Enhanced AC-3, also called Dolby Digital Plus or E-AC-3, extends AC-3 with higher bit
rates, more channels and independent substreams. It is defined in ATSC A/52 Annex E and
ETSI TS 102 366. Files of this kind usually have the extension .ec3 or
.eac3, but E-AC-3 streams also turn up with the .ac3
extension. The two are told apart by bsid, the 5-bit field at bits 40–44 of
the frame: 10 or lower is AC-3; 11 to 16 is Enhanced AC-3 (16 in current streams).
Sync frame header
E-AC-3 keeps the sync word but changes what follows it. All values are big-endian:
| Width in bits | Name | Meaning |
|---|---|---|
| 16 | syncword | 0B 77 |
| 2 | strmtyp | the stream type |
| 3 | substreamid | the substream number, 0 to 7 |
| 11 | frmsiz | the frame length in 16-bit words, minus one |
| 2 | fscod | sample rate code; 3 means a reduced rate in fscod2 |
| 2 | numblkscod, or fscod2 if fscod is 3 | blocks per frame, or the reduced rate |
| 3 | acmod | the channel layout, as in AC-3 |
| 1 | lfeon | 1 means the LFE channel is present |
| 5 | bsid | 11 to 16 |
Stream types
| strmtyp | Meaning |
|---|---|
| 0 | independent: a programme a decoder can play on its own |
| 1 | dependent: adds channels to the independent substream before it |
| 2 | converted: an independent stream converted from AC-3 |
| 3 | reserved |
Blocks per frame
| numblkscod | Audio blocks | Samples per channel |
|---|---|---|
| 0 | 1 | 256 |
| 1 | 2 | 512 |
| 2 | 3 | 768 |
| 3 | 6 | 1,536 |
Where a frame has fewer than six blocks, a frame set is the group of frames that together hold six blocks.
Reduced sample rates
When fscod is 3, fscod2 gives the rate, and each frame has six blocks.
| fscod2 | Rate |
|---|---|
| 0 | 24,000 Hz |
| 1 | 22,050 Hz |
| 2 | 16,000 Hz |
| 3 | reserved |
After the header
The rest of the header carries mix metadata, informational metadata, extra data and other fields, followed by the frame’s blocks. Those structures are specified in ATSC A/52 Annex E and ETSI TS 102 366, which this site could not read, so they are not reproduced here. E-AC-3 in other files is described in RFC 4598, and AC-3 itself is described on this site’s other pages. A bit in the additional bit stream information flags Dolby Atmos in E-AC-3 streams, a joint-object-coding layer that is proprietary and undocumented.