inSide SeT-Top Box - Electronics For You

Looking inside
inSide SeT-Top Box
STBs have enhanced our TV viewing experience through a host of value-added
services. This article takes you deep inside a set-top box to provide you a
complete overview of its architecture
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With the advent of digital
television, set-top box (STB)
has become a buzzword.
While transmission of television signals
has gone digital, most TVs are still
analogue. Therefore an STB is required
to convert the digital signal to analogue
signal so as to enable a viewer to watch
digital content on the TV set. However,
an STB is not merely a digital to analogue
signal converter, it also provides
a host of other services to enhance the
TV viewing experience—provision
of electronic programme guide to
keep track of the content is one such
example.
This article aims to take you deep
into the set-top box to enhance your
understanding about the STB hardware
architecture.
STB hardware components
The STB mainly consists of following
components:
Antenna. The antenna is not essentially
the part of a set-top box, as
it is fitted outside an STB. It contains
low-noise block (LNB) that receive
the satellite signal with a frequency of
approximately 12 GHz and performs a
down conversion to a frequency range
of 950 to 1450 MHz. Subsequently, the
signal is passed on to the ‘front end’
inside the set-top box.
Power supply. Power supply provides
the required voltage to run various
peripherals inside an STB. In some cases,
it can be used to power mechanised antennae
to align them in various directions
in order to receive different signals.
Front panel. It is a small microcontroller-based
hardware peripheral
inside an STB that receives the user
Fig. 1: Various hardware components inside an STB
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 present
on the outer frame of an STB.
Front end. It is also known as Network
Interface Module (NIM). Front end
is a peripheral device that interacts with
the antenna to receive the down converted
signal and send it to digital decoder
in the form of a transport stream. Basically,
the front end consists of two major
components—tuner and demodulator.
The tuner listens to the signal received
from the antenna and passes it on to the
demodulator for generating the transport
stream. Different types of NIMs are
satellite QPSK, cable QAM, terrestrial
COFDM, 8-VSB and telco ADSL.
Digital decoder. A digital decoder
receives the transport stream sent by the
demodulator inside the front end and
then demultiplexes and decompresses
the requested video by the user. It then
converts the digital stream to an analogue
signal, which is sent 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 a 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. It manages the Real-
Time Operating System (RTOS). The
processor also takes care of memory
devices and other forms of digital stor-
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Fig. 2: An example of a set-top box hardware
age 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 of any kind of data including
audio/video. It communicates with
the processor and is controlled by STB
software modules and storage drivers.
In fact, it is a hard disk which acts as a
digital storage media inside an STB.
CA module. The conditional access
(CA) module can be considered as the
most important component of a settop
box. This module is virtually the
lifeline of all the leading set-top box
solution providers. This peripheral,
called Integrated Conditional Access
Module (ICAM), is placed before the
demultiplexer to be used for descrambling
the encrypted signal, and also
provide a smartcard interface for various
security features.
Return path. This is an optional
component. 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
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to purchase PPV (pay per view) events
directly from the set-top box using
credit cards.
Other peripherals. There are other
optional peripherals present inside an
STB like RS-232 port, USB port, Wi-
Fi enablers, Bluetooth devices. These
peripherals are used inside an STB
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.
Flow of data
As you must have understood the roles
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 hardware while we
tune an STB to a particular channel.
The tuner receives modulated digital
transmission from the antenna and
passes it on to the demodulator. This demodulator
takes into account the type of
demodulation (like QPSK ) and forward
error correction to give out a transport
stream, which is a digital stream of bytes
known as data packets. According to
DVB standards, a transport stream is of
the size of 188 bytes. It then goes into the
demux where the content is separated
Looking inside
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Looking inside
in audio/video packetised elementary
stream (PES) and data packets known
as sections, as per DVB standards. PES
is a mechanism to carry audio/video
elementary streams in packet format
inside an MPEG-2 transport stream.
Sections are the data packets containing
information regarding the audio/video
content and other metadata. Once the
demux does its job, audio/video is sent
to MPEG-2/MPEG-4 decoder which
gives the output to the RF port to display
the video on television.
The data packets are sent to the
processor used by the STB software to
enable viewing. This data can be persistent
or kept in RAM as per the needs and
performance. Any kind of user request,
be it tuning or a purchase, goes through
the 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 STBs, 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 hackers from decrypting the signal.
These are developed in conjunction
with chipset vendors in a very secured
environment.
memory
Moving on further, let’s discuss some
of the memory types used inside an
STB. It contains various memory types
for different roles.
• 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 of the STB. Generally,
STB configurations are written into
NVRAM to keep the settings persistent
Fig. 3: Flow control between various STB hardware components
during power cycles and provide consistent
user experience. There are two
types of NVRAMs used inside a set-top
box, namely, Electrically Erasable Progammable
Read Only Memory (EEP-
ROM) and FLASH. EEPROM is phasing
out slowly as it is very expensive
to perform frequent read and writes,
whereas FLASH uses batch mode for
read and write and enhances 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 written on
to once in a lifetime of set-top box. It
contains some crucial software pieces
like boot loader and other securityrelated
code. The boot loader enables
the STB to check the integrity of the
software and let it 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 an STB design is ICAM
or CA module, which is responsible
for the viewer’s access to the 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 Entitlement Control Message
(ECM) and Entitlement Manage-
ment Message (EMM) acquisition and
management, descrambler and smart
card interface. ECMs and EMMs are
the ways 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 that uses the secure control
word to descramble the video. It is the
key to enable the video to be decoded.
The input transport stream is sent to
descrambler as an input having a specific
parity. Depending on this parity,
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 control word beyond silicon and it
does it much faster without stopping
the input stream.
In a nutshell, set-top box hardware
architecture is very similar to architectures
of any recorder and replay
device. But its uniqueness lies in its
security model and the enhanced interactivity
with viewers, enabling them
to perform various actions during live
video and also pay only for the content
they watch. �
The author is working with a leading digital television
solutions provider
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