JDSU Digital TV Re-Multiplexing at the CaTV Head-End

Communications Test & Measurement

Digital TV Re-Multiplexing at the CaTV Head-End

Digital TV benefits and constraints

To print the Application Note below, click here (61-KB file).

Multiple TV programs over a single transmission channel:

The main benefit of Digital TV is the combination of video compression and digital transmission which offers the opportunity to transmit 6 to 12 TV programs over a frequency previously used for a single analogue channel.

Not only does Digital TV allow for more TV programs – also known as virtual channels - but it can offer advanced services such as data, multi-language movie broadcast, multi-video sport programs and an unlimited offer of CD-quality radio programs.

Combining heterogeneous inputs into a consistent offer:

As described in the above figure, the offer to the subscriber is no longer a set of basic modulated TV channels. It includes digital services from various sources:

Redistribution of satellite and terrestrial feeds, already scrambled and multiplexed,
Local programming to be encoded and multiplexed,
Near Video on Demand services hosted on video servers,
Interactive applications and Electronic Program Guide.

The redistribution of multiple feeds from different sources generates a need for repackaging the corresponding programs - including Conditional Access and a common Electronic Program Guide – and finally re-multiplexing the various components.

Two-step DVB or ATSC Digital TV process

How does it work in DVB (Digital Video Broadcast) or ATSC systems? The figure below summarizes the operation for video (and audio) transport mechanism as defined in MPEG-2/DVB standards, using MPEG Packetized Elementary Streams structures for the transmission process.

Fragmentation into MPEG transport packets:

The 1 st step is a data fragmentation mechanism. Multi kilobyte pieces of data - such as a video frame - are fragmented in fixed-size (188 bytes) MPEG transport packets with minimal overhead. A series of transport packets for a given data stream – such as the video for one movie - is identified by a user-defined PID (Packet ID.) value, such as PID 0x0A00 as seen in the example.

Notice that each packet will automatically benefit from a standard Reed Solomon forward error correction mechanism later during the digital modulation to recover from most transmission errors.

Multiplexing the various data-streams:

The 2 nd step in the process is the multiplexing of the transport packets from multiple video and audio streams - possibly along with data streams for interactive applications - in a single MPEG TS (Transport Stream). A TS is a flagged bit-stream with a fixed global bit-rate corresponding to the performance of the digital modulation. As the rate of individual components (audio, video or data) is dynamically allocated by the multiplexer, stuffing with Null packets is provided to reach the fixed total bit-rate.

Critical Service-Information

Self-contained protocol:

The receiving equipment needs to filter the expected video and audio for a TV program from the entire multiplexed TS – without getting external information or pre-negotiation – Therefore, appropriate descriptive Service-Information is embedded in the multiplex along with the TV payload.

Such Service-Information has a specific structure – similar to file management tree structure on a PC disk – and uses specific PID values for transmission and identification by any receiving equipment. A root table lists the existing programs and services, and each service and its content is further described through additional sub-tables. These types of tables are named MPEG-PSI, DVB-SI and ATSC-PSIP depending on the standards, they are described in the table hereafter.

Award-winning test solution:

Only an MPEG-2 Transport Stream Analyzer such as the WG DTS-A offers access to these critical Service-Information status and decodes as well as PID-based bit-rate statistics and confirmation of encryption mechanisms.

DTS-A by Digital Transport Systems, a Wavetek Wandel Goltermann, Inc. company, received a "Best in Test" award from Test & Measurement Worldwide magazine for recognition as a unique MPEG-2/DVB and MPEG-2/ATSC analyzer.

PSI-SI-PSIP tables and descriptions

Purpose	Table	Link to
	MPEG-PSI tables
Lists the programs carried in the current TS	PAT – program association table	PMT and CAT tables
Lists the audio and video components for a program in the current TS	PMT –program map table	Elementary streams
Lists the data-streams with the access rights and keys for a program in the current TS	CAT – conditional access table	EMM and ECM encryption data
	DVB-SI tables
Describes all the TSs composing the entire service	BAT – bouquet association table NIT – network information table Including a descriptor for the modulation scheme and parameters.	SDT
Describes the programs contained in each TS	SDT – service description table	EIT
Describes the scheduled events for each program	EIT – event information table	Present and following events
Describes the changes in the scheduled events	RST – running status table
Date and time	TDT – time and date table TOT – time offset table
	ATSC-PSIP tables
Information for set-top-box operation	MGT – master guide table	EIT and ETT (PID)
Information for navigation and tuning	VCT - virtual channel table Including a field for the modulation mode and parameters.	EIT (program source identifier)
Information for the electronic program guide	EIT - event information table	Events per range of 3 hours
Extended message information to describe programs and events	ETT - extended text table
Parental guidance reference	RRT - rating region table
Date and time	STT - system time table

Main Service-Information tables

For more information, please send your query to mpeg@jdsu.com.