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.
Q: What bandwidths does subtitling require?
15-25 kB/s is a rule of thumb.
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 hard 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 the Unites States for the hard of hearing.
Q: How Many Languages of Subtitling Can Be Transmitted In One Channel?
This depends entirely on what transmission format that is being used.
Edgeware’s Cavena-based 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 our Cavena-based 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?
Edgeware can supply tools that read the video signal when played out from tape, extract the VBI (EBU Teletext or Line-21) and create subtitle files that can be used for file-based transmission in any format.
Edgeware’s Cavena-based transmission system can also be configured to extract VBI from tapes and transcode 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.