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FAQ

What is DVB subtitling?

What is DVB subtitling?
The DVB group have specified the means by which one or more subtitle stream(s) can accompany any or all video
services within a multiplex. This specification is known as ‘DVB Subtitling’ or ETSI 300 743 ‘DVB Subtitling
Systems’.
The specification details two methods of transmitting subtitles – ‘bit-map’ and ‘code based’.
DVB subtitling relies on DVB subtitle compliant solutions at both ends of the transmission chain. Both the subtitle
encoding and the decoding processes have to be DVB subtitling compliant. The presence of the DVB logo on
either encoder or decoder hardware is not proof that the equipment supports DVB subtitling.
Why are subtitles important?
For two reasons: firstly for the benefit of the hard of hearing viewing community and secondly for language
translation.
There is a significant percentage of any group of TV viewers who are either totally deaf or whose hearing is
impaired to the extent that they prefer to use subtitles in preference to, or to complement, the audio track. It is
becoming commonplace for governments to mandate increasing levels of hard of hearing subtitling as part of
digital TV transmissions.
Digital broadcasting is becoming a global business and satellite transponders have no respect for geographic
regions or country borders. Hence a broadcaster will often target TV channels at a viewing population who do not
necessarily share a common language. DVB subtitle compliant receivers can be configured to display subtitles in
a range of languages.
Where are DVB subtitles decoded and overlaid on the TV picture?
The obvious place and time to overlay DVB subtitles onto a TV picture is in the viewer’s home as he watches the
TV programme using a digital receiver. Only in this way does the viewer have complete choice over whether
subtitles are needed at all and if so in what language.
For this to happen there has to be a digital signal path all the way into each home. This could be direct-to-home
satellite, terrestrial or by digital cable television.
However it is often the case that digital services are down-linked from satellite and re-broadcast locally such that
the viewer receives the signal on a conventional analogue cable or terrestrial TV network. In a hybrid system such
as this, there is the opportunity to decode and overlay subtitles in one chosen language at the downlink location
prior to analogue re-broadcast. There are a number of professional DVB IRDs (integrated receiver decoder) that
support DVB subtitle overlay.
What is DVB bit-map subtitling?
The DVB bit-map method operates by converting each subtitle row into a graphics image and transmitting it as a
bit-map object.
Softel have adopted this bit-map form as the preferred standard for DVB subtitling because it passes all
responsibility and control for the subtitle size, font, colour, embellishments and outline to the broadcaster. Bitmaps
allow complete flexibility over how subtitles are presented to the viewer and any changes to this presentation
can be made independently of the receiving hardware.
What is DVB code-base subtitling?
The DVB code-based method operates by transmitting each character within a subtitle using a unique character
code. This method relies on the use of character code look-up tables that are shared between encoding systems
and the font processor of the receiving hardware and for there to be at least one subtitle font resident in the
receiver.
Although there is the payload advantage of requiring less transmission bandwidth than the bit-map method, the
code-based method is seen as being inflexible and is rarely used.
All subsequent sections of this document are based around the use of bit-map subtitling.
Are other forms of subtitling used in DVB networks?
There are two other means of subtitling; open ‘overlay’ subtitles and Teletext subtitles.
Open subtitles are keyed onto the video picture and are thus seen by all viewers. The disadvantages of open
subtitling are that the subtitles are a nuisance to those viewers that do not need them, and secondly there are
technical issues arising from MPEG encoding fast moving video sequences having static captions.
Teletext subtitles are a throwback to analogue days when VBI Teletext data was often used to provide a subtitling
service to viewers equipped with Teletext-capable televisions. The DVB broadcast standard continues to support
Teletext services (i.e. both information pages and subtitling) and DVB receivers may optionally either decode and
display Teletext services, or in the case of DVB set-top decoders they may re-insert the VBI Teletext data into the
analogue video signal passing to the TV set.
There are a number of disadvantages associated with Teletext subtitling of DVB services: (i) there are few DVB
receivers capable of supporting the display of Teletext services. (ii), a service operator cannot be sure of the
penetration of Teletext-capable televisions in viewer’s homes. (iii), Teletext decoders support only a subset of
international character sets depending on where they were purchased. (iv), the Teletext support for many Middle-
East and Asian character sets is either poor or non-existent. (v), Pre-prepared subtitle files may not be available in
a format suitable for Teletext transmission (for example, Teletext displays a maximum of 37 characters per row
whereas some subtitle workstations will create subtitle rows having 40 or more characters).
