Encapsulation of FLAC in ISO Base Media File Format
Version 0.0.4 (draft)

Table of Contents
1 Scope
2 Supporting Normative References
3 Design Rules of Encapsulation
  3.1 File Type Identification
  3.2 Overview of Track Structure
  3.3 Definition of FLAC sample
        3.3.1 Sample entry format
        3.3.2 FLAC Specific Box
        3.3.3 Sample format
        3.3.4 Duration of FLAC sample
        3.3.5 Sub-sample
        3.3.6 Random Access
            3.3.6.1 Random Access Point
   3.4 Basic Structure (informative)
        3.4.1 Initial Movie
   3.5 Example of Encapsulation (informative)
4 Acknowledgements
5 Author's Address

1 Scope

    This document specifies the normative mapping for encapsulation of
    FLAC coded audio bitstreams in ISO Base Media file format and its
    derivatives. The encapsulation of FLAC coded bitstreams in
    QuickTime file format is outside the scope of this specification.

2 Supporting Normative References

    [1] ISO/IEC 14496-12:2012 Corrected version

        Information technology — Coding of audio-visual objects — Part
        12: ISO base media file format

    [2] ISO/IEC 14496-12:2012/Amd.1:2013

        Information technology — Coding of audio-visual objects — Part
        12: ISO base media file format AMENDMENT 1: Various
        enhancements including support for large metadata

    [3] FLAC format specification

        https://xiph.org/flac/format.html

        Definition of the FLAC Audio Codec stream format

    [4] FLAC-in-Ogg mapping specification

        https://xiph.org/flac/ogg_mapping.html

        Ogg Encapsulation for the FLAC Audio Codec

    [5] Matroska specification

3 Design Rules of Encapsulation

    3.1 File Type Identification
    
        This specification does not define any brand to declare files
        which conform to this specification.  Files which conform to
        this specification shall contain at least one brand which
        supports the requirements and the requirements described in
        this clause without contradiction in the compatible brands
        list of the File Type Box.  The minimal support of the
        encapsulation of FLAC bitstreams in ISO Base Media file format
        requires the 'isom' brand.
        
    3.2 Overview of Track Structure

        FLAC coded audio shall be encapsulated into the ISO Base
        Media File Format as media data within an audio track.
        
            + The handler_type field in the Handler Reference Box
              shall be set to 'soun'.
            
            + The Media Information Box shall contain the Sound Media
              Header Box.
            
            + The codingname of the sample entry is 'fLaC'.
            
                This specification does not define any encapsulation
                using MP4AudioSampleEntry with objectTypeIndication
                specified by the MPEG-4 Registration Authority
                (http://www.mp4ra.org/).  See section 'Sample entry
                format' for the definition of the sample entry.
                
            + The 'dfLa' box is added to the sample entry to convey
                initializing information for the decoder.

                See section 'FLAC Specific Box' for the definition of
                the box contents.

            + A FLAC sample is exactly one FLAC frame as described
                in the format specification[3].  See section
                'Sample format' for details of the frame contents.

            + Every FLAC sample is a sync sample.  No pre-roll or
                lapping is required.  See section 'Random Access' for
                further details.

    3.3 Definition of a FLAC sample
    
        3.3.1 Sample entry format

            For any track containing one or more FLAC bitstreams, a
            sample entry describing the corresponding FLAC bitstream
            shall be present inside the Sample Table Box. This version
            of the specification defines only one sample entry format
            named FLACSampleEntry whose codingname is 'fLaC'.  This
            sample entry includes exactly one FLAC Specific Box
            defined in section 'FLAC specific box' as a mandatory box
            and indicates that FLAC samples described by this sample
            entry are stored by the sample format described in section
            'Sample format'.

            The syntax and semantics of the FLACSampleEntry is shown
            as follows.  The data fields of this box and native
            FLAC[3] structures encoded within FLAC blocks are both
            stored in big-endian format, though for purposes of the
            ISO BMFF container, FLAC native metadata and data blocks
            are treated as unstructured octet streams.
 
            class FLACSampleEntry() extends AudioSampleEntry ('fLaC'){
                FLACSpecificBox();
            }

            The fields of the AudioSampleEntry portion shall be set as
            follows:

            + channelcount:

                The channelcount field shall be set equal to the
                channel count specified by the FLAC bitstream's native
                METADATA_BLOCK_STREAMINFO header as described in [3].
                Note that the FLAC FRAME_HEADER structure that begins
                each FLAC sample redundantly encodes channel number;
                the number of channels declared in each FRAME_HEADER
                MUST match the number of channels declared here and in
                the METADATA_BLOCK_STREAMINFO header.

