.AC3 File Format – Metadata (EMDF)

The container

variable_bits(n) codes an unsigned number of any size in groups of n bits, each group followed by a read_more bit:

value = 0
do { value += read(n);  read_more = read(1);
     if (read_more) { value <<= n;  value += 1 << n; } } while (read_more)

So one group codes 0 to 2ⁿ − 1, two groups 2ⁿ upwards, and so on.

Payload configuration

Payload types

Every payload starts with a 2-bit version (3 meaning a 4-bit extended version follows and is added), except 3, 5, 6 and 7, and every payload ends with zero fill bits to a byte boundary.

1: programme loudness data

Measured loudness of the programme, and whether it was corrected before encoding, to guide loudness processing after the decoder. Version 1 in this syntax.

When a relative-gated (or, with dialogue gating, speech-gated) value is present, the stream’s dialnorm must match it.

2: programme information

Processing before encoding, the bands coded with special techniques, and the DRC profiles behind the stream’s compression words. Version 0 in this syntax.

upmixtyp in an AC-3 frame or an independent substream (acmod 7): 0 Dolby Pro Logic, 1 Pro Logic II movie mode, 2 Pro Logic II music mode, 3 Dolby Professional Upmixer, 4–15 reserved. In dependent substream 0: 1 Dolby Pro Logic IIx (only with Lrs/Rrs in the channel map), 2 Pro Logic IIz height (only with Vhl/Vhr), others reserved.

The DRC profiles (Table H.3.14). Boundaries are levels relative to the −31 dB reference; “None” is a constant 0 dB:

The printed table leaves the music light column of the cut section rows empty, so that profile has no cut section.

3: Enhanced AC-3 substream structure

Only in independent substream 0. num_ind_sub (3 bits) is one less than the number of independent substreams; then for each, dep_sub_exists (1) and, if set, num_dep_sub (3), one less than its number of dependent substreams. Fill bits end it.

4: dynamic range compression for portable devices

Version 0. portdrce (1) and portdrc (8): a gain word for very narrow dynamic range on portable devices, read like compr; then drcprofinfoe (1) and portdrcprof (3), its profile, coded as dynrngprof.

5: programme language

An IETF BCP 47 language tag, only in independent substreams. frame_sequence (1): if 1, the tag is spread over several frames, each with sequence_start (1), sequence_end (1) and a 16-bit langtag_chunk (the tag starting in its most significant byte, padded with a zero byte to even length); if 0, langtag_length (6; 0–40 mean 2–42 bytes, 41–63 reserved) and the whole langtag follow.

6: external data

User data with no payload type of its own. As the language payload: frame_sequence, then either sequence_start, sequence_end and a 16-bit external_data_chunk, or external_data_length (variable_bits(7), in bytes) and the external_data itself.

7: headphone rendering data