How is each subtitle encoded into DVB format?
Each character in a subtitle is rendered into a bit-map image following encoding rules defined locally by the DVB
subtitle processor. This establishes the height and width of each character as well as any other embellishments
such as italics, bold or underline. Character outlining and aliasing are also performed by the rendering process.
The colour of each pixel in the image is determined by a colour lookup table, which can contain up to 256 colour
definitions.
A ‘region’ has to be created on the TV display for each row of subtitles. A region will normally have a background
fill colour or transparency level associated with it.
FAQ – DVB Subtitling – Copyright Softel Ltd 2001
How is a DVB subtitle data stream constructed?
A typical DVB multiplexed signal carries six or more separate video channels, associated audio channels, and a
selection of service mapping tables and EPG information and a number of private data channels. Each of these
service components is carried as a ‘transport packet stream’ within a multiplexed stream and each service is
identified using a unique packet ID ‘PID’.
DVB subtitle data is carried within a private data stream. A single PID transport packet stream is generally used to
carry all the subtitles associated with a single video service although it is also possible for subtitle services for
multiple video channels to be carried within the same PID stream.
It is not within the remit of this document to describe fully the structure of a DVB subtitle data packet and interested
readers should obtain a copy of the ETSI specification documentation. It is sufficient to say that there are a
number of key components within a stream which together constitute a single subtitle service:
The PID of a subtitle transport packet stream is important as DVB decoders are aware from the DVB
service tables which PID streams carry subtitles for each video service in the multiplex.
Within a transport packet stream, each individual subtitle service is identified using a unique subtitle page
ID. The subtitle page carries information about the display of each subtitle such as the co-ordinates of the
subtitle regions.
The subtitles and other graphics are carried as Objects each having a unique ID so that they may be reused
where appropriate.
Each display object (including subtitle rows) has an associated Presentation Time Stamp (PTS) which
indicates to the DVB decoder at which point in time (relative to the main DVB PCR clock reference) the
object should appear on the screen. This mechanism optimises bandwidth and simplifies multi-lingual
service delivery by allowing subtitles to be pre-delivered to the decoder.
The important criteria when it comes to evaluating DVB subtitle products are (i) the quality and flexibility of the bitmap
subtitling rendering process and (ii) the way in which the PTS data is handled.
What is the typical bit rate of a DVB subtitle stream?
A single language bit-map subtitle stream typically requires a bandwidth of between 50 to 100 kbps depending on
the size and colour density of the subtitles.
To put this into perspective, an MPEG video stream might require between 2 and 4 Mbps and a complete DVB
multiplex might total 20-40 Mbps.
How are DVB subtitles broadcast?
Firstly, subtitles are encoded into a DVB format transport packet stream (using a subtitle processor) and then the
subtitle stream is multiplexed or ‘merged’ with all the other video, audio, private data and service information (using
a DVB multiplexer) to form a complete DVB multiplex signal ready for broadcast.
MPEG
Encoders
Subtitle
Processors
DVB
Multiplexer
Video & Audio
inputs
Subtitle inputs
DVB service
information
This simplified diagram shows how DVB subtitle processors contribute transport packet streams containing
subtitles to the multiplexer in the same way as MPEG encoded video and audio streams.
The data flow between subtitle processors and the multiplexer is shown as bi-directional because the subtitle
processors require a real-time feed of the master clock reference (PCR) generated by the multiplexer in order to
generate accurate presentation time stamps (PTS) for each subtitle.
What form of data interface is used between subtitle processor and multiplexer?
Data is most commonly exchanged beetween subtitle processor and multiplexer using ASI connections although
Ethernet TCP/IP connections are also used.
ASI (asynchronous serial interface) has become the industry standard means of carrying high speed digital TV
transport stream data locally between hardware units. ASI uses conventional video cable and BNC connectors
and it shares many of its characteristics with the commonplace 270Mbps serial digital video interfaces used in
video facilities. Although a subtitle stream occupies typically less than 100kbps, multiplexer inputs often require
higher data rates to keep them alive and thus null packets are used to fill up the unused capacity on the link.
