Open Subtitles, Closed Subtitles, Open Captions, and Closed Captions are each a type of Timed Text essence, where each type is targeted to a specific audience. Timed Text essence may be included in the Composition as a Timed Text Track File. While the essence is referred to as Timed Text, the actual track files may also reference and carry graphics.
The essence of Timed Text is defined in SMPTE ST 428-7 DCDM – Subtitle. However, the purpose of the Timed Text Track File is identified in the Composition Playlist (CPL). SMPTE ST 429-7 DCP – Composition Playlist defines the MainSubtitle element for Open Subtitle essence. This definition is extended in SMPTE ST 429-12 DCP – Caption and Closed Subtitle to include the additional elements of ClosedSubtitle, MainCaption, and ClosedCaption.
SMPTE Timed Text essence is wrapped as an MXF track file in accordance with SMPTE ST 429-5 DCP – Timed Text Track File. By doing so, SMPTE Timed Text assets can be encrypted and managed in the same manner as Picture and Sound track files. As explained below, Timed Text essence may have fonts and graphics associated with it. For SMPTE Timed Text, the associated files are wrapped with the essence in the Timed Text Track File. In contrast, Interop Timed Text essence are delivered in the package as an XML file, with the associated font and graphics essence also delivered as separate files.
Open Subtitles and Open Captions
Open Subtitles and Open Captions refer to text superimposed on the picture displayed in the auditorium. As such, they are visible to all members of the audience. Open Subtitles are typically displayed on-screen as text in a language other than the language of the movie’s sound track. The intent of the Open Subtitle is to translate the dialog of the sound track for the audience. Open Captions are also typically displayed on-screen as text, but in the same language as the movie’s sound track, with the intent to convey the dialog and action of the movie to those who are hearing-impaired or deaf. Graphics may also be displayed with on-screen timed text. Open Subtitle track files are identified in the CPL as MainSubtitle, and Open Caption track files are identified as MainCaption. A Composition may contain only one MainSubtitle, or one MainCaption file.
Timed Text is managed differently in SMPTE and Interop DCP. Both packaging methods employ XML for Timed Text essence, but each defines the XML differently. (The difference in Timed Text essence has proven to be one of the stumbling blocks in the adoption of SMPTE DCP.) In addition, Interop delivers Timed Text as an XML file in the Composition package, while SMPTE DCP wraps Timed Text XML essence in MXF. This is further explained below.
Interop Timed Text essence is based on a proprietary format introduced by Texas Instruments called CineCanvas™. The latest CineCanvas documentation is available on the Interop DCP page.. The CineCanvas file is in XML format, and included in the Composition as an XML file. Text properties and associated actions include language, font, timing of display, fade up/down properties, and the text position properties of direction, horizontal alignment and position (left, center, right), and vertical alignment and position (top, center, bottom). Image (graphics) properties include timing of display, fade up/down properties, horizontal alignment and position, and vertical alignment and position. Font and graphics files may be included with the distribution.
Timed text essence in SMPTE DCP is defined in SMPTE ST 428-7 DCDM – Subtitle. The SMPTE Subtitle format provides a more sophisticated control of display, placement, and font properties over CineCanvas, and includes 3D text. SMPTE Subtitle also includes Ruby text, small annotations used to guide pronunciation placed alongside logographic characters used in the Chinese, Japanese, and Korean languages. As with CineCanvas, horizontal alignment and position is described in terms of left, center, right, and vertical alignment and position in terms of top, center, bottom. Font and graphics files may be included with the distribution.
Closed Captions and Closed Subtitles
Closed Captions and Closed Subtitles are text-only recitations directed to a personal display, and not visible to the general audience (i.e., off-screen).
SMPTE ST 428-10 DCDM Closed Caption and Closed Subtitle sets forth a set of constraints applied to Timed Text essence for the off-screen applications of Closed Captions and Closed Subtitles. The constraints essentially limit the interpretation of positioning elements in the XML document to the vertical position. The same closed caption and closed subtitle constraints applied to SMPTE off-screen Timed Text may also be applied to CineCanvas™ off-screen applications in Interop distributions. Unlike Open Subtitles and Open Captions, Closed Subtitles and Closed Captions may carry up to six (6) versions of off-screen Timed Text Track Files in one Composition, one language per Track File. It is up to the personal display provider to provide on-demand access to the multiple track versions. The packaging of off-screen Timed Text Track Files deserves special attention, as explained in the Reel Flexibility for Timed Text section.
Interop DCP Closed Caption files are a constrained version of the CineCanvas format. The constraint documentation is available on the Interop DCP page.