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Encoding closed captions for digital television

The broadcasting industry is in a state of transition from analog to digital television, and further down the road, to high-definition TV. As part of this transition, closed captions must also be converted for service with digital. Closed captions, which are encoded in line 21 of the NTSC TV signal, display the dialogue, narration and sound effects of a TV program. Viewers can choose whether or not to display closed captions by using a decoder built into or attached to a television set, which “opens” the captions and shows them on the TV screen.

The move to digital TV and the implementation of closed captions has been part of an effort by the Federal Communications Commission. The group has mandated both requirements and forced broadcasters to adjust.

Reaching out with closed captions
Although legislation has been the major impetus for the adoption of closed captions, broadcasters and producers, supported by consumer research, are recognizing that there is a market need for them. Increasingly available on videos and DVDs, closed captioning serves a need in noisy environments, such as airports, where TV monitors provide ongoing public service information. Closed captioning has also benefited the nearly 32 million people in the United States who speak English as a second language (ESL). Studies show that captions can dramatically improve English vocabulary and comprehension among the ESL population. And, of course, closed captioning is a crucial aid for the hard-of-hearing population, the group for which the FCC regulation was originally intended.

The transition to digital TV
The closed caption standard used today is EIA-608, which specifies the use of closed captions in analog TV signals. As TV stations begin the transition to full digital environments, all programming will have to support the digital closed caption format, EIA-708. In the interim period, encoding and broadcasting equipment will have to support closed captions in both formats. The move from EIA-608 (analog) to EIA-708B (digital) brings with it many improvements. For example, viewers at home will be able to control the size of the caption text. EIA-708 also offers many more letters and symbols, as well as support for multiple fonts and colors for text and backgrounds. The traditional black box background can be replaced by a colored box, or done away with entirely in favor of edged or drop-shadowed text.

In addition, DTV captions contain more information. Currently, if we caption a show in two languages (CC1 and CC2) at 250 words per minute, we have hit the limit of what can be transmitted. There’s no room for text or Internet data. EIA-708 increases the data rate by 16 times.

EIA-608 captioning
Closed caption data is included in the vertical blanking interval (VBI) section of the NTSC analog TV signal, a part of the signal not displayed on TV screens. The VBI can also contain other signal information, such as teletext, time codes, and program delivery controls.

Production houses and TV studios take the closed caption data, encode it with the TV signal, and send the integrated TV signal to broadcasters for transmission.

This composite analog TV signal is fed into an MPEG encoding board, which extracts the closed caption stream and compresses the TV signal. It then multiplexes the closed caption signal and the encoded MPEG stream and outputs an MPEG-1 or MPEG-2 stream that includes the closed caption signal.

EIA-708B captioning


Figure 1. An analog TV signal that contains CC 608 data is fed into an MPEG encoder and a closed caption extractor box, then to a CC receiver that converts the 608 data to 708 format. Click here to see an enlarged diagram.

As mentioned before, analog closed caption data is located in the VBI of the analog stream. Digital TV signals do not have a vertical blanking interval. The digital EIA-708B standard offers an alternate way of including existing analog closed captions in the digital signal by transcoding or converting EIA-608 data to the EIA 708 standard via a closed caption conversion box that can receive either format. This saves production houses and studios from having to reproduce existing closed caption data in 708 format from scratch. In Figure 1, an analog signal containing CC 608 data is simultaneously fed into an MPEG encoding board and a closed caption extractor box. The closed caption extractor separates the CC data from the composite signal and feeds it into a closed caption receiver for conversion to the 708 format. Meanwhile, the MPEG encoder compresses the composite TV signal into MPEG. The MPEG signal and the 708 data are then multiplexed and output as an MPEG-1 or MPEG-2 stream.

Today, most DTV content still relies on EIA-608 captions that have been converted to 708 format, mainly because of the small consumer base of DTV receivers. When a larger number of consumers have receivers that are able to receive DTV signals, there will be more pressure on producers and broadcasters to include native EIA-708 closed captions.

No time to waste
Given the regulatory pressure to produce new programming with digital closed captions based on the EIA-708B standard, producers and broadcasters should be seriously thinking how to make this move. Although investing in new equipment may involve a learning curve and a capital investment, there is no doubt that the end product, in terms of quality and benefits for consumers, will be superior to that available today.

Danna Bethlehem is the publications and online marketing manager for Optibase.

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