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Case Closed
Posted Feb 1, 2003 Print Version     Page 1of 4 next »

With the FCC mandating Digital Closed Captioning support in all DTV-capable set-top boxes, producers must adjust to this new wrinkle in digital content development. What guidelines should you follow for how and when to implement it, and how can you use it advantageously in your own content development?

February 2003|On July 1, 2002, with little publicity or fanfare, the Federal Communications Commission (FCC) forever altered the appearance of television with a Digital Closed Captioning (DCC) mandate requiring that all set-top boxes with digital video output(s) support the Digital Television Closed Captioning (DTVCC) specification. In this article, we will examine the features and capabilities of DTVCC, explain why you should exploit it in your content, and provide guidelines for when and how you should start using it.

If it Works, Why Change it?
During the 1970s, the National Bureau of Standards (NBS) was experimenting with television signals to see if time information could be accurately transmitted over existing analog broadcasts. While these experiments failed, the NBS realized that the technology might be able to help the hearing impaired. Consequently, the National Captioning Institute (NCI) was formed. NCI's charter was to promote the use of closed captioning and facilitate the deployment of captioning technologies.

For ten years, the NCI fostered the growth of closed captioning by enabling both real-time and offline captioning technologies. However, it wasn't until the advent of the Television Decoder Circuitry Act that closed captioning became widely available. This act mandated that by 1993, all televisions 13 inches and larger be capable of decoding captioning information.

Since the caption text had to be inserted into existing analog video signals, the designers of this technology were required to preserve compatibility with older televisions. Fortunately, a National Television Standards Committee (NTSC) signal contains empty spots (also known as the Vertical Blanking Interval, or VBI) where captioning information can be inserted without degrading picture quality or affecting backwards compatibility.

Each VBI contains several "lines" of video and the NTSC has reserved the 21st line for captioning (this is why closed captioning is often referred to as line 21). Since NTSC signals are interlaced [see  ] line 21 content is transported in even and odd fields. Caption streams CC1 and CC2 and text channels 1 and 2 are transmitted in Field 1 while Caption streams CC3 and CC4 and any extended data services are transmitted in Field 2. Due to encoding restrictions, NTSC signals only contain two characters per line. Given that an NTSC signal has 30 frames or 60 fields per second, it is possible to transmit 120 characters (960 bits) of caption text per second.

Besides raw text characters, line 21 also contains primitive instructions on how to display the characters. For example, there are options to control the foreground and background colors and the screen position where the text is displayed. In addition, it offers three methods to manipulate how the characters are drawn: roll-up, pop-on, and paint-on.

Roll-up captions add words to the end of a line; when the line is full, it scrolls upward so a new line can be displayed. These captions are popular for sporting events and other live programs. By contrast, pop-on captions are immediately drawn onto the screen and erase whatever caption text was on the display. Paint-on captions are similar to pop-on captions except that they don't clear the captions that were previously drawn.

While line 21 captions were a significant advance for the deaf and hearing- impaired, the technology is stale in the age of digital video. For instance, one of the most significant problems with the line 21 approach is inconsistent presentation. Each television owns the look and feel for caption display (i.e., they are responsible for choosing fonts and colors, etc.), so the text presentation will vary wildly between manufacturers and even between different models for the same manufacturer. Further complicating the situation is uncertainty over text color controls. Since the FCC only recommends, and doesn't require, that televisions support a variety of text colors, few televisions offer the feature.

Another problem with line 21 is the paucity of its text and window attribute features. For instance, there is no control over the point size of the font. In addition, other than the roll-up, pop-on, and paint-on options, window display options are limited (i.e., it's impossible to perform a wipe, fade, or other window control primitives). While these may not initially seem like onerous restrictions, line 21 captioning is targeted at individuals with poor eyesight and other physical challenges. Consequently, it is essential that they be able to alter font size and display so they can enjoy the presentation.

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