            + samplesize:

                The samplesize field shall be set equal to the bits
                per sample specified by the FLAC bitstream's native
                METADATA_BLOCK_STREAMINFO header as described in [3].
                Note that the FLAC FRAME_HEADER structure that begins
                each FLAC sample redundantly encodes the number of
                bits per sample; the bits per sample declared in each
                FRAME_HEADER MUST match the samplesize declared here
                and the bits per sample field declared in the
                METADATA_BLOCK_STREAMINFO header.
                
            + samplerate:

                When possible, the samplerate field shall be set
                equal to the sample rate specified by the FLAC
                bitstream's native METADATA_BLOCK_STREAMINFO header
                as described in [3], left-shifted by 16 bits to
                create the appropriate 16.16 fixed-point
                representation.

                When the bitstream's native sample rate is greater
                than the maximum expressible value of 65535 Hz,
                the samplerate field shall hold the greatest
                expressible regular division of that rate. I.e.
                the samplerate field shall hold 48000.0 for
                native sample rates of 96 and 192 kHz. In the
                case of unusual sample rates which do not have
                an expressible regular division, the maximum value
                of 65535.0 Hz should be used.

                High-rate FLAC bitstreams are common, and the native
                value from the METADATA_BLOCK_STREAMINFO header in
                the FLACSpecificBox MUST be read to determine the
                correct sample rate of the bitstream.

                Note that the FLAC FRAME_HEADER structure that begins
                each FLAC sample redundantly encodes the sample rate;
                the sample rate declared in each FRAME_HEADER MUST
                match the sample rate declared in the
                METADATA_BLOCK_STREAMINFO header, and here in the
                AudioSampleEntry portion of the FLACSampleEntry
                as much as is allowed by the encoding restrictions
                described above.
                
            Finally, the FLACSpecificBox carries codec headers:

            + FLACSpecificBox
            
                This box contains initializing information for the
                decoder as defined in section 'FLAC specific box'.

        3.3.2 FLAC Specific Box
        
            Exactly one FLAC Specific Box shall be present in each
            FLACSampleEntry.  This specification defines version 0
            of this box.  If incompatible changes occur in future
            versions of this specification, another version number
            will be defined.  The data fields of this box and native
            FLAC[3] structures encoded within FLAC blocks are both
            stored in big-endian format, though for purposes of the
            ISO BMFF container, FLAC native metadata and data blocks
            are treated as unstructured octet streams.

            The syntax and semantics of the FLAC Specific Box is shown
            as follows.

            class FLACMetadataBlock {
               unsigned int(1) LastMetadataBlockFlag;
               unsigned int(7) BlockType;
               unsigned int(24) Length;
               unsigned int(8) BlockData[Length];
            }

            aligned(8) class FLACSpecificBox
                  extends FullBox('dfLa', version=0, 0){
               for (i=0; ; i++) { // to end of box
                  FLACMetadataBlock();
               }
            }

            + Version:

                The Version field shall be set to 0.

                In the future versions of this specification, this
                field may be set to other values. And without support
                of those values, the reader shall not read the fields
                after this within the FLACSpecificBox.

            + Flags:

                The Flags field shall be set to 0.

            After the FullBox header, the box contains a sequence of
            FLAC[3] native-metadata block structures that fill the
            remainder of the box.

            Each FLACMetadataBlock structure consists of three fields
            filling a total of four bytes that form a FLAC[3] native
            METADATA_BLOCK_HEADER, followed by raw octet bytes that
            comprise the FLAC[3] native METADATA_BLOCK_DATA.

            + LastMetadataBlockFlag:

                The LastMetadataBlockFlag field maps semantically to
                the FLAC[3] native METADATA_BLOCK_HEADER
                Last-metadata-block flag as defined in the FLAC[3]
                file specification.
                
                The LastMetadataBlockFlag is set to 1 if this
                MetadataBlock is the last metadata block in the
                FLACSpecificBox.  It is set to 0 otherwise.
               
            + BlockType:

                The BlockType field maps semantically to the FLAC[3]
                native METADATA_BLOCK_HEADER BLOCK_TYPE field as
                defined in the FLAC[3] file specification.

                The BlockType is set to a valid FLAC[3] BLOCK_TYPE
                value that identifies the type of this native metadata
                block.  The BlockType of the first FLACMetadataBlock
                must be set to 0, signifying this is a FLAC[3] native
                METADATA_BLOCK_STREAMINFO block.
               
