Subtitle Transcoding and Subtitling on Pass-Through Screen Subtitling Systems explains the process With 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 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
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 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.*** 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. 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
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. 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 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.
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 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.
So, why Transcode? Transcoding is often required where a service is first
Open or Closed Subtitles There are two main categories of
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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 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 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. 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 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. Advantages: • No separate subtitle inserter required. • Suitable for situations where the video encoding standard needs to be changed. • No special subtitle decoder required by viewers. Disadvantages: • The video must be decoded and recoded: - Cost of equipment - Loss of video quality - Delay in the video path
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