According to the Federal Communications Commission, Closed captioning (CC) displays the audio portion of a television program as text on the TV screen, providing a critical link to news, entertainment and information for individuals who are deaf or hard-of-hearing.
The term “closed” indicates that not all viewers see the captions—they are only to visible to those who choose to decode or activate them. This separates them from “open captions” which are visible to all viewers.
In addition to its fundamental use as a “tool”, allowing the deaf community or those individuals who have experience difficulties in hearing, CC can also help learning to read or speak non-native language, or in loud environment.
Closed Captioning system used in video broadcasting is mainly divided into two captioning standards:
• CEA-608 standard
It was developed during the 1970’s to carry caption and data services on NTSC video. It was encoded as a black and white series of pulses on line 21 VBI (vertical blanking interval).
There are two fields in Line 21: Field 1 has four data channels: two Captions (CC1, CC2) and two Text (T1, T2). Field 2 has five additional data channels: two Captions (CC3, CC4), two Text (T3, T4), and Extended Data Services (XDS).
The XDS data structure provides additional information about the transmission. It is optional, and can include channel name, program name, program category (drama, game show, etc.), V-chip (allows the blocking of programs based on their ratings category) and other.
CEA-608 provides controls for the color of the foreground and background of the text, underlining, blinking, and italics. The default color scheme is white characters on a black background, all opaque.
CEA-608 can be transmitted on analog & digital television. • CEA-708 standard
It was developed to replace 608 for HD television transmissions. 708 actually includes the 608 bytes as a compatibility layer for older decoding equipment, but also provides a completely new and enhanced captioning system.
The transmission of the packets is described in SMPTE 334-1 (VANC video packet) and as an ancillary packet in SMPTE 334-2.
Three streams are encoded in the video, two are backward compatible Line 21 captions (CEA-608 in CEA-708), and the third is a set of up to 63 additional caption streams encoded in CEA-708 format. CEA-708 has constant data rate of 9600 bps.
708 caption services are labeled CS1, CS2,… CS63 or S1, S2,… S63.
CEA-708 can only be transmitted on digital television.
CC handling by ATHENA products
ATHENSA products process Closed Captions for analog TV (CEA-608, Line 21) by using a DeckLink card, while digital TV (CEA-608 in CEA-708) Closed Captioning delivery is executed with both DeckLink and DELTACAST card. CC data in MPEG2 TS and H.264 TS are supported too.
Closed Captioning is supported by both ATHENSA Ingest and Playout solutions in the following ways:
• Inserted into the MPEG file as CC data.
• Created in a separate MCC file for use in postproduction and editing.
• Capability to read CC data inside MPEG files.
• Capability to read QuickTime MOV CC data.
• Capability to injest CC data based on a MCC file (both the actual media file and mcc file have to have the same name).
• If the video file has embedded CC and there is external MCC file available AirBox will use CC from the MCC file which allows fast correction of badly embedded CC.
Definitions and Requirements:
• CC data contains byte pairs for Field1 and for Field2
• CC data rate is always 29.97 pairs / second. That does not depend on the video standard. This rate should be provided within the whole CC data handling chain – from the Input through the Output.
• It is possible to have 1 or 2 CC data streams.
• The calculations are always based on 29.97 byte pairs per second. Some of the pairs may not contain data or may be missing. Missing byte frames should be filled with empty pairs.