            + Length:

                The Length field maps semantically to the FLAC[3]
                native METADATA_BLOCK_HEADER Length field as
                defined in the FLAC[3] file specification.

                The length field specifies the number of bytes of
                MetadataBlockData to follow.

            + BlockData

                The BlockData field maps semantically to the FLAC[3]
                native METADATA_BLOCK_HEADER METADATA_BLOCK_DATA as
                defined in the FLAC[3] file specification.

            Taken together, the bytes of the FLACMetadataBlock form a
            complete FLAC[3] native METADATA_BLOCK structure.

            Note that a minimum of a single FLACMetadataBlock,
            consisting of a FLAC[3] native METADATA_BLOCK_STREAMINFO
            structure, is required.  Should the FLACSpecificBox
            contain more than a single FLACMetadataBlock structure,
            the FLACMetadataBlock containing the FLAC[3] native
            METADATA_BLOCK_STREAMINFO must occur first in the list.

            Other containers that package FLAC audio streams, such as
            Ogg[4] and Matroska[5], wrap FLAC[3] native metadata without
            modification similar to this specification.  When
            repackaging or remuxing FLAC[3] streams from another
            format that contains FLAC[3] native metadata into an ISO
            BMFF file, the complete FLAC[3] native metadata should be
            preserved in the ISO BMFF stream as described above.  It
            is also allowed to parse this native metadata and include
            contextually redundant ISO BMFF-native repackagings and/or
            reparsings of FLAC[3] native metadata, so long as the
            native metadata is also preserved.

        3.3.3 Sample format
        
            A FLAC sample is exactly one FLAC audio FRAME (as defined
            in the FLAC[3] file specification) belonging to a FLAC
            bitstreams. The FLAC sample data begins with a complete
            FLAC FRAME_HEADER, followed by one FLAC SUBFRAME per
            channel, any necessary bit padding, and ends with the
            usual FLAC FRAME_FOOTER.

            Note that the FLAC native FRAME_HEADER structure that
            begins each FLAC sample redundantly encodes channel count,
            sample rate, and sample size.  The values of these fields
            must agree both with the values declared in the FLAC
            METADATA_BLOCK_STREAMINFO structure as well as the
            FLACSampleEntry box.

        3.3.4 Duration of a FLAC sample

            The duration of any given FLAC sample is determined by
            dividing the decoded block size of a FLAC frame, as
            encoded in the FLAC FRAME's FRAME_HEADER structure, by the
            value of the timescale field in the Media Header Box.
            FLAC samples are permitted to have variable durations
            within a given audio stream.  FLAC does not use padding
            values.

        3.3.5 Sub-sample

            Sub-samples are not defined for FLAC samples in this
            specification.

        3.3.6 Random Access
        
            This subclause describes the nature of the random access
            of FLAC sample.

            3.3.6.1 Random Access Point
            
                All FLAC samples can be independently decoded
                i.e. every FLAC sample is a sync sample. The Sync
                Sample Box shall not be present as long as there are
                no samples other than FLAC samples in the same
                track. The sample_is_non_sync_sample field for FLAC
                samples shall be set to 0.

    3.4 Basic Structure (informative)

        3.4.1 Initial Movie
        
            This subclause shows a basic structure of the Movie Box as follows:

            +----+----+----+----+----+----+----+----+------------------------------+
            |moov|    |    |    |    |    |    |    | Movie Box                    |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |mvhd|    |    |    |    |    |    | Movie Header Box             |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |trak|    |    |    |    |    |    | Track Box                    |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |tkhd|    |    |    |    |    | Track Header Box             |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |edts|*   |    |    |    |    | Edit Box                     |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |elst|*   |    |    |    | Edit List Box                |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |mdia|    |    |    |    |    | Media Box                    |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |mdhd|    |    |    |    | Media Header Box             |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |hdlr|    |    |    |    | Handler Reference Box        |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |minf|    |    |    |    | Media Information Box        |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |smhd|    |    |    | Sound Media Header Box       |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |dinf|    |    |    | Data Information Box         |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |dref|    |    | Data Reference Box           |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |    |url |    | DataEntryUrlBox              |
            +----+----+----+----+----+----+ or +----+------------------------------+
            |    |    |    |    |    |    |urn |    | DataEntryUrnBox              |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |stbl|    |    |    | Sample Table                 |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |stsd|    |    | Sample Description Box       |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |    |fLaC|    | FLACSampleEntry              |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |    |    |dfLa| FLAC Specific Box            |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |stts|    |    | Decoding Time to Sample Box  |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |stsc|    |    | Sample To Chunk Box          |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |stsz|    |    | Sample Size Box              |
            +----+----+----+----+----+ or +----+----+------------------------------+
            |    |    |    |    |    |stz2|    |    | Compact Sample Size Box      |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |    |    |    |stco|    |    | Chunk Offset Box             |
            +----+----+----+----+----+ or +----+----+------------------------------+
            |    |    |    |    |    |co64|    |    | Chunk Large Offset Box       |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |mvex|*   |    |    |    |    |    | Movie Extends Box            |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |trex|*   |    |    |    |    | Track Extends Box            |
            +----+----+----+----+----+----+----+----+------------------------------+

