scope:

This document is intended as a definition of DTV Closed Captioning (DTVCC) and provides specifications and guidelines for caption service providers, DTVCC decoder and encoder manufacturers, DTV receiver manufacturers, and DTV signal processing equipment manufacturers. This specification includes the following:

• a description of the transport method of DTVCC data in the DTV signal

• a description of DTVCC specific data packets and structures

• a specification of how DTVCC information is to be processed

• a list of minimum implementation recommendations for DTVCC receiver manufacturers

• a set of recommended practices for DTV encoder and decoder manufacturers

The use of the term "DTV" (Digital Television) throughout is intended to include, and apply to, HDTV (High Definition Digital Television) and SDTV (Standard Digital Television) which use the digital data stream specified in ATSC A/53 and related ATSC A/54.

Overview

DTV Closed Captioning is a migration of the closed-captioning concepts and capabilities developed in the 1970's for NTSC television video signals to the high-definition television environment defined by the ATV (Advanced Television) Grand Alliance and standardized by the Advanced Television Systems Committee (ATSC). This new television environment provides for larger screens and higher screen resolutions, and higher data rates for transmission of closed-captioning data.

NTSC Closed Captioning consists of an analog waveform inserted into Line 21 of the NTSC Vertical Blanking Interval (VBI). This waveform provides a transport channel which can deliver 2 bytes of data on every field of video. This translates to 120 bytes per second (Bps), or 960 bits per second (bps). In contrast, DTV Closed Captioning is transported as a logical data channel in the DTV digital bit stream. Of the DTV bitstream bit rate (which is 19.4 Mbps for terrestrial broadcast, and 38.4 Mbps for cable), DTV-specific closed captioning is allocated 9600 bps. This increased capacity opens the possibilities for the simultaneous transmission of captions in multiple languages and at multiple reading levels.

The DTV standard accommodates a variety of increased vertical and horizontal screen resolutions (e.g., 480 x 704, 720 x 1280 and 1080 x 1920), versus the single 525 vertical scan line format for NTSC. These added resolutions provide for more defined representations of character fonts and other on-screen objects.

The heart of DTVCC caption display is the caption "window" which is identical to the window concept found in all computer Graphical User Interfaces (GUI). Windows are placed within the DTV screen, and caption text is placed within windows. Windows and text have a variety of color, size and other attributes.

This document describes the above issues in a reverse-hierarchical (i.e., low-to-high level) fashion. It follows an "Open Systems Interconnect (OSI) Reference Model"-type protocol stack for layered protocols. DTVCC consists of 5 protocol layers: the Transport Layer, the Packet Layer, the Service Layer, the Coding Layer, and the Interpretation Layer. The discussion of the first 3 layers is a detailed presentation of data transport and organization issues. The discussion of the last 2 layers provides a more informative presentation of the unique aspects of closed captioning. Some readers may wish to start with these last 2 layers first, beginning in section 7.

Throughout EIA-708-B, in concert with ATSC A/53, the lowest numbered bit in a multibit numbered value is considered to be least significant (uimsbf).