Amit’s lab

Update: This article has been published in India’s leading Electronics magazine “Electronics For You (EFY)” january’2010 edition.

With the advent of Digital Television, Set Top Box (STB) has become the buzzword. The transmission of television signals has gone digital whereas most of the TV’s at home are still analog. So, an STB converts the digital signal to analog signal so as to enable a viewer to watch content on television. An STB is not merely a digital to analog signal convertor, but, it provides a host of services to enhance television viewing experience. For example, it provides an Electronic Program Guide to keep track all the content or a viewer can record/rewind/pause a program as per his needs.

In this article, Let us dive deep into the Set Top Box Hardware architecture.

Figure 1: various hardware components inside an STB

The STB consists, mainly of, following components.

• Antenna: The antenna is not essentially a part of a Set Top Box as it resides outside an STB, generally, outside the house of the television viewer. The antenna contains Low Noise Blocks (LNB) which receives the signal from the satellite with a frequency of approximately 12 GHz and performs a down conversion to a frequency range of 950 – 1450 Mhz. Then, the signal is passed on to the ‘Front End’ inside the set top box.
• Power Supply: Similar to every other device, even an STB needs a power supply to run itself. The power supply provides the voltage to run various peripherals inside an STB. In some cases, this power supply can be used to give power to mechanized antennas to align themselves in various directions to receive different signals.
• Front Panel: A front panel is a small micro controller based hardware peripheral inside the Set Top box which takes of the user input and interacts directly with the main CPU control (processor). It contains an IR receiver to listen to Remote Control input. Also, it is directly linked to some input buttons or keys visible on the outside frame of an STB.
• Front End: It is also known as NIM (Network Interface Module). Front End is the peripheral device which interacts with the antenna to receive the down converted signal and give it to “digital decoder” in the form of a Transport Stream. Basically, Front End contains two major components, namely, a Tuner and a Demodulator. The Tuner listens to the signal from the antenna and passes it on to demodulator for getting the Transport stream. Among the different types of NIMs are Satellite QPSK, Cable QAM, Terrestrial COFDM and 8-VSB, and telco ADSL.
• Digital Decoder: A Digital decoder receives the Transport Stream given by the demodulator inside the Front End and then, demultiplexes & decompresses the requested video by the user. It then, converts the digital stream to analog signal to be given to RF port. The task of the demultiplxer or a demux is to separate all the data streams packaged together by the HeadEnd in the specified format. The data contains audio, video and DVB (Digital Video Broadcasting) specified data like SI/PSI tables. After the demuxing, decompression is done by the specified decoder in specified formats like MPEG-2 or MPEG-4. The specific data or information is given to CPU for processing for advanced features and memory storage and to be used in graphics.
• CPU (Processor): Just as in a regular PC, The processor inside the CPU takes care of interactions of all the hardware peripherals and software modules inside a Set Top Box. The processor takes care of managing the RTOS (Real Time Operating System). The processor also takes of the memory devices and other forms of digital Storage like hard disks or flash drives.
• Digital Storage: Digital Storage is another optional component inside a Set Top Box. It is needed for persistent storage for any kind of data including audio/video. It communicates with the processor and is controlled by the STB software modules and storage drivers. Genreally, it is a hard disk which acts as a digital storage media inside an STB.
• CA Module: The CA (Conditional Access) module can be called as the most important component of a Set Top Box. This module virtually forms the lifeline of all the leading Set Top Box solution providers. This peripheral called ICAM(Integrated Conditional Access Module) is placed before the Demultiplexer and is used for descrambling the encrypted signal and also, provides a smart card interface for various security features.
• Return Path: This is an optional component in an STB. A return path is used by an STB to communicate back with the HeadEnd and send data packets. The return path can be present in various forms like a PSTN line connection, a cable modem in case of Cable STBs or an Ethernet jack in IP Set Top box using ADSL broadband modems. For Example: A return path becomes a necessary component if a user needs to purchase PPV (Pay per view) events directly from the set top box using credit cards.
• Other Peripherals: There are other various optional peripherals present inside a Set Top Box. For example: an RS-232 port, a USB port, Wi-Fi enablers, Bluetooth devices. These peripherals are used inside a Set Top box for enhanced interactivity with the consumer and provide better user experience. Basically, These components or peripherals can be used for creating advanced features like File sharing and plugging in external devices.