                    Figure 1 - Basic structure of Movie Box

            It is strongly recommended that the order of boxes should
            follow the above structure.  Boxes marked with an asterisk
            (*) may or may not be present depending on context.  For
            most boxes listed above, the definition is as is defined
            in ISO/IEC 14496-12 [1]. The additional boxes and the
            additional requirements, restrictions and recommendations
            to the other boxes are described in this specification.
            
    3.5 Example of Encapsulation (informative)
        [File]
            size = 17790
            [ftyp: File Type Box]
                position = 0
                size = 24
                major_brand = mp42 : MP4 version 2
                minor_version = 0
                compatible_brands
                    brand[0] = mp42 : MP4 version 2
                    brand[1] = isom : ISO Base Media file format
            [moov: Movie Box]
                position = 24
                size = 757
                [mvhd: Movie Header Box]
                    position = 32
                    size = 108
                    version = 0
                    flags = 0x000000
                    creation_time = UTC 2014/12/12, 18:41:19
                    modification_time = UTC 2014/12/12, 18:41:19
                    timescale = 48000
                    duration = 33600 (00:00:00.700)
                    rate = 1.000000
                    volume = 1.000000
                    reserved = 0x0000
                    reserved = 0x00000000
                    reserved = 0x00000000
                    transformation matrix
                        | a, b, u |   | 1.000000, 0.000000, 0.000000 |
                        | c, d, v | = | 0.000000, 1.000000, 0.000000 |
                        | x, y, w |   | 0.000000, 0.000000, 1.000000 |
                    pre_defined = 0x00000000
                    pre_defined = 0x00000000
                    pre_defined = 0x00000000
                    pre_defined = 0x00000000
                    pre_defined = 0x00000000
                    pre_defined = 0x00000000
                    next_track_ID = 2
                [iods: Object Descriptor Box]
                    position = 140
                    size = 33
                    version = 0
                    flags = 0x000000
                    [tag = 0x10: MP4_IOD]
                        expandableClassSize = 16
                        ObjectDescriptorID = 1
                        URL_Flag = 0
                        includeInlineProfileLevelFlag = 0
                        reserved = 0xf
                        ODProfileLevelIndication = 0xff
                        sceneProfileLevelIndication = 0xff
                        audioProfileLevelIndication = 0xfe
                        visualProfileLevelIndication = 0xff
                        graphicsProfileLevelIndication = 0xff
                        [tag = 0x0e: ES_ID_Inc]
                            expandableClassSize = 4
                            Track_ID = 1
                [trak: Track Box]
                    position = 173
                    size = 608
                    [tkhd: Track Header Box]
                        position = 181
                        size = 92
                        version = 0
                        flags = 0x000007
                            Track enabled
                            Track in movie
                            Track in preview
                        creation_time = UTC 2014/12/12, 18:41:19
                        modification_time = UTC 2014/12/12, 18:41:19
                        track_ID = 1
                        reserved = 0x00000000
                        duration = 33600 (00:00:00.700)
                        reserved = 0x00000000
                        reserved = 0x00000000
                        layer = 0
                        alternate_group = 0
                        volume = 1.000000
                        reserved = 0x0000
                        transformation matrix
                            | a, b, u |   | 1.000000, 0.000000, 0.000000 |
                            | c, d, v | = | 0.000000, 1.000000, 0.000000 |
                            | x, y, w |   | 0.000000, 0.000000, 1.000000 |
                        width = 0.000000
                        height = 0.000000
                    [mdia: Media Box]
                        position = 273
                        size = 472
                        [mdhd: Media Header Box]
                            position = 281
                            size = 32
                            version = 0
                            flags = 0x000000
                            creation_time = UTC 2014/12/12, 18:41:19
                            modification_time = UTC 2014/12/12, 18:41:19
                            timescale = 48000
                            duration = 34560 (00:00:00.720)
                            language = und
                            pre_defined = 0x0000
                        [hdlr: Handler Reference Box]
                            position = 313
                            size = 51
                            version = 0
                            flags = 0x000000
                            pre_defined = 0x00000000
                            handler_type = soun
                            reserved = 0x00000000
                            reserved = 0x00000000
                            reserved = 0x00000000
                            name = Xiph Audio Handler
                        [minf: Media Information Box]
                            position = 364
                            size = 381
                            [smhd: Sound Media Header Box]
                                position = 372
                                size = 16
                                version = 0
                                flags = 0x000000
                                balance = 0.000000
                                reserved = 0x0000
                            [dinf: Data Information Box]
                                position = 388
