVB-T are exactly identical as satellite models, except for the reception front-end (an insignificant detail for the final user); there are different frequencies involved and a different modulation (COFDM vs. QPSK). For instance, the MPEG2 decoder and the control and management SW (basic EPG) are identical.

With free-to-air STBs you can watch all program “free”, but not the MHP applications that might be associated to them, however this does not affect the reception quality.

With STBs with CA and a smart card enabled for the head end, you can watch also encrypted programs, but not the MHP. It is worth noticing that, with both types of decoders, you can have additional information, real program guides thanks to the resident SW that generates the EPG with the information fed by the multiplexer into the PSI/SI of the TS.

Finally, with MHP STBs, you can view all information available in digital TV.

DVB-MHP is a very recent but stable standard, and many research centers and manufacturers are working around it.

It defines three interactivity profiles (another one is being defined):

1) Enhanced Broadcasting.
2) Interactive Broadcasting.
3) Internet Access.

The first profile refers to applications that require limited interactivity in the user-STB domain. An example is Superteletext, an application that delivers real-time news constantly updated by the OC generator through a sort of multimedia editorial office, which we can show in operation. The news are supplied in a readable format, complete with pictures, allowing to browse the menus interactively and quickly normally only with the TV remote control.

The second profile refers to full interactivity between user and broadcaster through the so-called return channel. At present, the return channel is implemented by means of a PSTN modem. With this tool, you can make transactions, fix dates, online booking, t-commerce and everything offered by new business models.

The third profile, according to the approach of the DVB project, is an open door into the future, when more and more advanced hardware and software configurations will allow a strong convergence between TV and the Internet.