Closed Captioning for Broadcast using Apple Compressor


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    Live Captioning Support

    While Compressor is a good inexpensive option in many cases, there are some issues with complex input and output scenarios.  Among the known issues:

    • Before encoding, check to make sure the start time of your media and the start time of your caption file are the same. By default all caption files created by AST CaptionSync will have a start time of 00:00:00.  Some versions of Final Cut will export to Compressor with a default start time of 01:00:00.  If this is the case for your media, you can either a) change the start time of the media file to 00:00:00 before proceeding with the encode, or b) do a free redo to get a new caption file with a one hour offset.  
    • Compressor limits you to certain frame sizes when adding closed captions. The geometry tab under the A/V options tab is greyed out, meaning that non-standard frame sizes cannot be selected.
    • On some server setups Compressor may complete the transcode but fail to add captions to the video, without providing any error message.  One customer found this tool useful for repairing the links between Compressor and Qmaster:
    • Encoding captions in a ~60 fps video seems problematic.  The captions will be encoded, but captions near the beginning of the video may be skipped.
    • CEA 608 versus CEA 708 captioning: Apple notes in the Compressor 4 user manual: "For MPEG-2 program and transport stream outputs: Compressor embeds the closed caption data in program and transport MPEG-2 streams using the EIA-708 ATSC protocol."  Technically this is correct, but in practice the compatibility of captions encoded into HD video with Compressor depends on your station and their broadcast equipment.  Some stations have equipment that will take captions that are encoded in this way and make them compatible with both CEA 608 and CEA 708 equipment on broadcast.  Other stations can accept your video, along with a .SCC or DFXP file containing the captions, without any encoding required.  If your station requires encoded HD video with captions and you intend to use Compressor for the encoding, best practice is to send them a short sample file first for testing. You can download the MPEG-2 Program Stream sample file, generated using the method described in this article, from the "Video Files" section above.
    • If your station does not accept video encoded with the method above (or alternatively delivery of a sidecar file in a format like DFXP or .SCC), then you will need to consider more expensive software or hardware alternatives, such as those from Harmonic, Blackmagic Design, or AJA.
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