Redistributing Free-To-Air Content for IP Networks

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Redistributing Free-To-Air Content for IP Networks

Jonathan Peacock
Product Manager - Digital & Broadcast, Sematron UK Ltd

August 2009

Content

Introduction

Digital TV – Important Parameters

Encoding MPEG-2 MP@ML

Encoding MPEG-4 H.264/AVC

Multiplexing & Transport Containers

Modulation

MPEG PSI & DVB SI

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Introduction

Making use of multiple free-to-air services received from satellite or terrestrial
transmissions has never been easier and the technology is available to distribute
some or all of the services received across buildings or campus networks. This
paper highlights some of the technologies used to implement these gateway
services for IP distribution and these are found within Sematron’s Semstream
Gateway DVB to IP device.

Semstream Gateway’s design criteria was a simple and intuitive user interface that
could harness cost-effective IT components, such as industrial PC hardware and
plug-in DVB-T, DVB-S and DVB-S2 tuner cards, and allow these to be
automatically tuned, and the resulting services processed so that they could be
distributed via unicast or multicast routing and selected at display screens across
the organisation.

Figure 1. Service Delivery Model

Recent industry trends have been to make free to air and scrambled Digital TV and
Radio available on multiple platforms. For example in 2009 the UK had 6
multiplexes transmitting approximately 32 services free to air over the DVB

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Terrestrial network. The Services referred to here are made up from streams such
as video, audio, data etc. A key requirement of a receiver is that it needs to be able
to distinguish between individual services and must have knowledge about stream
information. Figure 1 shows the Service Delivery Model for digital Broadcasting [1].

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Digital TV – Important Parameters

Digital TV has embraced the MPEG-2 standard and in particular its parts describing
video coding, audio coding and systems in ISO/IEC 13818. These standards have
helped drive compression applications which provide acceptable pictures and
sound at a much reduced bit rate compared to the original uncompressed material.
The multiplexing techniques, specified at the systems layer in ISO/IEC 13818,
ensure that the picture and sound information are combined into a common
bitstream and digital modulation is utilised to reduce the RF bandwidth to a level
appropriate for the service carrying the MPEG data. Error correction systems within
the transmission standard are designed to be able to cope with recovering low
signal strength signals to give acceptable pictures and sound [2].

Characteristics common to both MPEG-2 and MPEG-4 compression systems are
profiles and levels and the compression system architect would typically pick the
best tool to use from the toolbox provided for the intended application. For
example digital television makes extensive use of MPEG-2’s Main Profile and Main
Level (MP@ML). The benefit of specifying profiles and levels is that they specify
conformance points to help interoperability in the real world.

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Encoding MPEG-2 MP@ML

Some common parameters found in Main Profile includes I, P and B frame picture
coding types and Group of Pictures or GOPs, Chroma Format of 4:2:0 and several
aspect ratios [3]. Generally the profile specifies a set of coding tools that can be
used in generating a conforming bitstream.

Several examples of the constraints in Main Level are a Frame rate of 25 Hz, a
maximum horizontal resolution of 720 pixels (compared with 1920 pixels for High
Level), a maximum vertical resolution of 576 lines (compared with 1152 for High
Level) and a maximum bit rate of 15 Mbit/s (compared with 80 Mbit/s for High
Level). Generally the level places constraints on certain key parameters of the
bitstream.

Other parameters that are used within TV services are support for interlace video,
8-bit video sampling and standard definition video at resolutions of 720, 704, 544 ×
576 pixels for a frame rate of 25 Hz. An example of the use of different resolutions
can be found on the UK’s DVB-T multiplex where lower priority channels use sub-
sampled video (only 544 pixels per line rather than 704 or 720 pixels) in order to
reduce the bitrate for that service [4]. Other variable quality factors also appear to
have been applied to the BBC channels.

As technology develops and advances business applications are keen to see
improvements in compression performance and the efficiency of the delivery
mechanism. For example the UK’s UHF Band IV/V is divided into 49 channels and
before digital switchover 46 of them were used for both analogue and digital
television broadcasting whereas after 2012 the six existing television multiplexes will
occupy 32 channels giving a surplus equivalent to 14 spectrum television channels,
each containing 8 MHz i.e. a surplus of 112 MHz [5]. The spectrum used for
broadcasting must utilise advances in technology, such as DVB-T2 and H.264
compression, to aid the efficiency of the delivery mechanism.

