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The Basics of 608 vs. 708 Captions

June 18, 2009
The Basics of 608 vs. 708 Captions

Last Friday (June 12, 2009) was the official transition of analog television being replaced by digital television (DTV) in the United States. With this having taken place, I thought it would be the appropriate time to discuss 608 captions (captions for analog TV) and 708 captions (captions for DTV).  If you didn't know that two types of captioning standards existed, you most definitely will want to read on.

Please note that in this article I simplified the terms to 608 or 708 captions, but you will often see these terms referred to as EIA-608 or EIA-708 after its developer Electronic Industries Alliance.

A LITTLE BIT ABOUT 608 CAPTIONS (LINE 21 CAPTIONS)

608 captions, which are commonly referred to as Line 21 captions, used to be the standard for closed captioning for NTSC TV analog broadcasts in the United States and Canada.

What do 608 captions look like?

These are the captions you are mostly likely used to.  They are known for their limited display capabilities: white letters, black-box background.  Typically the letters are in all uppercase as this has become a North-American standard for 608 captions.

How does 608 captioning work?

608 captions are embedded in the television signal and become visible when the viewer uses a decoder, either as a separate device or built into a television set. Without this decoder, the captions are not viewable.

Line 21 captions are hidden in--you got it--the Line 21 data area found in the VBI (vertical blanking interval) of the television signal, which is a non-visible, active video data area.  There are two fields in Line 21.  Field one contains CC1, CC2, T1, and T2 (the latter being text services). Field 2 contains CC3, CC4, T3, and T4. CC1 is most often used to carry English captions and CC3 is increasingly being used for Spanish captions and captions edited for young children's reading speed.

608 Language Capabilities

Line 21 captions are mainly limited to be encoded in these languages: English, Spanish, French, Portuguese, Italian, German, and Dutch.

608 captions for DTV

Seems kind of like an oxymoron, but even though Line 21 captions are for analog broadcasts, they are also sometimes present in the picture user data in digital transmissions.

Future of 608 captions

Don't think they are going to disappear this year, but because of the transition to DTV last week, 608 CC is becoming less prevalent.  That being said, producers are still sending their programs to TV stations with only Line 21 captioning.  They may not even know that this is what their captioning company is providing them with as the stations are not always demanding otherwise.  This is because 608 captions are still supported in DTV by carrying them inside ancillary data channels. TV stations haven't completely addressed this DTV captioning issue.  My bet is they have enough on their plate right now with all of the industry changes.  Only time will tell what must be done, but if you are a television producer, ask your closed-captioning service provider if they are providing your digital programs in a 708 CC format.  There is a good chance that they are not.  In fact, most CC viewed on DTV signals is 608 CC up-converted to 708 CC.  That being said, the visa-versa is not true: There is no standard way to down-convert 708 captions to 608 captions as there are many features of the 708 format that are not supported in 608 captions.

A LITTLE BIT ABOUT 708 CAPTIONS (DTV CAPTIONS)

708 captions are the standard for all digital television, whether that mean standard-definition digital broadcasts or high-definition broadcasts.  I have found that many people think that 708 captions are only for hi-def channels, but this couldn't be further from the truth.

What do 708 captions look like?

This is where things get fun.  Are you sick of the same old look?  Like things your way?  You'll be happy because the at-home viewer of DTVCC has some control on how they view the captions. The 708 technology for DTV captioning allows for up to eight different fonts, and can be displayed in three different sizes. DTV captioning also allows for up to 64 different colors of text on up to 64 different colored backgrounds. Another nice feature is that the background can be made translucent or even omitted entirely with text displayed as edged or dropshadowed characters.  North American 708 captions can have a nicer look than 608 captions and if captioned in the pop-on style, they can have a similar look to that of subtitles you may find on a DVD or Blu-ray disc.  One thing the viewer cannot change is the captioning style.  For example, roll-up captions cannot be reformatted to pop-on captions.  This is in the control of the captioner who created the captions. These 708 capabilities can only be obtained if you have access to a new digital television set with a built-in decoder which supports these functions.  Although 708 captions were designed with all these features in mind, each decoder manufacturer has created their decoders with different capabilities, features, and functions, so your control over the captions will widely depend on the specific decoder you are using.

708 Language Capabilities

Unlike Line 21 captions, there are many more language and character-set capabilities for 708 captions.  Due to their universal character set (unicode) and complete range of special symbols, 708 captions were designed to support any character in any alphabet in the world.  With 608 captions the viewer chooses between CC1, CC2, CC3, CC4 to view different language captions,  many times only accurately allowing for up to two languages due to overlapping signals. In the 708 caption technology, different languages are transmitted as CS1, CS2, CS3, CS4, CS5, CS6...  This easily allows for closed captioning the same program in multiple languages, broadening the scope of a program's audience.

How does 708 captioning work?

DTV does not have a VBI (vertical blanking interval) like analog video, but rather 708 captions are place into MPEG-2 video streams in the picture user data. Known as the DTVCC Transport Stream, the packets are in picture order, and need to be rearranged like picture frames are.  In the fixed-bandwidth channel there is space allocated for both Line 21 and DTVCC: 960 bit/s are allocated to be compatible with Line 21 captions and 8640 bit/s is allocated for 708 captions.

The future of 708 captions

More and more DTV viewers will start demanding true 708-type captions, rather than accepting the older up-converted 608 captions.  They will see DTV captioning on some programs and love them, and want them for all their programming. Even so, my inkling is that 708 captions won't be the only type of captions needed until analog television sets no longer exist. People with analog television sets can still receive a digital signal by using an external converter box and this population will still need access to 608 caption data.  So to be on the safe side, I recommend you provide both 608 and 708 captions that can be embedded in the MPEG-2 video streams of digital video, so everyone is happy and most importantly, everyone has access.

Read more about the basics of closed captioning and learn more about the services we offer here: Captioning Services | Aberdeen Broadcast Services.

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