Set Top Box

Assignment : 

Assignment By, Srinivas V

Set–Top Box (STB) : 

Set–Top Box (STB)

What is STB? : 

What is STB? It is an electronic device that is connected to a communication channel, such as cable television line and produces output on a conventional television screen. Set-top boxes are commonly used to receive and decode digital television broadcasts and to interface with the Internet through the user’s television instead of a PC. A typical digital STB contains one or more microprocessors for running the operating system, it also includes RAM, MPEG decoder chip, and more chips for audio decoding and processing.

Architecture : 

Architecture

Slide 5: 

STB Software Driver Adaptation Layer Middleware Device Drivers (Manufacturer) CAS Application Layer Loader

Software Layers : 

Software Layers An operating system is the most important piece of software in a STB. OS is the one which talks to the STB hardware and manage their functions such as scheduling real time tasks, managing limited memory resources, etc. A STB OS is arranged in layers with each layer adding new capability STB OS contains the kernel layer stored in ROM, which is responsible for managing memory resources, real time applications and high-speed data transmission. The kernel supports multi threading and multi tasking which allows a STB to execute different sections of a program and different programmes simultaneously.

OS : 

OS STB needs a ‘loader’ to enable the TV operator to upgrade ‘resident applications’ or download ‘OS patches’ to STB. A resident application is a program or a number of programs that are built into the memory of the STB. The STB also requires ‘drivers’ to control the various hardware devices. Every hardware component in the STB must have a driver. A driver is a program that translates commands from the TV viewer to a format that is recognizable by the hardware device. STB OS needs to incorporate a set of Application Programme Interfaces which are used by the programmers to write high-level applications for a specific API.

Hardware Architecture : 

Hardware Architecture

Digital Video Broadcasting (DVB) : 

Digital Video Broadcasting (DVB)

What is DVB? : 

What is DVB? DVB (Digital Video Broadcast) is a set of standards for the digital transmission of video and audio streams, and also data transmission. DVB systems distribute data using a variety of approaches that includes, Satellite (DVB-S) Terrestrial (DVB-T) Cable (DVB-C) These standards define the physical layer and data link layer of the distribution system . These distribution systems differ mainly in the modulation schemes used and error correcting codes used . DVB uses MPEG TS as a standard format for transmission and storage of audio and video data.

MPEG : 

MPEG

MPEG : 

MPEG The Moving Picture Experts Group (MPEG) is a working group of experts that was formed by ISO and IEC to set standards for audio and video compression and transmission. The MPEG compression methodology is considered asymmetric as the encoder is more complex than the decoder. The encoder needs to be algorithmic or adaptive whereas the decoder is 'dumb' and carries out fixed actions . The MPEG standards consist of different Parts. Each part covers a certain aspect of the whole specification

MPEG Standards : 

MPEG Standards MPEG -1 : Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbit/s . MPEG-2: Generic coding of moving pictures and associated audio information. Transport, video and audio standards for broadcast-quality television supporting and interlace and high definition. MPEG-4: Coding of audio-visual objects. It uses further coding tools with additional complexity to achieve higher compression factors than MPEG-2.

MPEG - 2 : 

MPEG - 2 MPEG-2 is widely used as the format of digital television signals that are broadcast by terrestrial (over-the-air), cable and direct broadcast satellite TV systems. MPEG-2 standards were published in nine parts. Each part covers a certain aspect of the whole specification. It has higher Bandwidth (up to 40Mbits/sec) and wider range of frame sizes (including HDTV) and can deal with interlaced video . MPEG -2 stream consists of the Program Stream and the Transport Stream. Each is optimized for a different set of applications.

MPEG – 2 System : 

MPEG – 2 System

Program Stream (PS) : 

Program Stream (PS) Combines one or more Packetised Elementary Streams (PES), which have a common time-base, into a single stream Designed for use in relatively error-free environments and suitable for applications which may involve software processing Program stream packets may be of variable and relatively great length

Transport Stream(TS) : 

Transport Stream(TS) Transport stream (TS) is a standard format for transmission and storage of audio, video, and data and is used in broadcast systems such as DVB . Combines one or more Packetised Elementary Streams (PES) with one or more independent time bases into a single stream (sometimes called multiplex) Designed for use in environments where errors are likely, such as storage or transmission in lossy or noisy media The transport stream is made of packets with fixed length of 188 bytes

TS Packets : 

TS Packets A packet is the basic unit of data in a transport stream. It consists of a sync byte, whose value is 0x47, followed by three one-bit flags and a 13-bit Packet Identifier (PID). This is followed by a 4-bit continuity counter. The rest of the packet consists of payload. Packets are 188 bytes in length . The communication channel may add some error correction bytes to the packet . Each table or elementary stream in a transport stream is identified by a 13-bit packet ID (PID).

