Content - Closed Captions and Subtitles

Closed Captions

Swank encodes closed captioning to CEA-608 and CEA-708 for the majority of the available encodes. For specific encode information click more info for the desired encode listed here: Encode Specifications 

CEA‑608 (EIA‑608) – Legacy Analog Closed Captions

Definition
CEA‑608 defines the original closed captioning standard for analog television (NTSC). Caption data is encoded into the video signal itself, specifically on line 21 of the vertical blanking interval (VBI). This would only be available when being distributed via COAX, SDI, Etc. HDMI and IP broadcast do not support 608 closed captions over line 21. The only way to use these transmission types with closed captions would be to have the source overlay the CC information on the picture prior to transmitting to the display over the interconnect's video output. When CC information is encoded in a digital data stream, only the device that decodes the data has access to the closed caption information; there is no standard for transmitting the CC information to a display monitor separately. 

Key Characteristics

• Analog-based transport
• Encoded as byte pairs (2 bytes per caption packet)
• Limited to:
  • Basic text (uppercase-dominated historically)
  • Minimal styling (italics, underline, limited colors)
  • Fixed positioning (rows)
• Typically rendered as:
  • White text on a black box
  • No advanced styling or repositioning

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CEA‑708 (EIA‑708) – Digital Closed Captions

Definition
CEA‑708 defines the digital closed captioning standard used in ATSC (DTV) broadcasts, significantly expanding on 608 capabilities.

Key Characteristics

• Does NOT use line 21
• Carried inside the video transport stream (ATSC)
• Packaged as Caption Distribution Packets (CDPs)
• Fully digital transport (ATSC A/53 stream)
• Supports: Multiple caption services (languages, channels)
  • Full styling (fonts, colors, transparency)
  • Precise positioning

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608‑over‑708 (Compatibility Layer)

The CEA‑708 transport stream encapsulates CEA‑608 byte pairs internally for backward compatibility. This is referred to as 608‑over‑708.

How it works

• CEA‑608 data is placed into**“608 compatibility bytes”** inside the 708 stream
• Allows:
  • Legacy 608 decoders → still display captions
  • Modern 708 decoders → optionally read either source

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Timed Text (tx3g – MPEG‑4 / 3GPP Timed Text)

Definition
tx3g is the four-character code identifier for MPEG‑4 Part 17 Timed Text, also known as 3GPP Timed Text.

Standards

• ISO/IEC 14496‑17 (MPEG‑4 Timed Text)
• 3GPP TS 26.245

These are the same format when used in MP4 containers

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Key Differences (608 / 708 / tx3g)

Feature	CEA‑608 (EIA‑608)	CEA‑708 (EIA‑708)	Timed Text (tx3g)
Transport	Analog (NTSC)	Digital (ATSC)	MP4 container
Data Format	Byte pairs (line 21)	Packetized (CDP)	Text track
Styling	Very limited	Advanced	Basic–moderate
Positioning	Fixed rows	Full screen placement	Region-based
Services	4 CC channels	Multiple services	Multiple tracks
Compatibility	Legacy systems	Includes 608 support	Separate (not CC)
Use Case	Broadcast legacy	Broadcast digital	Streaming / MP4 playback

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Closed captioning can be included in various types of video encodes, regardless of the resolution, including HD (High Definition) video.

In most cases closed captions will be in English formatted for ATSC (North America) standards. The ADA calls for English and that is what Swank supplies when available. Closed captioning standards differ between ATSC (North America) and DVB (Europe), making them incompatible on EU TVs.

Subtitles

If an srt/subtitle file is available, it will be in a separate file in the Content folder as shown below. 

• Content
  • (Order #)
    • (Shipment #)
      • (Line #)
        • (productNumber).mp4
        • (productNumber).MediaFileInfo.xml
        • (If available .srt files will be included in this directory)

We cannot currently guarantee that all titles have a specific language at this time for srt/subtitle files.