this one's for GB
New Era
Subtitling for UHD
Peter Cherriman, Chair, DVB TM-SUB
New subtitling standards tend to appear rarely and the existing standards have a long life. There are two DVB subtitling standards in common usage today; both are around 20 years old.
EN 300 472 was first published in 1994; it defines the mechanism for carrying ITU-R System B Teletext subtitles (also known as EBU Teletext EN 300 706) within DVB transport streams. EN 300 472 enabled Teletext subtitles, as used in analog television to be used with digital television. The Teletext characters being rendered by fonts within the receiver or television.
EN 300 743 was first published in 1997, it defines a method by which subtitles, logos and other graphical elements are carried within a DVB transport stream. This specification took a different approach. At the time it was judged that there were several advantages to performing the font rendering at the broadcast headend and sending line-by-line bitmap representation of the subtitles to receivers. It reduced receiver costs since preinstallation of fonts for subtitling was no longer required. It also allowed the broadcasters to have closer control of the positioning and look of the subtitles. However the bitrate requirements for transmission are not constant, but variable and content dependent.
The first two revisions of the EN 300 743 specification were designed for standard definition digital television. In 2006 the EN 300 743 specification was updated to version 1.3.1 for HD television, allowing support of multiple HD formats including 1920x1080, 1440x1080, and 1280x720. The final two revisions of EN 300 743 (latest being v1.5.1) added support for subtitling plano-stereoscopic (i.e. 3D) television, by allowing the broadcaster to signal the disparity at which the subtitles should be displayed, so they appeared at a suitable depth.
Next Generation subtitles
In 2014 DVB started looking at commercial requirements for subtitles for Ultra High Definition TV (UHDTV). The existing subtitle specifications were now quite old, technology had moved on and a more efficient and flexible subtitle system was desirable for both current and future broadcast systems.
There was concern the bitrate requirements for bitmapped subtitles for UHDTV would be significant. It was felt a move back towards text-based subtitles was now possible since modern receivers included high quality font rendering engines for rendering web pages and user interfaces. While bitmap subtitles have to be rendered at the broadcaster headend for one particular resolution, text-based subtitles can be rendered at the optimal resolution for the graphic capabilities of each receiver/display. Text-based subtitles have other advantages: they could allow user customization (e.g., size, color) and are more easily converted to other formats, perhaps being used by text to speech engines in future receivers.
The commercial requirements produced required a bitrate efficient system suitable for use with SD, HD and UHD services. The solution needed to support compositing of subtitles which may be defined in a different color-space to the video. The solution also needed to support downloadable fonts via broadcast or the internet. The downloadable font feature enables receivers to acquire fonts “in the field” rather than having the fonts preinstalled.
Due to continued interest in bitmap subtitles in regions of the world that needed to support many languages and characters, in 2016, DVB produced a further set of commercial requirements concerning bitmap-based subtitles for UHDTV. These commercial requirements require a backward compatible update to the EN 300 743 specification. The requirements are expected to be delivered by specifying HD resolution subtitles on UHDTV services.
Ongoing Technical work
The DVB technical subtitling group (TM-SUB) that has been working on the text-based subtitle requirements has decided to use a TTML based format, either EBU-TT-D or the text profile of the recently published IMSC1 standard. DVB had previously adopted EBU-TT-D within the DVB profile of MPEG DASH (TS 103 285). TM-SUB is currently working towards a PES based encapsulation and signaling for DVB transport streams.
The work on a new revision of EN 300 743 is yet to start, but some ambiguities with the current specification have already been found and will be corrected with the update for UHDTV.
When do we plan to be finished?
The goal is to complete both specifications to meet both sets of commercial requirements in October 2016 to align with a new TS 101 154 specification. However the Subtitling Group within DVB is quite small, so this is a challenging deadline to meet. So the Group would welcome any additional help from members (particularly those with implementation experience of PES packetization or font rendering) in order to complete these important specifications.
