Subtitling Question & Answers

In this section you find the most frequently asked questions, FAQ, with answers regarding subtile preparation and subtitle transmission.

Subtitling preparation

Q: What is a prep station?

A prep (short for preparation) station is a specialized product for translators and subtitle editors.

 

Aside from being a basic text editor, it also has many special functions to make editing of subtitle texts efficient. A prep station also handles editing of time codes for subtitles in a file. This is often synchronized directly to video that is played within the prep station. Old systems relied on analogue video, thus requiring extra hardware. Modern prep stations like Tempo uses Windows Media and MPEG video files, for example. This allows for even faster production of subtitles. It also means there is no requirement for additional hardware, allowing the users to run the Tempo subtitle preparation station on a laptop.

Q: How to temporary change VPOS, font, size, background color etc?

We get many questions about vertical position, VPOS, font, size, background color etc and subtitles, here follows a summary.


The main content of a subtitle files is text and time code (in- and out-time of each subtitle). Some subtitling files are made in Word and contains no further information, some in special formats contains more info. Lets presume the translator, working from his/hers PC wants to lift the subtitle text in the picture, temporary and reason being not to cover burned in text. Another temporary request could be to change font or background or color, this is often used with hard of hearing subtitles to indicate a background noise or similar.

Q: How do we achieve this temporary change of font, size, background color etc?

By assigning attributes to each individual subtitle or text row, a translator is in full control to improve the overall quality of the programme by making subtitles cover a foreign language text, or the opposite to move a subtitle so it doesn’t cover areas of the picture normally used by the subtitles. To make this work these attributes must be stored in the subtitle file. Most text formats only store text and timecode and nothing more, but of course it’s possible to implement attributes for this. But this is the reason there already exists subtitle file formats that support various attributes for this purpose. The EBU subtitle file format supports vertical position, justification and colors, for example. The Cavena 890 format adds exact line vertical position, background type (like black box or not), transparency level, dual font support and Asian language support.

 

Fonts are generally not controlled by the translator, but is set by the broadcaster as it is often a channel branding factor. However, in the Cavena Tempo and 890 format it is possible to switch between a main and secondary font, typically an open subtitle and closed subtitle font in a mixed file containing both open and closed subtitles. Mixed files are not supported in the EBU file format, mixed files are used in countries that normally use subtitles for translated text and the programme have local language spoken, which is then subtitled via closed transmission formats like teletext.

 

Override or do all setting at the broadcasters site will not give us the possibility for required temporary changes.

 

The solution is a combination, where the translators prep tool works with default values on all attributes, so default vertical position, default background etc. The actual vertical position, background etc is then set by the broadcaster in the transmission equipment. Now if the translator lifts the subtitle vertical position, or sets a black box background etc, those specific attributes are fixed for that specific subtitle and overrides the transmission settings.

 

So for proper handling and display of subtitles, giving higher quality experience for the viewers, text and time code is not enough. Therefore use of better subtitle file formats than simple text documents is needed.

Q: What is the difference between captioning and subtitling?

Subtitling is the basic term for conveying the dialogue of a program using text displayed in the picture. Captioning is a term used for subtitling for hard of hearing, mainly in USA.

Q: There are many subtitle file formats, 890, CIP etc, which one should I use?

Subtitle are stored within a file, typically all subtitles for a specific program for one language in one file.


There is no universal standard that everyone uses. STL is a format defined by EBU targeting the use of subtitles in the teletext transmission format, primarily. Different manufacturers have often created their own formats to better support other features and languages. The 890 format is the file format defined and used by Cavena from translator products to transmission products. It is a widely used format in the subtitling market. the CIP format is the format used by the modern preparation station Tempo. The conversion between file formats, and the conversion between different TV standards and time code standards, can be done using the Toolbox product from Cavena.

Q: What are the hardware requirements for PCs?

Standard x86 based PC running Microsoft Windows is required. All products runs on Windows XP and Windows Vista, Windows 7

 

The transmission products also support Windows 2000. Some of the optional hardware requires XP/Vista, though. All current hardware drivers require 32-bit Windows. Cavena can supply turn-key ready systems, or ship software and optional hardware separately for installation on PCs purchased and supported locally.

Subtitling transmission

Q: What are the external interfaces to a Cavena system?

There are a number of input and output interfaces of a Cavena system, pls read more here in a specific document about “Interfaces to external systems”.

Q: What is in-vision subtitling?

In-vision means having a video signal pass through a subtitle inserter that inserts subtitle text into the picture.

 

The output of the subtitle inserter is the video signal with text overlayed. When this is done in an offline production environment, it is also called burn-in subtitling, as the result is a new recording with the subtitle texts in the video picture.

 


What bandwidths does “subtitling” require?

15-25 kB/s is one rule of thumb, for more information please read following document: “Bandwidth analysis for embedded DVB subtitles”.

Q: Why so many transmission formats like DVB, teletext etc?

When transmitting subtitles closed (as separate data from the video) there are various protocols depending on the transmission format.

 

The most commonly used are: The EBU teletext protocol for the VBI of a TV signal can be used to transmit subtitles, this is widely used in Europe for heard of hearing subtitles and also for multi-lingual subtitling.

 

DVB Subtitling is used in digital DVB transmissions. Since it is bitmap based, it supports any language and is therefore not only used in Europe, but also commonly in Asia and other parts of the world. Line-21 closed captioning is also using the VBI in TV signals to transmit subtitles. This standard is mainly used in Unites States for heard of hearing people.

 

Read more about subtitle file formats and subtitle transmission formats.

Q: How many languages of subtitling can be transmitted in one channel?

This depends entirely on what transmission format is used.

 

The Cavena transmission system has no limit on the number of languages and formats than can be transmitted. For example, in EBU teletext, theoretically you can have 800 languages. However, as languages are transmitted on separate teletext pages and pages are transmitted serially, each language added causes a delay to when it is actually displayed. Therefore there is a practical limitation in teletext to the number of languages that can be used for subtitling.

Q: How do we transition our existing transmission system to HD?

When using a Cavena transmission system to playout subtitles from files, synchronized to video using time code, it is very easy to change the transmission formats.

 

Converting, or simply adding, HD subtitling can be done using only configuration changes and only add hardware if required by the signal formats. For example, if the time code feed is changed from SD to HD, a timecode reader board supporting HD signals must be used. If transmitting in-vision subtitles, an HD compatible inserter must be installed if not present. For DVB transmissions, changing to HD requires only configuration changes. No changes are required to the subtitle files, the same files can be used for any transmission format.

Q: Can we re-use subtitles in the VBI on a tape?

Cavena can supply tools that reads the video signal when played out from tape, extracts the VBI (EBU Teletext or Line-21) and creates subtitle files that can be used for file based transmission in any format.

 

The Cavena transmission system can also be configured to extract VBI from tapes and trans code the subtitles live into any transmission format, including DVB, in-vision, HD etc. The transmission system can also swap between file based playout and transcode mode for the same channel, thereby allowing a playout using both subtitle files and VBI material from tape or servers.