Closed Caption Support for DTVkit
Published on Monday, 8 February 2021 16:10
This month we’d like to give you some insight into some work that we have been doing for DTVKit with Closed Captions. As part of its DTV software offering to its members DTVKit provides a DVB stack, DVBCore. DVBCore has many compelling features that have led to it being used by manufacturers in millions of DTV products worldwide. Included in these is support for both DVB subtitles and EBU digital teletext subtitles (and full teletext), but not ‘Closed Captions’. Closed Caption are widely used in some regions, and adding support to DTVkit will be a benefit to its members targeting those areas with their products.
On the surface, Closed Captions appear to be similar to DVB or digital teletext subtitles, but if you dig under the hood you’ll find they are implemented in a completely different manner. The data for DVB subtitles and digital teletext are carried as separate Packetised Elementary Streams (PES) within a transport stream, however, Closed Caption data is actually carried within the private user data field of the MPEG video stream. Closed Captions, much like teletext, originated in analogue TV, and broadcasters and content owners were keen to find a way to maintain compatibility as things moved into the digital age. Although digital Closed Captions provide more flexibility including support for a wider range of languages, fonts, text size/colour, to regenerate the caption data for all the legacy content out there would be an expensive business. So the digital standard for Closed Captions (CEA-708) also includes a mechanism for carrying older, analogue style ( CEA-608) caption data, known as ‘608 over 708’.
Accordingly, we have been busy over the past couple of months adding support for Closed Captions to DVBCore, for both the digital 708 and legacy ‘608 over 708’ formats. DVBCore relies on the platform to extract the raw data from the MPEG video, but it performs the decoding of the caption data and rendering. Rendering uses the existing DVBCore HAL graphics APIs so is compatible with all platforms currently running DVBCore, e.g. Android, Linux, RTOS. We are in the process of testing the Closed Caption implementation at the moment and aim to upstream it to DTVkit by the end of Q1.
For further information please email info@oceanbluesoftware.co.uk or contact your DTVKit representative.