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Encoding MPEG-4 H.264/AVC

The H.264/AVC specification was published in 2003 by ITU-T as Recommendation
H.264 and by ISO/IEC as 14496-10 and are jointly maintained so that they have
identical technical content. H.264 was developed for a wide variety of applications
for example digital storage media, TV broadcasting and Internet streaming. These
applications covered a wide range of bit rates, resolutions, qualities and services.
Eleven profiles are defined in ISO/IEC 14496-10:2008 of which four are: Baseline,
Main, Extended and High. The relationship between these profiles in terms of the
major tools available in the toolbox is shown in Figure 2 [4] [6].

Figure 2. Relationship Between High Profile And The Three Original
Profiles

The most powerful variable length coding scheme, CABAC, can be seen to be
available as a tool in Main Profile and High Profile and it has been estimated that
this gives about 10 to 15% saving in bit-rate compared to the simpler CAVLC [4].
In the DVB audio-visual coding specification, TS 101 154, High Profile support is
required for HDTV decoders which implement H.264. Remember that MPEG only
defines the syntax and semantics of the bitstream and not the actual encoder
implementation and this leaves scope for better compression implementations as
technology improves.

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Multiplexing & Transport Containers

The delivery mechanisms typically employ MPEG Transport Streams (TS) as inputs
into the RF modulation systems at the transmitters. Video and audio channels are
coded as MPEG Elementary Streams and several of these streams are typically
combined into a continuous sequence of Transport Stream (TS) packets shown in
Figure 3 [7]. Each packet has a length of 188 bytes with the first 4 bytes containing
the header of the TS packet and the following 184 bytes used for the payload. The
most important components of the header are the synchronization (sync) byte and
the Packet ID (PID).

Figure 3. Transport Stream Packet Structure Overview

The PIDs which identify the numerous Elementary Streams within an MPEG TS are
easily identified by giving each a unique thirteen-bit code.

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Modulation

The DVB standard is actually a group of ETSI standards. For example EN 300 468
is a specification for Service Information (SI) in DVB Systems and EN 300 744
describes DVB-T: Framing Structure, Channel Coding and Modulation for Digital
Terrestrial Television. DVB uses an MPEG-2 TS to convey compressed digital
video, compressed digital audio and data over a channel. The MPEG-2 TS has a
maximum bit rate of about 24.1 Mbps for an 8 MHz DVB-T channel. DVB-S
channel rates are dependent on the transponder bandwidth and code rates used
and can approach 54 Mbps [5].

The DVB-T Terrestrial transmission format in the UK uses a multiplexing method
known as COFDM (Coded Orthogonal Frequency Division Multiplexing). The data is
distributed over many carriers as opposed to using a single carrier and as such the
relative data rate transmitted on each COFDM carrier is relatively low. Typical
carrier values are 2048 and 8192. Standard modulation methods are used to
modulate the COFDM carriers such as QPSK, 16-QAM and 64-QAM.

Table 1. Useful Data Rates (Mbps)

The DVB-T specification has a range of different transmission parameters that can
be chosen to give different trade-off points of robustness versus bit-rate. Two
different parameter sets, chosen from the available combination in Table 1 [8], have

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been used in the UK giving Multiplex capacities of between 18.10 Mbps and 24.13
Mbps each respectively. For example typical values for Multiplexes are 16QAM
modulation, 1/32 Guard Interval, 3/4 FEC Rate and 64QAM modulation, 1/32
Guard Interval, 2/3 FEC Rate [4].

DVB-S satellite systems typically make use of QPSK for broadcast applications.
The more recent DVB-S2 standard makes use of the latest modulation and coding
techniques to deliver performance that approaches the theoretical limit for such
systems. DVB-S2 supports four modulation modes with QPSK and 8PSK intended
for broadcast applications in non-linear satellite transponders driven close to
saturation and 16APSK and 32APSK mainly targeted at professional applications
such as news gathering and interactive services [9].

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MPEG PSI & DVB SI

Figure 4. Tables Within the MPEG TS

Once a receiver has achieved sufficient signal strength to lock onto the carrier and
correct any errors introduced by the transmission medium it then tries to
demultiplex the streams. The Program Specific Information (PSI) is important
because it enables the receiver to know what streams are present and which audio
belongs to what video etc.

