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.

Table 1. Sample table box hierarchy for encrypted Dolby AC‑4 audio tracks
Nesting level

Reference

4 5 6 7 8
stbl ISO/IEC 14496-12
stsd

AC4SampleEntry

(header type set to enca)

AC4SpecificBox

(header type set to dac4)

Annex E.4 of ETSI TS 103 190-2
sinf ISO/IEC 14496-12
frma
schm
schi
stts
stsc
stsz
stz2
stco
co64
library_booksNote: Although the order of the dac4 and sinf boxes is explicitly defined for Dolby AC‑4 bitstreams in the table, this order may not be the same for other encrypted video or audio tracks that may be present in an ISO base media file delivered using MPEG-DASH. As a result, parsers must also be capable of processing ISO base media files in which the sinf box is the first box inside the protected sample entry box.