Custom fonts for streaming subtitles
Posted by Matt Juggins on , last updated
Subtitles provide an on-screen text representation of our content and are used not only by viewers with hearing loss but by many other sections of our audience. Ensuring the readability of our subtitles on both broadcast and internet delivered services across the the different languages used within the UK is therefore of critical importance and the choice of font used has a significant impact on accessibility and legibility. Βι¶ΉΤΌΕΔ Research & Development has recently contributed support for the DVB Font Downloading mechanism to the dash.js project to enable the Βι¶ΉΤΌΕΔ to use our preferred Βι¶ΉΤΌΕΔ Reith sans font.
On broadcast platforms, content providers have full control over the appearance of subtitles as they are broadcast in bitmap form. Subtitles for our internet streams work differently, with the client device rendering the subtitle text. Typically, content providers don’t control the platforms to which they deliver media, and so they have limited influence over the default font used to display subtitles. There is, however, a mechanism standardised by DVB that allows the content provider to request that a downloadable font be used with the subtitles in a DASH stream (or even to indicate that the font must be used).
Βι¶ΉΤΌΕΔ Research & Development's Distribution team have added support for the DVB Font Downloading mechanism into the latest release of , an open source JavaScript library for MPEG DASH streaming on browser-based devices. In addition, we’ve also provided an .
To confirm this new font downloading functionality works in dash.js and to allow other content providers to test their own MPEG DASH players, we’ve added new test material to our website, covering both live and on-demand streams. We’ve also made a number of these test streams available in the .
Each stream tests a different facet of the “DVB Font Downloading” mechanism, including amongst others:
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Support for different font file formats
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Situations where incorrect or malformed URLs are supplied for font files
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Situations where subtitles must not be presented if the specified font is not available
These streams use a (hand drawn!) test font we created in-house called “Βι¶ΉΤΌΕΔRD_Massif”. Whilst it won’t be winning any awards for design, the font has been designed to perform some useful character re-mapping. The test streams contain the words “WRONG font”; this means that when any other font is used, either because of a failed download or the client ignoring download instructions, those words will be visible. However, when “Βι¶ΉΤΌΕΔRD_Massif” is correctly downloaded and used for display, the words “RIGHT font” are rendered on screen. Other useful indicators appear on-screen too, such as a timestamp and the format of the font in use.
We hope that these test streams and the addition of support for DVB Font Downloading in dash.js will enable content providers using MPEG DASH to improve subtitle delivery for their audiences. Particularly, we hope this can improve subtitle delivery on TV devices, as more and more services move towards IP delivery, for example, with .