July 2017


This Technical Article Provides An Overview Of The Sections Of A Digital Set Top Box.

Over the past few years, Television broadcasting is rapidly shifting digital. These digital Broadcast can be over cable, via satellite, terrestrial broadcasts or even internet compatible IPTV.

However, there are over 1 billion analog TV sets in homes worldwide. These TV sets cannot receive digital broadcasts. A Digital Set Top Box (STB) connected between the incoming digital signal and the analog TV set, enables digital TV reception.

D i g i t a l transmissions offer better picture quality, larger number of channels in the same bandwidth and several other functions, such as secure encryption. Clearly there are several advantages to digital despite the additional cost of a digital STB. There are several different digital standards adopted worldwide. However, the DVB (Digital Video Broadcast) standards adopted in Europe and India, and of course several other countries has now emerged as the world's most popular digital standard.


The DVB project group has formulated separate technical standards for optimum transmission via cable, satellite & terrestrial. These standards are :

Cable TV : DVB-C Satellite : DVB-S Terrestrial : DVB-T

Digital STBs provide the interface between any of the above DVB standards and an analog TV set.

The channel coding and modulation for DVB-C/S/T are different (See Box Below on Digital Transmission) but the MPEG-2 or MPEG- 4 compression used is the same in all transmission modes. This makes it easy for chip manufacturers to develop standardised Integrated Circuits (ICs) for MPEG decoding. Due to very large quantity requirement for the MPEG chips, economies of scale ensure that the very complex MPEG decoder chips can be produced at relatively low cost.

Only the initial tuner section of a digital STB will vary, depending on its application for Cable TV, Satellite/DTH or terrestrial transmission.

Figure-1 provides a Simplified Block Diagram Of A Digital STB.


Cable TV, satellite / DTH and terrestrial digital transmissions each need to be optimised for the conditions under which they are transmitted. A different set of engineering comprises have been created for each transmission type.


Cable TV signals are transmitted through optical fibre and coaxial cable. This provides high signal strength as well as relatively low external noise. As a result cable TV downstream (headend to consumer) signals are transmitted using QAM modulation. QAM (Quadrature Amplitude Modulation) enables transmission of large volume of digital data, at high speeds, using small bandwidth. This is the most efficient form of digital modulation. Where ever signal is extremely strong with very little noise, high density QAM modulation such as QAM-256 or even QAM-512 can be deployed.

For CATV applications, the upstream/reverse path signal (from the consumers TV set back to the headend) has to contend with high levels of noise. In such an environment, a lower level of QAM modulation such as QAM16, QAM-64 or in some cases even QPSK modulation is deployed.


A STB can be considered to be a collection of a number of small functional blocks or modules, with each module performing a different, well defined function.

All STBs have multiple 'modules.' A module may be created out of pure hardware or pure software or some combination of hardware + software.

The design of each module and the overall design of an STB is left to the manufacturer. The MPEG standard does not dictate the overall architecture of the STB. For this reason, STBs from different vendors may or may not work together in a Cable TV or DTH network.


A Ku Band satellite transponder emits approximately 50 watts of power. This signal suffers approx. 200 dB of attenuation as it travels 36,000 kms. through the atmosphere to reach a satellite dish. The signal reaching the satellite dish is only a few Pico watts i.e. 1 billionth of a watt ! To ensure that such a small signal still provides a crystal clear TV picture, QPSK (Quadrature Phase Shift Key) modulation is used.

QPSK modulation is practically unaffected by external noise but requires a much larger bandwidth to transmit each TV channel.


Analog terrestrial transmissions have always been plagued by multiple images or "Ghosts" due to the TV signal bouncing of different objects and multiple signals arriving at the TV antenna. The DVB-T standard utilises OFDM (Orthogonal Frequency Division Multiplexing) modulation. This modulation uses several thousand carriers (usually 8000 carriers) to transmit 1 digital stream. This completely avoids multiple images or ghosts.

Satellite & Cable TV magazine has carried a detailed article explaining the different types of digital modulation. The article can be read free from the "Technical Articles" Section of our website or in our "How To...." Books.


A STB is said to have an 'Open' architecture, only if the functionality of every module is available in the public domain in the form of published international standards or de-facto industry standards.

Any technical work is generally called "Open" if the IPR (Intellectual Property Rights) and the technical information needed to implement, compliant products is available under fair, reasonable and nondiscriminatory (FRND) terms. IPR holders are generally required to accept the FRND terms set by the recognized international standardization bodies.


Two STBs are interoperable if they can both receive all services from any network.

It is possible to have such an interoperable STBs, even though they may have proprietary encryption systems.

A standardised "Conditional Interface (CI)" is introduced between the Tuner + MPEG decoder chips and the rest of the STB. This is shown in Figure-2


The STB selects the appropriate TV channel, through its Tuner ( which is different for DVB: C/S/ T STBs.

The information in the selected (tuned) RF channel is then processed by the demodulator to produce an MPEG (it could be MPEG-2 or MPEG-4) Transport Stream (TS) containing the audio, video and other information that relates to the selected TV programme.

While the MPEG transport stream's structure is standardised by the DVB standard, the TS content may be encrypted to prevent customers who have not paid for a particular channel/service from being able to view it.