New Era
Subtitling for UHD
Peter Cherriman, Chair, DVB TM-SUB
New subtitling standards tend to appear rarely and the existing standards have a long life. There are two DVB subtitling standards in common usage today; both are around 20 years old.
EN 300 472 was first published in 1994; it defines the mechanism for carrying ITU-R System B Teletext subtitles (also known as EBU Teletext EN 300 706) within DVB transport streams. EN 300 472 enabled Teletext subtitles, as used in analog television to be used with digital television. The Teletext characters being rendered by fonts within the receiver or television.
EN 300 743 was first published in 1997, it defines a method by which subtitles, logos and other graphical elements are carried within a DVB transport stream. This specification took a different approach. At the time it was judged that there were several advantages to performing the font rendering at the broadcast headend and sending line-by-line bitmap representation of the subtitles to receivers. It reduced receiver costs since preinstallation of fonts for subtitling was no longer required. It also allowed the broadcasters to have closer control of the positioning and look of the subtitles. However the bitrate requirements for transmission are not constant, but variable and content dependent.
The first two revisions of the EN 300 743 specification were designed for standard definition digital television. In 2006 the EN 300 743 specification was updated to version 1.3.1 for HD television, allowing support of multiple HD formats including 1920x1080, 1440x1080, and 1280x720. The final two revisions of EN 300 743 (latest being v1.5.1) added support for subtitling plano-stereoscopic (i.e. 3D) television, by allowing the broadcaster to signal the disparity at which the subtitles should be displayed, so they appeared at a suitable depth.
Next Generation subtitles
In 2014 DVB started looking at commercial requirements for subtitles for Ultra High Definition TV (UHDTV). The existing subtitle specifications were now quite old, technology had moved on and a more efficient and flexible subtitle system was desirable for both current and future broadcast systems.
There was concern the bitrate requirements for bitmapped subtitles for UHDTV would be significant. It was felt a move back towards text-based subtitles was now possible since modern receivers included high quality font rendering engines for rendering web pages and user interfaces. While bitmap subtitles have to be rendered at the broadcaster headend for one particular resolution, text-based subtitles can be rendered at the optimal resolution for the graphic capabilities of each receiver/display. Text-based subtitles have other advantages: they could allow user customization (e.g., size, color) and are more easily converted to other formats, perhaps being used by text to speech engines in future receivers.
The commercial requirements produced required a bitrate efficient system suitable for use with SD, HD and UHD services. The solution needed to support compositing of subtitles which may be defined in a different color-space to the video. The solution also needed to support downloadable fonts via broadcast or the internet. The downloadable font feature enables receivers to acquire fonts “in the field” rather than having the fonts preinstalled.
Due to continued interest in bitmap subtitles in regions of the world that needed to support many languages and characters, in 2016, DVB produced a further set of commercial requirements concerning bitmap-based subtitles for UHDTV. These commercial requirements require a backward compatible update to the EN 300 743 specification. The requirements are expected to be delivered by specifying HD resolution subtitles on UHDTV services.
Ongoing Technical work
The DVB technical subtitling group (TM-SUB) that has been working on the text-based subtitle requirements has decided to use a TTML based format, either EBU-TT-D or the text profile of the recently published IMSC1 standard. DVB had previously adopted EBU-TT-D within the DVB profile of MPEG DASH (TS 103 285). TM-SUB is currently working towards a PES based encapsulation and signaling for DVB transport streams.
The work on a new revision of EN 300 743 is yet to start, but some ambiguities with the current specification have already been found and will be corrected with the update for UHDTV.
When do we plan to be finished?
The goal is to complete both specifications to meet both sets of commercial requirements in October 2016 to align with a new TS 101 154 specification. However the Subtitling Group within DVB is quite small, so this is a challenging deadline to meet. So the Group would welcome any additional help from members (particularly those with implementation experience of PES packetization or font rendering) in order to complete these important specifications.
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