What is DVB subtitling?
The DVB group have specified the means by which one or more subtitle stream(s) can accompany any or all video services within a multiplex. This specification is known as "DVB Subtitling" or "ETSI 300 743 DVB Subtitling Systems". The specification details two methods of transmitting subtitles - bitmap- and code-based. DVB subtitling relies on DVB subtitle compliant solutions at both ends of the transmission chain. Both the subtitle encoding and the decoding processes have to be DVB subtitling compliant. The presence of the DVB logo on either encoder or decoder hardware is not enough proof, that the equipment actually supports DVB subtitling.
Where are DVB subtitles decoded and overlaid on the TV picture?
The obvious place and time to overlay DVB subtitles onto a TV picture is in the viewer's home as he watches the TV program using a digital receiver. Only in this way does the viewer have complete choice over whether subtitles are needed at all and if so in what language. For this to happen there has to be a digital signal path all the way into each home. This could be a direct-to-home satellite, terrestrial or by digital cable television. However, it is often the case that digital services are downlinked from satellite and re-broadcasted locally such that the viewer receives the signal on a conventional analogue cable or terrestrial TV network. In a hybrid system such as this, there is the opportunity to decode and overlay subtitles in one chosen language at the downlink location prior to analogue re-broadcast. There are a number of professional DVB IRDs (integrated receiver decoder) that support DVB subtitles overlay.
How is each subtitle encoded into DVB format?
Each character in a subtitle is rendered into a bit-map image following encoding rules defined locally by the DVB subtitle processor. This establishes the height and width of each character as well as any other embellishments such as italics, bold or underline. Character outlining and aliasing are also performed by the rendering process. The color of each pixel in the image is determined by a color lookup table, which can contain up to 256 color definitions. A region has to be created on the TV display for each row of subtitles. A region will normally have a background fill color or transparency level associated with it.
How is a DVB subtitle data stream constructed?
A typical DVB multiplexed signal carries six or more separate video channels, associated audio channels, and a selection of service mapping tables and EPG information and a number of private data channels. Each of these service components is carried as a transport packet stream within a multiplexed stream and each service is identified using a unique packet ID PID. DVB subtitle data is carried within a private data stream. A single PID transport packet stream is generally used to carry all the subtitles associated with a single video service although it is also possible for subtitle services for multiple video channels to be carried within the same PID stream. It is not within the remit of this document to describe fully the structure of a DVB subtitle data packet and interested readers should obtain a copy of the ETSI specification documentation. It is sufficient to say that there are a number of key components within a stream which together constitute a single subtitle service. The PID of a subtitle transport packet stream is important as DVB decoders are aware of the DVB service tables which PID streams carry subtitles for each video service in the multiplex.
Within a transport packet stream, each individual subtitle service is identified using a unique subtitle page ID. The subtitle page carries information about the display of each subtitle such as the co-ordinates of the subtitle regions. The subtitles and other graphics are carried as Objects each having a unique ID so that they may be reused where appropriate. Each display object (including subtitle rows) has an associated Presentation Time Stamp (PTS) which indicates to the DVB decoder at which point in time (relative to the main DVB PCR clock reference) the object should appear on the screen. This mechanism optimizes bandwidth and simplifies multi-lingual service delivery by allowing subtitles to be predelivered to the decoder. The important criteria when it comes to evaluating DVB subtitle products are the quality and flexibility of the bitmap subtitling rendering process and the way in which the PTS data is handled.
How are DVB subtitles broadcasted?
Firstly, subtitles are encoded into a DVB format transport packet stream (using a subtitle processor) and then the subtitle stream is multiplexed or merged with all the other video, audio, private data and service information (using a DVB multiplexer) to form a complete DVB multiplex signal ready for broadcast. DVB subtitle processor (Subtitle Plus DVB Server) contributes transport packet streams containing subtitles to the multiplexer in the same way as MPEG encoded video and audio streams. The data flow between subtitle processor and the multiplexer is bi-directional because the subtitle processors require a real-time feed of the master clock reference (PCR) generated by the multiplexer in order to generate accurate presentation time stamps (PTS) for each subtitle.
What form of data interface is used between the subtitle processor and multiplexer?
Data is most commonly exchanged between subtitle processor and multiplexer using ASI connections although Ethernet TCP/IP connections are also used. ASI (asynchronous serial interface) has become the industry standard means of carrying high-speed digital TV transport stream data locally between hardware units. ASI uses conventional video cable and BNC connectors and it shares many of its characteristics with the commonplace 270Mbps serial digital video interfaces used in video facilities. Although a subtitle stream occupies typically less than 100kbps, multiplexer inputs often require higher data rates to keep them alive and thus null packets are used to fill up the unused capacity on the link. Where an ASI connection is used to feed the PCR clock reference data back to the subtitle processor, the signal often contains the complete multiplexer transport stream output. The subtitle processor then needs to extract the PCR from the stream. The ASI interface is easier than TCP/IP to configure where there is a master/standby fault-tolerant hardware because ASI signals can be routed through conventional video signal matrices.
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