                                size = 36
                                [dref: Data Reference Box]
                                    position = 396
                                    size = 28
                                    version = 0
                                    flags = 0x000000
                                    entry_count = 1
                                    [url : Data Entry Url Box]
                                        position = 412
                                        size = 12
                                        version = 0
                                        flags = 0x000001
                                        location = in the same file
                            [stbl: Sample Table Box]
                                position = 424
                                size = 321
                                [stsd: Sample Description Box]
                                    position = 432
                                    size = 79
                                    version = 0
                                    flags = 0x000000
                                    entry_count = 1
                                    [fLaC: Audio Description]
                                        position = 448
                                        size = 63
                                        reserved = 0x000000000000
                                        data_reference_index = 1
                                        reserved = 0x0000
                                        reserved = 0x0000
                                        reserved = 0x00000000
                                        channelcount = 2
                                        samplesize = 16
                                        pre_defined = 0
                                        reserved = 0
                                        samplerate = 48000.000000
                                        [dfLa: FLAC Specific Box]
                                            position = 484
                                            size = 50
                                            version = 0
                                            flags = 0x000000
                                            [FLACMetadataBlock]
                                                LastMetadataBlockFlag = 1
                                                BlockType = 0
                                                Length = 34
                                                BlockData[34];
                                [stts: Decoding Time to Sample Box]
                                    position = 492
                                    size = 24
                                    version = 0
                                    flags = 0x000000
                                    entry_count = 1
                                    entry[0]
                                        sample_count = 18
                                        sample_delta = 1920
                                [stsc: Sample To Chunk Box]
                                    position = 516
                                    size = 40
                                    version = 0
                                    flags = 0x000000
                                    entry_count = 2
                                    entry[0]
                                        first_chunk = 1
                                        samples_per_chunk = 13
                                        sample_description_index = 1
                                    entry[1]
                                        first_chunk = 2
                                        samples_per_chunk = 5
                                        sample_description_index = 1
                                [stsz: Sample Size Box]
                                    position = 556
                                    size = 92
                                    version = 0
                                    flags = 0x000000
                                    sample_size = 0 (variable)
                                    sample_count = 18
                                    entry_size[0] = 977
                                    entry_size[1] = 938
                                    entry_size[2] = 939
                                    entry_size[3] = 938
                                    entry_size[4] = 934
                                    entry_size[5] = 945
                                    entry_size[6] = 948
                                    entry_size[7] = 956
                                    entry_size[8] = 955
                                    entry_size[9] = 930
                                    entry_size[10] = 933
                                    entry_size[11] = 934
                                    entry_size[12] = 972
                                    entry_size[13] = 977
                                    entry_size[14] = 958
                                    entry_size[15] = 949
                                    entry_size[16] = 962
                                    entry_size[17] = 848
                                [stco: Chunk Offset Box]
                                    position = 648
                                    size = 24
                                    version = 0
                                    flags = 0x000000
                                    entry_count = 2
                                    chunk_offset[0] = 686
                                    chunk_offset[1] = 12985
            [free: Free Space Box]
                position = 672
                size = 8
            [mdat: Media Data Box]
                position = 680
                size = 17001

4 Acknowledgements

    This spec draws heavily from the Opus-in-ISOBMFF specification
    work done by Yusuke Nakamura <muken.the.vfrmaniac |at| gmail.com>

    Thank you to Tim Terriberry, David Evans, and Yusuke Nakamura
    for valuable feedback. Thank you to Ralph Giles for editorial
    help.

5 Author Address

    Monty Montgomery <cmontgomery@mozilla.com>