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Build a hybrid IPTV-DTV/DVB STB

Posted: 01 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:hybrid STB? IPTV? DTV/DVB STB?

By Richard Lane
Micronas USA

Of all the ongoing developments in the field of television distribution, none is likely to have as large an impact as the advent of IPTV. A combination of network-bandwidth availability, the program-content owner's desire to widen distribution, less copy-prone methods of doing so, and a consumer desire for more tailored programming will combine to ensure that within a few short years, IPTV will have become the way we normally watch television. A visit to any TV industry tradeshow in the past 24 months demonstrated that this change is coming, with large floor-area taken up by STB manufacturers, Digital Rights Management (DRM) software vendors and telecommunication companies talking about TV programming.

Analog's retirement
Meanwhile, the 50-plus-year-old technology of broadcasting analog TV signals is coming to an end, with the United States and virtually all countries in Europe planning to shut off their transmitters and go digital by the end of the decade. In Europe, the broadcasting of DVB-T terrestrial signals is now commonplace in all countries, with Germany the first to shut off the analog PAL transmitters in certain cities. In the United States, ATSC digital transmission has been underway for a few years with mandated broadcasting of MPEG-2 encoded streams. In many cases, the premium "prime-time" programming is transmitted in high-definition (720p or 1080i) resolutions.

At the moment only about 14 percent of TV-equipped households in the United States use an antenna for primary reception. This is likely to rise as knowledge of the high quality programming increases!and accelerate when the mandated change to digital transmission is finalized. In Europe, the free-to-air broadcasting of digital content via DVB-T is well established, with the consumer well aware of the high-quality programming available on the national TV networks. Thus, any business model for IPTV is made more attractive by offering this content in addition to the new IPTV content.

Therefore, during this transition period, where the IPTV business is in its infancy and the terrestrial transmission of digital signals takes over from analog, there is a need for a STB that is a hybrid of the two sets of requirements!i.e., that can receive, decode and display terrestrial DTV signals!as well as provide a platform for the viewing of IPTV services.

Standards galore
One aspect of the IPTV business that will be challenging to engineers designing the STBs is the wide proliferation of standards involved in all stages of the process. For instance, the TV picture can be captured at one of several resolutions from standard definition (which can be 720x480 or 720x576) up to high definition (720p or1080i). It can be interlaced or progressively encoded. It can be encoded into one of many video codecs: MPEG-2, MPEG-4, H.264/AVC, WMV9/VC-1, etc. It can then be encrypted into one of many DRM schemes (WMDRM or many others), wrapped into one of several containers (MOV, ASF) and transmitted via several variations on IP protocol (RTP, RTSP). This is all before the challenge of audio encoding is added to the mix.

Therefore, the chosen decoder device in any STB has to possess a degree of programmability to implement the many required standards. Using a fully programmable device, where even the actual video codec is programmed such as in a DSP device, is not usually feasible, since a HD decoder, the necessary OS, an audio decoder and the user interface and viewer application all share the same processor core. The current ASIC offerings provide hardwired blocks of logic for the video decoders and decryption blocks, with programmable cores for de-multiplexing, decoding audio, running the OS and running user applications.

In this way, the heavily computational aspects of decoding modern video codecs, which are well-standardized, is offloaded to dedicated logic, thereby freeing the CPU(s) for the fast-changing requirements of OS, user interface, transport protocols and audio standards.

Another main complication that designers need to take into account is the lack of IPTV standards worldwide. For the digital terrestrial transmissions, the world is slightly simpler. Within each country at least, there is standardization. The ATSC standard adopted by the United States uses the 8VSB modulation scheme with MPEG-2 encoding (though the picture may be encoded in one of many, many resolutions). The bitrates are high, up to 19.3Mbit, with excellent video quality. In Europe the DVB-T standard is well established with SD resolutions and MPEG-2 encoding. It seems likely that for HD terrestrial broadcasts in Europe, H.264 encoding will be adopted.

For the hybrid STB then, in the digital domain the greatest challenges lie on the IPTV side with the wide range of requirements. For terrestrial digital TV reception, the set of codecs required to be decoded is a subset of those required for IPTV. The challenge for terrestrial reception lies more with the appropriate choice of tuner, filtering and demodulator required to make the most of a potentially low-strength signal. "Can Tuners" (i.e. a small module containing analog and digital tuning components) have been the most commonplace solution until quite recently.

Today there are several suppliers of silicon tuner devices, the best have relaxed requirements for their board layout (no longer requiring the 45< inductors of yesteryear) and provide a compact footprint, ideal for the size constraints in an STB. The chosen demodulator must be capable of pulling out the digital carrier from a high-noise signal subject to heavy ghosting in built-up environments.

The usual connection between the demodulator and the decoder SoC (since it's established that a CPU and dedicated decoder logic are both required) is via a Transport Stream Interface or TSI. This is the typical input interface to any STB decoder SoC and there is usually more than one of them on any single device.

Thus the decoder must be capable of receiving incoming streams over both its Ethernet interface (wither wireless or wired) and its TSI inputs.

Figure 1: Video decoder SoC (click on image)

The block diagram of the typical IPTV STB therefore will resemble Figure 1. The decoder SoC is highly integrated with only memory, Ethernet connection and output interfaces needed to create a fully working system. The tuner module required to create the hybrid box will typically be added as a separate module to enable variations for multi-country deployment!but this function could also be integrated onto the main board. Future developments in decoder SoCs will involve further integration of the PHY layer (analog interfaces) of the various I/Os.

Figure 2: Decypher 8100 block diagram (click on image)

In the case of terrestrial DTV reception, the most challenging aspect to designing the box can be the software. There are several types of auxiliary data broadcast with the TV signal! these would include but not be limited to closed caption data for subtitling, Electronic Program Guide (EPG) data, and "teletext" data for auxiliary displays of news and weather. Display of EPG information is considered a standard feature and will be required in most applications.

This discussion of required software raises the subject of the quality of experience that the user will expect. He/she is watching TV, an experience that for decades has displayed almost bullet-proof reliability. A typical test of a prototype STB would involve the box being connected immediately to a server and run for 72hrs. Any crash or fault during that time would result in failure.

The advent of IPTV and its necessarily software-heavy design requires a level of robustness that cannot be achieved without extensive QA testing and multi-city or multi-country verification. The task to create, for example, the required "middleware" for ATSC reception should never be underestimated. Typically this function would be implemented in conjunction with a partner.

To deliver a great hybrid STB, the designer has to come up with the winning combination that addresses the various components of the streaming landscape like the resolution, video codec, container format, standard protocol and the networking interface. The designer has to consider the diversity of the streams that need to be decoded and address the complete matrix.

To summarize, from a systems hardware perspective, a hybrid STB involves very little modification to the existing IPTV STB and hardly escalates the cost. What the designer needs to focus on is the development planning around the complexity of the software to ensure added robustness to the middleware.

About the author
Richard Lane
is the director of product marketing at Micronas USA, and is responsible for the Cypher and DeCypher lines of audio/video SoCs.

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