In today’s modern broadcast stations where DVB is used, a multiple channel playout system will look as follows;
A file based playout system consists of a Subtitle Archive Management, SAM, a Subtitle Transmission Controller, STC, and one Subtitle Transmission Unit, STU, per channel. For a DVB system, in order not to use too many ASI ports on the video multiplexer there is also a S-MUX to multiplex the subtitles from multiple channels. There are also redundant systems, for SAM, STC and S-MUX, here we use a 1 + 1 solution and for the STU we recommend an N+1 redundancy although the system can also use a 1+1 solution or a mix of both.
Multiple channel subtitle playout system
The subtitle files are entered to the system through the Cavena Subtitle Archive and Management, SAM. The SAM used for import is normally placed on the office network. SAM can import files arriving as a mail attachment or saved to a directory on the network. SAM reports back to the designated persons and in the case of a mail import also to the person sending the file. The report can be “file accepted” or “file not accepted” with a log file showing the errors. There is an advanced safety system included for the mail-in function. The SAM can send reports to external system allowing third party Material Management systems to include the information of subtitle import to SAM. Once a subtitle file is imported to the first SAM it is automatically forwarded to the second SAM which is placed on the transmission LAN.
The Cavena subtitle transmission controller (STC) can in the standard configuration handle subtitling control for up to 24 channels of programs. The STC uses playlists to retrieve the correct files from SAM. The playlists are text files containing one line per program. Each line has columns for Event time, Name of event, Name of subtitle file (optional for those not using the same name for event and subtitle file), SOM, Start Of Message, for event, EOM, End Off Message, for event, language codes (for the languages used in the event) and name of program. The playlist can be made with a standard text editor or received from the automation system but best is to receive the playlist from the Traffic system, since this will allow the playlist to be available many days in advance. Once the playlist is available STC will start to pull files from SAM. The STC has a built-in WEB-server and this is used among other things to make available a web-page containing a missing files list. This list shows the missing files sorted in date and time of day allowing the supplier of subtitle files to the system to control that all files required for the programs for a certain day are available in the system. The STC handles updates of the playlist by reading the new playlist from the same directory. A playlist updated a minute before a program change will allow the STC to pull the file and put it on air in time. Furthermore as long as a program is in the playlist the subtitle playout for that program will start even if there are no subtitle files available and when these subtitle files become available, imported to SAM, they will immediately go on air without a requirement for a second start of event.
The STC stores the settings for all Cavena Subtitle Transmission Units, STU and can download the setting used for a faulty STU into the redundant STU. The STC also controls the routing of the time code to the redundant unit.
The Cavena STU is the unit that plays out the subtitle files. The STU reads the time code off program video and plays out the subtitles synchronised to the program. There is one subtitle file per language. Each subtitle language can be played out in one or more formats. Some customers use this to play out the same subtitle file for different feeds with different formats for instance in-vision for the local feed and closed captions for the satellite feed others use it to transmit the subtitles for both the 625 and the 525 feed from the same system. It is possible to replace one language, while the rest are playing. To replace a subtitle file it is enough to place the edited copy of the file in the SAM. The system will proceed to exchange the file on air and this will only take a couple of seconds. The old file will keep playing until the new file replaces it on air so there is no visible loss of subtitles. The STU output formats are teletext for analogue and SDI, US closed captions for analogue and SDI, DVB teletext, DVB Bitmap for SD and HD, US HD formats for closed captions and finally in-vision subtitling for both SD and HD SDI. The format is selected per language in the STU and it is possible to use one or several formats for each language. Each format has separate settings for delay and pre-transmission. Each language and format can be transmitted to one or multiple ports for instance it is possible to transmit the same data to a main and a redundant DVB multiplexer.
The Cavena subtitle multiplexer (S-MUX) is used to multiplex the subtitle streams from several STUs in order to minimize the number of ASI ports used on the video multiplexer. S-MUX also has a throttling function to control the bandwidth used in the total stream.