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Environment: HLS Playback session, full segments HEVC Segments CMAF/fmp4 CEA-608 caption ingest using Video payload PREFIX_SEI HEVC NALUs Safari Native HLS Player Problem description When playing the HLS HEVC based content on Safari, the SEI NALU embedded CC doesn't render ( although it is selectable by the Safari native player ) Analysis For AVC based 608 muxed SEI NALUs, Safari successfully render CCs When concatenating the Init segment and concatenating w/ the CMAF segments, the resulting file IS PLAYABLE using VLC where 608 CC is properly rendered Having in mind "HLS Authoring Specification for Apple Devices", it is indicated that "For HDR10 video, the SEI NAL units (that is, static metadata) SHOULD be in the HEVC Configuration Box ('hvcC') and not in the individual sample data. Adding CC as SEI NALUs to the hvcC atom, didn't have any affect, CCs were still not playing... Why can't the 608 CC play on the Native Safari HLS Player ? For reference, here is the concatenated file: https://drive.google.com/file/d/1NGLRtPGdQMHEO6E12DnF3cEV01ESSQm_/