Where an ASI connection is used to feed the PCR clock reference data back to the subtitle processor, the signal
often contains the complete multiplexer transport stream output. The subtitle processor then needs to extract the
PCR from the stream.
The ASI interface is easier than TCP/IP to configure where there is master/standby fault tolerant hardware
because ASI signals can be routed through conventional video signal matrices.
Where do programme subtitles come from?
At any point in a broadcast schedule there will either be no accompanying subtitles, or there may be subtitles that
have been pre-prepared for the programme, or the programme may be subtitled ‘live’.
Where programmes are pre-subtitled, the subtitles may already be present on the associated video feed (ie.
subtitles stored as VBI Teletext data) or they may exist as a timecoded subtitle data file (or files in the case of
multi-lingual subtitles).
A DVB subtitle processing system has to be designed on a case by case basis to with regard to the source or
sources of programme subtitles.
What DVB subtitle processing products do Softel offer?
Softel have supplied subtitling preparation, transmission, and character generator products to both broadcasters
and subtitle facilities around the world for over ten years. It was therefore a natural decision for the company to
develop subtitling solutions for digital TV as soon as the DVB standards were defined around 1995. Indeed, Softel
were one of the first companies to offer DVB solutions and most of the early digital TV network adoptors around
the world such as On Digital here in the UK, Danmarks Radio in Denmark and the Australian Broadcast networks
all make use of Softel DVB subtitling processors.
Softel’s DVB subtitle processor (DiSP) is part of the company’s Swift range of subtitling products (Swift subtitle
workstations, Swift Character Generators and Swift TX subtitle transmission units).
Softel recognised that no two DVB subtitle systems would ever be the same and therefore the DVB product
needed to be both modular and scalable in order to meet the requirements of individual installations. Thus, in each
case a single or group of Softel Swift TX transmission units may be configured from the following hardware and
software components:
Software
Product Description
DiSP Digital Subtitle Processor – A PC Windows application responsible for converting
subtitle rows into DVB bit-map format and creating a DVB compliant subtitle
transport packet stream output. The DiSP receives subtitle input from a number of
possible sources including (i) Teletext subtitles, (ii) the file-based output from a
local schedule client (see below), (iii) real-time subtitles received via local Ethernet
or serial ports. A single DiSP supports one or more subtitle language streams
associated with one video service. Each DiSP currently requires its own PC
hardware although this may change in the future.
Config
Client
A configuration application used to define the characteristics of each DiSP. Each
processor is configured with settings such as DVB addressing, subtitle font and
presentation information. Some of the DiSP settings are stored in pre-configured
style sheets so that settings may be altered as required.
The configuration application may be run either on the local hardware or on any
other networked PC.
Schedule
Client
The scheduling client application is used in cases where some or all of the subtitle
output is derived from pre-prepared subtitle files. In such cases, the schedule client
maintains a subtitle playlist defining the order in which files are transmitted.
In Manual Mode, the schedule application allows files to be loaded into a playlist
and for playlist events to be started and stopped by the user.
In Automatic Mode, the schedule application creates a playlist derived from a 3rd
party automation system, and events are initiated and stopped under automation
control.
ASI
concentrator
A Windows application designed to concentrate a number of DVB transport packet
streams from individual DiSPs across a LAN into a single multi-PID transport
stream suitable for ASI connection to a DVB multiplexer.
Hardware
Product Description
2U PC
Chassis
A professional 2U rackmount PC with front panel display, hard disk, LS120 super
drive and slots for up to three plug-in interface cards.
1U PC
Chassis
A professional 1U rackmount PC with hard disk and a slot for one plug-in interface
card
VBI
Decoder
A plug-in Teletext decoder card and driver software designed to extract one or
more Teletext subtitle streams from a video signal. Factory settings for PAL or
serial digital video input.
Timecode A plug-in dual mode (VITC or EBU) Timecode decoder card and driver designed for
use when there is local playout of timecoded subtitle files using the schedule client
application.
ASI RW A plug-in ASI interface card capable of both reading and writing ASI streams.
