There is no standard way to downconvert 708 captions to 608 format

Although the transition from analog to digital TV offers some wonderful new capabilities for closed captioning, there are two issues that are causing confusion, and it could get worse . They deal with the must-carry decision to deliver analog programs after the 2009 end of the digital transition, and the limitations of the popular HDMI interface.

Analog captions conform to CEA-608-D “Line 21 Data Services.” CEA-608 (“native 608”) closed captions are captions designed for the analog television system and carried on the two fields of line 21 of the vertical blanking interval. The common look and feel of native 608 captions is limited to white block letters within a box-like black background field. Color (non-white) characters, while possible, are only occasionally used due to the 13-year legacy of set-top decoders and TV sets which cannot display colored captions.

CEA-708-C “Digital Television (DTV) Closed Captioning” is the standard for captions carried in a digital television signal. Because the DTV signal does not have a vertical blanking interval, the captions are carried as data within the MPEG-2 bitstream. CEA-708-C captions have greatly enhanced formatting and display capabilities compared to native 608 captions. “Native 708” captions are those authored and encoded for CEA-708-C use.

In addition, the CEA-708-C standard provides a way to carry native 608 captions within the 708 captioning data stream. Most digital cable programs today carry both the native 708 captions, which are displayed on digital TV receivers, and the native 608 captions, which are displayed on analog TV sets, within their data stream.

One issue, which will become more apparent after the cessation of over-the-air analog broadcasting, is that there is no standard way to downconvert 708 captions to the 608 format, and there are many features of 708 captions that are simply not supported in the 608 format. This limitation is particularly important during the three years that cable operators have agreed to deliver analog versions of the broadcast signals to subscribers with analog TV sets that understand 608 captions but not 708 captions.

Broadcasters might think about no longer carrying 608 captions when they discontinue analog broadcasts. That would be a big mistake. They must continue to dual carry both the native 708 digital captions and the native 608 analog captions carried with the 708 stream, if they want captions to be displayed on analog TV sets. Cable boxes should be expected to pass through 608 captions if they are present in the 708 stream, but they should not be expected to create 608 captions if only native 708 captions are transmitted by the broadcaster.

While this was never raised as an issue in the Digital Must-Carry proceeding, it was hiding in the background. If, after the transition, a subscriber files a complaint with the FCC because there are no analog closed captions on the analog TV set, it will emerge on the front burner.

The bottom line – the FCC may have to order broadcasters to continue to deliver native 608 captions as long as there are analog TVs.

The other issue is with us today, and it’s a problem. The HDMI interface has become the most popular interface for carrying digital video from a cable set-top box or a DVD player to a DTV receiver or display device. The HDMI interface carries digital video, not at the MPEG-2 compressed video rate of a few Megabits per second, but at the uncompressed rate of several Gigabits per second. It also carries digital audio and some control signals, such as the Active Format Descriptor, which is supposed to optimize the use of letterboxing and pillar bars when converting between 4:3 and 16:9 aspect ratios.

But the HDMI interface does not have a way to carry digital closed captions as a digital bitstream. That means that to display closed captions, the decoding must be accomplished in the set-top box or DVD player. The HDMI video bitstream thus carries open captions, if it carries captions at all.

Unfortunately, neither set-top box instruction manuals nor DTV receiver manuals do a good job of explaining this. In fact, the set-top box I have makes things particularly difficult. Pushing the “menu” button on the remote control does not bring up any relevant option. I now know that I have to first turn the set-top box “off” (it’s not really off, just in a standby mode) and then push the “menu” button. This displays a scary setup screen with black letters on a white background. Only then can I select the open caption display mode. Other brands of boxes are a little more user-friendly.

The FCC Consumer Advisory on this subject (http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-273678A1.doc) is useless. It never even mentions the HDMI interface.

Viewers who are used to having the caption decoding done in their analog TVs are mystified when they get a new DTV receiver and a digital set-top box. While the consumer electronics industry, the cable industry and the FCC have done much to educate the viewers on the transition from analog to digital, this particular problem has fallen by the wayside.

 jkrauss@krauss.ws