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Subtitling - Transmission
FAB DVB Interface is a software package for PC that can be used to receive DVB Subtitles over TCP/IP from FAB Subtitler BCAST/XCD and converts and synchronizes them to a PCR present on ASI/IP input and transmits them on the ASI/IP output to a DVB multiplexer.
Instead of using ASI cards one or more additional network cards can be used in the PC to use ASI over IP without any special hardware. One FT-DVB software license is required for every DVB Multiplexer present on the ASI/IP input. Multiple ASI cards and network cards are supported per PC.
Main functionalities of FAB DVB Interface:
Figure 1: DVB Subtitling Transmission for multiple channels and up to two DVB multiplexers
Figure 2: DVB Subtitling Transmission for multiple channels and more than two DVB multiplexers
DVB subtitling is used in satellite and terrestrial DVB (Digital Video Broadcast) transmissions. Only Set Top Boxes (DVB decoders) that support DVB subtitles are capable of displaying DVB subtitles.
DVB subtitles are transmitted in a separate DVB stream with a separate PID. Every subtitle contains a PTS (presentation time stamp) which is defined by the subtitle transmission system. The PTS is calculated from the PCR/video stream which comes from the DVB ASI output of the DVB multiplexer and is connected to the input of FAB DVB Interface.
The DVB encoder/multiplexer must be configured so that the subtitle streams with correct PIDs are passed through and the PMT (Program Map Table) must be configured correctly so that Set Top Boxes will associate subtitle streams to video streams correctly.
Multiple FT-DVB licenses with multiple ASI cards can be installed in one PC: One FT-DVB license is required for every connection to a DVB multiplexer.
FAB DVB Interface supports a special feature for PTS restamping of DVB subtitling and teletext streams. This function receives multiple DVB subtitling and teletext streams and calculates the time and PTS offset between the packets of the received DVB subtitle/teletext stream and the referenced PCR stream. The DVB subtitle/teletext stream is retransmitted on the output but referenced with the same time and PTS offset to a different PCR stream. The DVB subtitling/teletext stream can also be delayed for a constant time defined in miliseconds.
This function is necessary when video/audio is received, reencoded and retransmitted so that the PTS of the retransmitted DVB subtitling/teletext stream is correctly referenced to the PCR of reencoded video/audio.
Figure 3: PTS restamping for DVB Subtitle and Teletext Streams
In the above example 2 ASI/IP inputs and one ASI/IP output are necessary:
- One ASI/IP input is used for receiving the DVB stream from the DVB receiver. The stream must include the PCR/Video PID and the DVB Subtitling or Teletext PID that should be restamped.
- The connection to the DVB Multiplexer is using one ASI Input to read the PTS and PCR from the Video PID to which the DVB Subtitling or Teletext PID should be restamped. The ASI Output will contain only the DVB Subtitling or Teletext PID with correctly restamped PTS so that they will correspond to the PCR/PTS of the Video PID on the DVB Multiplexer.
FAB DVB Interface can receive teletext over SD/HD-SDI and convert it to ASI/IP synchronized to a PCR which is present on the ASI/IP input. One SDI input can be converted to multiple ASI/IP output streams which are synchronized to different PCRs. One or more FAB FT-HT 11 DCD cards must be installed in the PC. Each card has two SD/HD-SDI inputs.
Figure 4: SDI Teletext to ASI/IP Conversion
FAB DVB Interface always uses the ASI ports on the FT-ASI cards so that one is input and one is output. The FT-ASI 1 card has one input and one output. The FT-ASI 2 card has two inputs and two outputs. However a license for FT-DVB software is only required if the output is actually used. Therefore the above configuration requires only one FT-DVB license. Additional FT-DVB licenses are required for every DVB Multiplexer to which the subtitles have to be sent to. Additional ASI/IP inputs for receiving DVB Subtitling or Teletext Streams do not require additional FT-DVB licenses.
FAB DVB Interface can transmit signalisation for hbbtv applications and transmission of stream events as defined in ETSI TS 102 809 can be triggered by a simple HTTP request.
Multiple hbb-tv apps can be included in the signalization within the transmitted TS with the link to the App URL, App ID and all other necessary parameters:
- AIT PID, App language, App name, App URL, App ID, Boundary extensions, Control code, Priority, Service bound, Digital text, Object carousel, Component tag, Remote, ONID, TSID, SID
To signalize a stream event the parameters in the HTTP request define the type and content of the stream event. The following parameters can be defined for a stream event:
- Events PID, Component tag, Event range
Example of a trigger for a hbbtv stream event:
Detailed parameters for stream events are defined in the configuration of FAB DVB Interface.