Teletext
inserter
A plug-in Teletext inserter card and driver software designed to provide an
analogue Teletext VBI subtitle data output for installations requiring simulcast
transmission of both digital and Teletext subtitles.
Which subtitle disk formats can be loaded into a Swift TX unit?
Swift TX units are primarily designed to import industry standard EBU3264 subtitle files. In the case of non-Latin
character sets (such as Asian, Chinese, and Arabic), files can also be transferred from Softel’s Swift subtitle
preparation workstation using a Softel file format (based on the ESEF extended subtitle exchange format).
Softel can also provide file format converter utility software that supports a wide range of 3rd party subtitle file
formats (for example RAC, PAC, Cheetah, NCI Cap)
How do you configure a Swift TX Teletext to DVB subtitle transcoder?
Unit Software Hardware
2U Swift TX DiSP
Config Client
2U chassis
VBI Decoder
ASI RW
The configuration application is used to specifiy the Teletext subtitle page number(s) to be converted into DVB
format and the presentation styles.
There is a finite delay between decoding each subtitle and outputting it to the multiplexer, however the delay is
certainly less than the time taken to MPEG encode the video so overall the subtitles are presented at the correct
time.
What are the advantages in using Teletext to DVB subtitle transcoders?
If the subtitles are already present as VBI Teletext data within the video signal as it reaches the MPEG encoding
process, then there is no need for the subtitle processors to interface to the station playlist automation systems.
Instead, the subtitles are detected automatically as they arrive and transcoded in realtime. This can simplfy the
complexity of the subtitling installation and makes the subtitle units independent from the automation systems.
Where the video channels are coming off local video servers or VTRs, the subtitles may be pre-encoded into the
video as vbi Teletext data using a conventional Swift TX subtitle disk reader and VBI inserter. Where video
programmes are already Teletext-subtitled, or where the broadcaster has access to subtitle files for the programme
in Teletext format, then Softel would recommend the standard Teletext subtitle VBI data format be used. If,
however, the subtitle disks are received in a more general format (especially if they are in a non-latin character set,
or if subtitle rows exceed the maximum 37 characters allowed in Teletext subtitles), then Softel would suggest that
the subtitles are VBI encoded using a unicode-based Teletext private data format.
What is the most effective way of configuring more than one channel of Teletext subtitle
trancoders?
Digital TV systems are very rarely limited to a single video channel, so Softel have ensured that there is a scalable
solution for multi-channel installations. Simply replicating the single channel Swift TX transcoder in one possible
solution, but, considering both physical rackspace and the number of ASI connections to the multiplex hardware,
Softel offer an alternative approach:
Swift TX
ASI subtitle
output
ASI PCR
feedback
Video & Teletext
subtitles
Swift TX & ASI
combiner
ASI combined
subtitle output
ASI PCR
feedback
Video & Teletext
subtitle
inputs
Swift TX
Swift TX
Swift TX
Swift TX
In this configuration, the 2U Swift TX subtitle unit is acting as both subtitle processor and ASI concentrator. The
remaining 1U Swift TX units act as single channel DVB transcoders but their DVB transport packet stream outputs
are routed via a local LAN to the ASI concentrator which then outputs the individual streams together on a single
ASI connection to the DVB multiplexer.
The ASI concentrator also reads the multiplex PCR clock reference and distributes PCR across the LAN to the
other Swift TX units so that they can correctly timestamp each subtitle.
The configuration of these units is as follows:
Unit Software Hardware
2U Swift TX & ASI
combiner
DiSP
Config Client
ASI combiner
2U chassis
VBI Decoder
ASI RW
1U Swift TX DiSP
Config Client
1U chassis
VBI decoder
How can you protect a multi-channel configuration against unit failure?
The operator has first to decide how important the subtitling service is to his customers and his business, and then
design the system architecture to meet this objective.
If the operation is manned at all times, and a subtitle service failure of 30 minutes is acceptable, then a warm
standby replacement unit would suffice. If a subtitle service failure is not accetable, then a hot-standby
configuration is required.
In the multi-chanel configuration above a second 2U Swift TX unit could be pre-configured with style sheets for
each of the units in the system. In the event one of the units failed, the warm standby could be installed and the
appropriate style sheet selected.
