Signaling a Dolby AC‑4 bitstream in an encrypted ISO base media format file

When a Dolby AC‑4 bitstream is included in an encrypted file, the header type value of the AC4SampleEntry box is enca (a protected audio sample entry). The original sample entry type (ac-4) is then stored in a box called the OriginalFormatBox within the ProtectionSchemeInfoBox.

The AC4SpecificBox (which describes the configuration of the Dolby AC‑4 audio stream) is immediately followed by a protection scheme information (sinf) box, which describes the type of encryption used, and the type and location of the license-key management system that must be used to decrypt the file. The sinf box also contains an original format box (frma), which is used to describe the format of the original unencrypted track. The value of the frma box must be set to ac-4 for an encrypted Dolby AC‑4 audio track.

The locations and hierarchy of the boxes that must be included in the stbl box to identify a Dolby AC‑4 bitstream within an encrypted MPEG-DASH compliant ISO base media file, together with the reference to the definition for each box, are listed in Sample table box hierarchy for encrypted Dolby AC‑4 audio tracks. The value of the nesting level listed for each box is based on the structure of the complete ISO base media file.