TS Packet : 

TS Packet Header A F Pay Load 188 Bytes 4 Bytes TS packet is a fixed length of 188 bytes. Packets contain one type of data only i.e, video ,audio, data, programme guide information. It may also contain timing information (PCR).

TS Packet Header : 

TS Packet Header Every 4 Byte Header Contains following fields: Sync bit 0X47. Transport error indicator. Payload Unit start indicator. Transport Priority. PID Scrambling control. Continuity counter. Adaptation field control.

Program Clock Reference (PCR) : 

Program Clock Reference (PCR) It is used to Enable the MPEG decoder to synchronise to the encoder. 42 bit sample of the STC (System Time clock) at the encoder indicates to the demultiplexer what the STC time should be at the decoder when each clock reference is received. Synchronization errors arise if the PCR value generated by the multiplexer is inaccurate or if it is received late because of network delays.

DTS and PTS : 

DTS and PTS DTS is the Decode Time Stamp. It is a 33 bit data field indicating the appropriate time according to the STC that a particular AU should be decoded. PTS is the ‘Presentation Time Stamp’ derived from the same clock as the DTS. It controls when a decoded access unit (ie a presentation unit) is presented to the viewer. Both timestamps are found in the PES header packet

Frames : 

Frames MPEG has several frame types that serve different purposes. Different types of frames are : I – frame (Intra frame) P – frame (Predicted frame) B – frame (Bidirectional frame)

Program Specific Information (PSI) : 

Program Specific Information (PSI) PSI table contains : Services that are present in a Transport Stream The component in a TS that constitute a service. The condition access system Used, ECM and EMM. These information is contained in the following four PSI tables. Program Association Table (PAT) Program Map Table (PMT) Conditional Access Table (CAT) Network Information Table (NIT)

PAT : 

PAT It lists all programs available in the transport stream . The PAT in a Transport Stream can best be regarded as a directory listing of the services present in that Transport Stream. Each of the programs listed in PAT has an associated value of PID for its Program Map Table (PMT). It also provides the each service’s Unique Identifier ( Service ID),

PMT and NIT : 

PMT and NIT Program Map Tables (PMT) contain information on the components (audio, video) of the service. The PMT provide information on each program present in the transport stream, including the program number, and list the elementary streams that comprise the described MPEG-2 program. NIT provide information about the networks and frequency. It specifies the characters of the network infrastructure (cable, satellite or terrestrial) used by TS.

CAT : 

CAT CAT provides information about the CA system used by the service in a TS. MPEG-2 Transport stream carries encrypted or scrambled services, the TS also carries two types of messages . EMM (Entertainment Management Message) 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 on the other hand carries a data element called control word (CW), which is used by the ‘descrambler’ in the STB to descramble the picture and make it intelligible again.

MPEG – 2 PSI Structure : 

MPEG – 2 PSI Structure

Service Information Tables (SI) : 

Service Information Tables (SI) Service information is the DVB data that identifies what parts of the transport stream belong to particular program. This information is contained in the following tables. Event Information table (EIT). Service Information table (SIT). Time descriptor table (TDT). Time offset table (TOT). Bouquet Association table (BAT). Network information table (NIT).

EIT and SIT : 

EIT and SIT EIT provides the characteristics of the event that make up the service. It also provides information like start time, end time, name , description about the event. SIT provides information about the services available in the network including the name of the service provider and the textual description of the channel.

TDT,TOT and BAT : 

TDT,TOT and BAT TDT specifies the Universal time coordinate (UTC), provides the present date and time which can be adjusted according to the time zone. TOT provides the local offset relative to UTC, gives information about at what time the program has to be broadcasted at different regions. BAT describes a number of services from different TS, it gives the bundle of services that can be purchased as a single product.

Scanning : 

Scanning

Scanning : 

Scanning Scanning is a method for displaying, storing or transmitting moving images . Basically there are two types of scanning : Interlaced Scanning Progressive Scanning

Interlaced : 

Interlaced It uses the technology developed for cathode ray tube (CRT) based TV monitor displays . Interlacing divides the horizontal lines (row of pixels) into odd and even lines and then alternately refreshes them at 30 frames per second. The slight delay between odd and even line refreshes creates some distortion or 'jaggedness'. The effects of interlacing can be somewhat compensated by using de-interlacing.

Progressive : 

Progressive Progressive scan is a system in which the image is displayed on a screen by scanning each line (or row of pixels) in a sequential order. In progressive scan, the image lines (or pixel rows) are scanned in numerical order (1,2,3) down the screen from top to bottom.  By progressive scanning smoother, more detailed, image can be produced on the screen that is perfectly suited for viewing fine details, such as text, and is also less susceptible to interlace flicker.

Slide 36: 

THANK YOU