contribution
Subtitle transcoding and
subtitling on pass-through
Experts at Screen
Subtitling Systems
guide users through the
intricacies of the process.
this White Paper will concentrate on the ‘real
time’ option. Here ‘real time’ refers to the fact
that the transcoding is done directly in the
transmission path as quickly as possible so to
appear seamless to the viewer. See the separate
White paper on File Based Subtitle Format
Conversion.
satellite there is often a need to decode, recode
or retime the subtitles and captions.
Throughout this piece we will attempt
to describe a range of methods by which
this can be achieved and the advantages
and disadvantages of each and we’ll cover
both SD & HD. In some cases there will be a
requirement to transcode from one subtitle
format to another.
ith laws surrounding the provision
of TV access services being
introduced a few years ago across
Latin American countries, as a subtitling
technology company, Screen has been keeping
a keen eye on how it’s working out and the
challenges that are being faced.
It appears that more and more services
that are sourced from countries outside the
region are being rebroadcast alongside advert So, why Transcode?
Transcoding is often required where a service
is first broadcast with subtitles in one format
but downstream is re-broadcast and requires
subtitles in a different format. A common
example is where analogue services are
simulcast as digital services. The analogue
services require subtitles in Teletext format but
the digital service requires DVB subtitles. In
this case the Teletext subtitles are transcoded Open or Closed Subtitles
There are two main categories of subtitles,
open and closed.
Open subtitles are added into the visible
part of the video image and cannot then be
changed. The process of adding or ‘burning in’
the subtitles can be done before transmission
by the original broadcaster or during
transmission at one or more of the headends.
Open subtitles are only suitable where a single
language is required and in the context of this
whitepaper this is one option for the output
from the headend.
Closed subtitles (or captions) are subtitles
that are carried in part of the broadcast signal
insertion and video re-encoding.
With captions for the hard-of-hearing
now mandatory on some content, closed
caption/subtitle insertion and retiming is also
inevitable. With many legacy set-top-boxes
(STBs) still prevalent in the region there
appears to be an ever increasing requirement
to transcode the captions too. Whilst many of
the newer STBs can handle this, there is still an
abundance of old models which struggle with
the process leading to captions being displayed
badly placed, in the wrong colour or even not
appearing at all.
Screen has a wealth of working experience
of not only transcoding the caption/subtitle
file but also correcting them to overcome these
problems. to DVB. The delay in the signal due to the video
MPEG encoding can be used to allow time
for the subtitle encoding so the presentation
timing of the DVB subtitles closely matches
that of the original Teletext subtitles. that is not normally visible to the viewer.
Special receiving equipment either at a
headend or at the viewer’s site is required to
select and display the subtitles.
Closed subtitling has the advantage that
Transcoding subtitles and processing them
for pass through is complex but here we’ll
attempt to demystify the process.
As possibly a simpler process to explain,
let’s begin by covering off Transcoding.
In the context of subtitling, transcoding
refers to the process of taking subtitle data in
one format and recoding it into another format.
This can be considered a special case of pass-
through subtitling but we’ll get onto that in a
moment.
Although the transcoding process can
be done ‘off line’ by receiving, recording,
editing and then re-transmitting the data, There are a number of subtitling formats and
it is not possible to transcode from all types.
The following table shows the options and
feasibility of transcoding. more than one language can be supported
as well as hard of hearing subtitles (closed
captions). In the context of this whitepaper
closed subtitles are used to deliver several
simultaneous subtitle languages to the headend
or directly to the viewer.
W
12 LATAM Briefing
Subtitling on Pass-Through Channels
Again, in the context of subtitling, ‘pass-
through channels’ refer to channels that are
received and retransmitted either in real time
or with a short delay. This configuration is used
extensively by cable and satellite operators
where channels from other broadcasters are
combined onto their platform at a headend.
Where multiple services are received off
Let’s Look at Some Examples
Open Subtitles – using a professional
decoder (IRD) at a headend.
Closed subtitles for a range of languages are
supplied by the original broadcaster in a
suitable format such as Teletext, DVB bitmap,
Imitext or SCTE 21 (EIA-608 & 708) closed
captions.
At each headend a professional integrated
receiver/decoder (IRD) decodes the video and
adds the required subtitle language into the
picture. The resulting combined video with
open subtitles is then re-encoded for broadcast.