For an N+1 hot standby configuration a second 2U Swift TX unit would be installed on the network along with a
signal router to take care or re-routing signal inputs and outputs when the warm standby was required to take over
from a failed unit in the system:
The management of a hot-standby implementation requires some additional software and hardware product items:
Swift TX & ASI
combiner
(Standby)
ASI combined
subtitle output
ASI PCR
feedback
Video & Teletext
subtitle
inputs
Swift TX
Swift TX
Swift TX
Swift TX
Swift TX & ASI
combiner
(Master)
Signal
Routing
Matrix
Software
Product Description
Status
Monitor
A process that checks and reports the health status of other local Swift TX units.
The process will normally run on the standby unit so that take-over can be initiated
and also on one other unit to monitor the status of the standby unit.
Matrix
control
The standby unit runs a process which re-routes matrix signal inputs and outputs in
the event of take-over. If one of the 1U Swift TX units fails, the appropriate Teletext
subtitle inputs signals needs to be switched to the input of the standby unit.
If the Master 2U Swift TX ASI combiner fails, the hot switch-over becomes more
complex: both ASI outputs from Master & Standby ASI combiners are routed
through the matrix so the standby signal would be selected, in addition to re-routing
the Teletext subtitle input.
Hardware
Product Description
Routing
Matrix
Softel will recommend router matrix hardware that most suits each configuration.
Most matrix hardware supports both ASI and serial digital video connections but the
combination of ASI and analogue video inputs will require bespoke solutions.
How do you configure a subtitle file-based DVB subtitle system?
Unit Software Hardware
2U Swift TX DiSP
Config Client
Schedule client
2U chassis
Timecode
ASI RW
In manual mode, the Swift TX may be used to load one or more subtitle file into a local playlist and control subtitle
output using simple start, pause and stop commands. Once a subtitle event is started, the unit will output the
subtitle that matches the timecode input. The subtitle files must be loaded to the unit hard disk before transmission
(either by floppy input or by LAN file transfer). The Swift TX front panel LCD display and keypad is designed to
allow the unit to be used in simple one-shot manual mode without the need for an external VGA monitor.
It is more likely, however, that the DVB subtitle units will be used in automatic mode in an on-air transmission suite
where the units interface to the master programme playlist automation server. This interface allows the Swift TX
unit to construct a local playlist of all programme events that are due to be subtitled and it responds to event ‘play’,
‘pause’ and ‘stop’ commands from the remote system. An operator alarm is raised if a one or more subtitle files for
a subtitled programme is not found.
For more information on third party automation system playlist interfaces supported by Swift TX, please refer to the
Technology Partners section on www .softel.co.uk
Swift TX
ASI subtitle
output
ASI PCR
Programme feedback
Timecode
Playlist Interface
How does the file-based Swift TX configuration scale in a multi-channel environment?
The disk-based subtitle solution scales in the same way as the Teletext to DVB product. The difference is that
both 1U and 2U devices are fitted with the Timecode hardware option in place of the VBI decoder and that the
schedule client software is required on all units.
In a multi-channel file-based system, Softel recommend the use of a subtitle file store unit. This is a rack-mounted
Windows PC with a hard disk drive large enough to accommodate storage for all the programme subtitle files used
across the system. Prior to transmission, each Swift TX transfers a copy of the file(s) to local disk so that the
service is not dependant on a single point of failure.
How do you protect a file-based transmission system against unit failure?
The same warm or hot standby unit options are available as for the Teletext to DVB transcoder units. The only
diference is that, in the case of the hot-standby solution, provision has to be made through the matrix for routing
the appropriate timecode and playlist connection to the hot-standby unit in the case of unit failure.
How does Swift TX support both Teletext transcoding and file-based DVB subtitling?
In some cases it is necessary for the DVB subtitle process to accept subtitles from various different
sources depending on the actual TV programme or event. One example would be the case where
commercials are subtitled using Teletext data present in the video input but programmes are subtitled
using local subtitle files.
Unit Software Hardware
2U Swift TX DiSP
Config Client
Schedule client
2U chassis
Timecode
VBI Decoder
ASI RW
The schedule client has to understand when to select Teletext input and when to select file-based
output. This may optionally be controlled by the playlist interface, or one or other of the sources can be
programmed to overrule the other when there is input conflict.
 

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