While The Structure Of The Transport Stream Is Standardised, Its Content Maybe Encrypted

The MPEG de-multiplexer selects and decrypts the compressed audio and video for the particular programme that the viewer wishes to watch, using decryption keys supplied by the Conditional Access System (CAS). The MPEG decoder then de-compresses the audio and video information for the selected programme.

The Central Processing Unit (CPU) controls the whole operation.

All STBs generally use an Operating System (OS). The OS like in a PC, is software that provides a working environment for all the hardware, and even sb-software. This is very similar in function to say a Windows XP operating system on a PC.

There is no widespread standard operating system for digital STBs. Some of the popular operating systems for STBs include Power TV OS, Vx Works pSO System, Microware's DAVID OS9, Android, Linux etc.


Some STBs are capable of supporting simultaneous internet connectivity, or even authorisation of some services such as instant Video-On-Demand. This requires the capability for the STB to not only receive information from the headend, but also to send back digital data to the headend. This requires a 'modem' to be built into the STB. For a Cable TV STB, the modem will be a cable Modem. cable modems receive data from the headend using QAM but return path data is sent using either QAM or QPSK. The DOCSIS Cable modem standard does not permit QPSK for reverse path.


Whenever an MPEG TS carries encrypted (or scrambled) services, the Transport Stream also carries 2 types of messages.

These are called the EMM (Entertainment Management Message) and ECM (Entitlement Control Message).

An EMM carries a list of Pay TV services which the owner of that STB is entitled to view and also the date upto which he is entitled to receive them.

The ECM carries data called 'Control Word' (CW), which is used by the 'descrambler' in the STB to descramble the picture and make it intelligible again.

Both these messages are carried in the TS in an 'encrypted' form.

Even though DVB has standardized the scrambling algorithm (known as DVB common scrambling algorithm, DVBCSA), algorithms used for ECM/EMM encryption are not standardized for obvious reasons.

Hence DVB compliant CAS systems available in the market use different ECM/EMM encryption algorithms.

The security of any CAS system depends primarily on the efficiency of the algorithm used for ECM, EMM encryption. Obviously, such algorithms are closely guarded secret.


The Conditional Access (CA) module inside each STB contains the relevant ECM, EMM decryption algorithm (formula).

Since each CA system is different from its competitor, if a STB has a CA module embedded inside it, the STB can no longer be considered to have an "Open Architecture". Such an STB will also not be 'Interoperable' across different networks which use different CA systems.


To make an STB interoperable, so that different CAS systems can be used with the same STB, DVB has formulated its 'DVB Common Interface (DVB CI).'

The block diagram of an Interoperable STB is shown in figure 2.

The STB contains only those modules required to receive FTA (Free-To-Air) broadcasts.

The CA system is contained in an external module, which is attached to the STB via the common interface connector provided on the STB.

Hence the same STB can be used with different CAS systems such as Viaccess, Irdeto etc. The consumer needs to procure the Viaccess or Irdeto CA modules and plug them into his STB.

If such STBs were provided by DTH platforms, consumers could shift their STB from one to another DTH platform, by simply pulling out and replacing the CA module !


We will now take a very brief overview of the different software used in an STB. The different software on an STB can be considered to be installed in 'layers' one on top of the other.


The inner most and most essential layer is the Operating System (OS). As mentioned earlier, each STB must have an Operating System (e.g. Android), that provides a suitable environment for all the modules to interact with each other. The STB's operating system itself has 2 layers, a core 'Kernel' is stored permanently on a chip called the ROM (Read Only Memory). The kernel is read, first thing, when the STB is powered on. The STB 'boots up' similar to a PC, before it can commence operation. The OS is in constant use until the STB is shut off.

In addition to the kernel, a STB needs a 'loader' to enable the network to upgrade 'applications' or download 'OS patches' to STB. The applications are programs that are built into the memory of the STB.

Similar to a PC, the STB also requires 'drivers or software' to control each of the hardware devices. Every hardware component in the STB must have a driver.

Finally a STB OS needs a set of Application Programme Interfaces (APIs) which are used by the programmers to write applications.


Just like in a PC, an STB too runs its own 'applications or programs.'

All the applications can broadly be classified into 2 categories: Enhanced and Interactive.


An Enhanced TV application is the one which is based on 'local interactivity' and which does not require a return path back to the service provider.

As an example, consider a DTH service offering an educational quiz. The child is presented with a question, and a choice of 3 answers. The correct answer is also transmitted with the question, but the child sees the answer, only after he has selected his option.


An 'Interactive Application' compulsorily requires 2 way interactivity, where data needs to be sent back to the headend.

An instant "Pay Per View" service, authorised from the STB could be an example. Currently, no DTH or Cable TV service in India supports true interactive applications. Some DTH platforms incorrectly publicise their enhanced learning applications as interactive, but this is false and misleading.


Often there is a layer of software between the OS and the applications. This is called 'Middleware.'

Middleware is used to isolate set top applications from the details of the underlying hardware.

This makes it easy for develop new applications, and all applications can be created using a common API. The terms API (Application Programme Interface) and middleware are sometimes interchangeably used.

There are several custom created API, but Java based API is rapidly gaining popularity, since Java is based on an 'Open Architecture.'