Signaling Dolby AC‑4 bitstreams in an ISO base media file format

Dolby AC‑4 uses the basic structures defined within ISO/IEC 14496-12 to signal audio tracks and uses specific extensions within compliance to ISO base media file format to provide detailed information on the characteristics of a Dolby AC‑4 bitstream.

The information is included in the stbl box of the moov box of the ISO base media file. The locations and hierarchy of the boxes that must be included in stbl to identify a Dolby AC‑4 stream within an ISO base media file are listed in the table.

Table 1. Sample table box hierarchy for Dolby AC‑4 audio tracks
Nesting level
4 5 6 7 Reference
stbl ISO/IEC 14496-12
stsd

AC4SampleEntry

(header type set to ac-4 or enca)

ETSI TS 103 190-2

AC4SpecificBox

(header type set to dac4)

ETSI TS 103 190-2
     

Zero or more AC4PresentationLabelBox

(header type set to lac4)

ETSI TS 103 190-2
stts ISO/IEC 14496-12
stsc
stsz
stz2
stco
co64

In this table, the value of the nesting level provided for each box is based on the structure of the complete media file, beginning with a nesting value of 0 for the ftyp and moov boxes. The table also includes a reference to where each box is defined.

The value for the AC4SampleEntry box header type depends on whether the file is encrypted. For an unencrypted file, the AC4SampleEntry box header type value is ac-4; for an encrypted file, the AC4SampleEntry box header type value is enca.

The value of the AC4SpecificBox header type is dac4.

The stbl box of a Dolby AC-4 audio track must contain a sync sample box (stss), unless all samples are sync samples (I-frames). The stss box must reference all the sync samples. Random access points are signaled by the sync sample box (stss) in an ISO base media file, or by setting the tr_flags in the track run box in ISO base media segments.

Unless otherwise stated here, all requirements specified in ISO/IEC 14496-12 and Annex E of ETSI TS 103 190-2 must be followed.