A decoder can establish the relationship that exists for the services in the TS by
searching for all packets with a PID of zero as shown in Figure 4 [10]. PID zero
identifies the PAT and this regular stream of information contains a list of all the
programs in the transport stream. De-multiplexing is impossible without the PAT
and the lockup speed is a function of how often the PAT packets are sent, typically
no longer than 0.5 seconds.

Each program is described further by its own PMT and this gives an overview of all
components belonging to a TV channel. This leads the receiver to identify what
video, audio and data streams are available. The transport mechanism is based on

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either packetized elementary streams (PESs) or tables that are mapped into the TS
packets.

The Service Information (SI) forms part of the DVB bitstream and is a small
collection of hierarchical associated tables designed to extend the MPEG-2 PSI
tables. It provides information on what is available on other transport streams and
even other networks. There are four mandatory tables in DVB systems: The EIT,
NIT, SDT and Time and Data Table (TDT). The other tables in the SI include MPEG-
2 defined tables and DVB optional tables. Just like MPEG-2 the DVB uses
descriptors to add new functionality. In addition to various MPEG-2 descriptors
one or more of these DVB-specific descriptors may be included within the PMT or
one or more SI tables to extend data within the tables. A descriptor not recognised
by a decoder must be ignored by that decoder. This enables new descriptors to be
implemented without affecting receivers that cannot recognise and process the
descriptors. Two examples of descriptors are the AC-3 Descriptor indicating Dolby
Digital audio is present and the CA Identifier Descriptor which indicates whether a
bouquet, service or event is associated with a conditional access system and if so
identifies conditional access used.

The EIT table consists of one part containing information about present and
following event and one part containing schedule information. The event
information is name and description of the event, start time, duration and category.
For DTT networks, it is normal practice for a multiplex to carry EPG data (actual and
other EIT tables) for the programmes on the other multiplexes in addition to its own
services. Thus local insertion on one multiplex will also require changes to the other
multiplexes on air in this region. The EIT table can contain a large amount of data,
so it is important to control how EIT data is inserted. A well designed solution may
free a considerable amount of bandwidth which can instead be used for extra TV or
radio services.

Other information, such as tuning information, can be described in the NIT which is
referenced within program zero of the PAT. For a satellite broadcast the NIT would
describe the modulation parameters of the TSs, such as the orbital position,
polarization, carrier frequency and modulation scheme and it can be used to
convey changes of tuning information to the receivers. The service description
table (SDT) mainly translates program numbers into service names.

The Conditional access table (CAT) is used for conditional access to the channels
and provides information about entitlement messages and this area is beyond the
scope of this document.

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References

[1] EN 300 468 V1.9.1 Digital Video Broadcasting (DVB);
Specification for Service Information (SI) in DVB systems, ETSI 2009.

[2] Watkinson J., 2000. The Art of Digital Video Third Edition, Focal Press

[3] Jack K., 2005. Video Demystified Fourth Edition Elsevier Inc.

[4] McCann K., 2007. Review of DTT HD Capacity Issues An Independent
Report from ZetaCast Ltd. Commissioned by Ofcom.
URL http://www.ofcom.org.uk/consult/condocs/dttfuture/report.pdf (Last
accessed 27 July 2009).

[5] Unknown Author, September 2006. BBC The Spectrum and Its Uses A
simple guide to the radio spectrum.
URL http://www.bbc.co.uk/rd/pubs/spectrum/bbc-the-spectrum-and-its-
uses.pdf (Last accessed 28 July 2009)

[6] DVB Document A084 r3, 2009. Digital Video Broadcasting (DVB);
Specification for the use of Video and Audio Coding in DVB services
delivered directly over IP protocols
URL http://www.dvb.org/technology/standards/a084r3.tm2821r17.ts102-
005v1.4.1.pdf (Last accessed 25 August 2009)

[7] Information technology — Generic coding of moving pictures and associated
audio information: Systems, ISO/IEC International Standard 13818-1, 2000.

[8] Digital video broadcasting (DVB); Framing structure, channel coding and
modulation for terrestrial television, ETSI EN 300 744 V1.5.1, European
Telecommunications Nov. 2004.

[9] Morello A. and Mignone V., 2004. DVB-S2 - ready for lift off. EBU Technical
Review.

[10] Ladebusch U. and Liss C. A., 2006. Terrestrial DVB (DVB-T): A Broadcast
Technology for Stationary Portable and Mobile Use. Proceedings of the
IEEE, Vol. 94, No. 1.

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