Figure 2: An Example of a Set Top Box Hardware

As you must have understood the roles and responsibilities of various hardware modules inside an STB, Let’s move on to the next level, where we’ll go through the flow of data between these various hardware devices resulting in tuning to a particular channel.

Figure 3: Flow control between various STB hardware components

Basically, the tuner receives the modulated digital transmission from the antenna and passes it on to the demodulator. This demodulator takes into account the type of demodulation (For example: QPSK ) and FEC (Forward Error Correction), in turn, giving out a Transport Stream. A Transport Stream is a digital stream of bytes known as data packets. According to DVB standards, a Transport Stream is of the size of 188 bytes. This Transport Stream goes to the demux to separate out the content in the form of Audio / Video PES (Packetized Elementary Stream) and Data Packets known as Sections as per DVB standards. PES. PES is a mechanism to carry A/V elementary streams in packet format inside an MPEG-2 transport stream. Sections are the data packets containing information regarding the A/V content and other metadata. Once the Demux does its job, A/V is given to MPEG-2/MPEG-4 decoder which gives the output to the RF port to display the video on television. The data packets are given to the processor used by the STB software to enable the viewing experience. This data can be persistent or kept in RAM as per needs and performance reasons. Any kind of user request, be it tuning or a purchase goes through CPU. In between, the most important functionality of descrambling is performed by a descrambler embedded inside the ICAM part of the chipset. In some variants of an STB, a descrambler can be a part of a decoder or it can exist independently. The descrambler takes care of decrypting the encrypted Transport Stream using the Control Word technique. Generally, these descramblers and Control Word algorithms are closely guarded secrets with CA solution providers to prevent the hackers from decrypting the signal. These are developed in conjunction with chipset vendors in a very secured environment.

Moving on further, let’s discuss some of the memory types used inside a Set Top Box. Set Top Box contains various memory types for different role plays.

• DRAM (Dynamic Random Access Memory) is used to store all the software needs and volatile working area like static variables.
• Video DRAM is used in an STB for storing the decoded video image that is being output to television or video recorder.
• NVRAM (Non Volatile Random Access Memory) is needed for persistent memory needs for the Set Top Box. Generally, STB configurations are written into NVRAM to keep the settings persistent during power cycles and provide consistent user experience. There are two types of NVRAMs used inside a Set Top Box, namely, EEPROM (Electrically Erasable Progammable Read Only Memory) and FLASH. EEPROM is phasing out slowly as it is very expensive to perform frequent read and writes where as FLASH uses batch mode for read & write enhancing the life of NVRAM. Also, Flash can be used for storing parts of code segment for software performance reasons.
• ROM / OTP (Read only Memory / One Time Programmable): As the name suggests, this is the memory which is written on to once in a lifetime of Set Top Box. Basically, it contains some crucial software pieces like Boot Loader and other security related code. The boot loader enables the Set Top Box to check the integrity of the software and let the STB work only if it contains authentic software. This is one of the key memory areas which protect the STB from hackers to write illegal software into the box.

As mentioned earlier, the most important part in a Set Top Box design is ICAM or conditional access module. Basically, this module is responsible for viewer’s access to content available on air. It maintains all the transactions and performs validations to check the entitlement of the viewer to selected programme. ICAM module consists of ECM (Entitlement Control Message)& EMM(Entitlement management message) acquisition and management, descrambler and smart card interface. ECM & EMM’s are the way of communication used by broadcaster to an STB. Smart card Interface provides access to secured information like passwords, Parental Lock information, Purchase Information and all other kinds of transactional details. The most important part is the descrambler which uses the secure ‘Control Word (CW)’ to descramble the video. CW is the key to enable the video to be decoded. The input Transport Stream is given to Descrambler as an input which has a specific parity. Depending on this parity, a respective Control Word is generated and used to descramble the video. The descrambling is done in hardware due to performance and security reasons. Doing it in hardware does not expose the CW beyond the silicon and it does it much faster without even stopping the input Transport Stream.

In a nutshell, Set Top Box hardware architecture is very similar to architectures of any recorders and replay devices like DVD players. But the significance lies in its security model and the enhanced interactivity of the viewer enabling him to perform various actions during live video and also, pay only for the content he/she watches.

Quote of the Day:"Too bad drinking scotch isn't a paying job or Kenny's dad would be a millionare!" -- Cartman

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