Reference
What is AC-3 / Dolby Digital?
AC-3 is the codec/bitstream format commonly branded as Dolby Digital. It became a core surround format for DVD, digital television and home-theater delivery.
AC-3 is: a lossy multichannel audio codec. Dolby Atmos is not another name for AC-3: Atmos metadata/object information may be delivered through other Dolby formats such as Dolby Digital Plus or TrueHD.
Common Dolby Digital specifications
| Property | Dolby Digital / AC-3 |
|---|---|
| Channels | 1.0 to 5.1 discrete |
| Compression | Lossy |
| Typical DVD 5.1 bitrate | 384 or 448 kbps |
| Blu-ray 5.1 bitrate | Up to 640 kbps |
| Metadata | Dialogue level, channel mode, LFE, dynamic-range and downmix control metadata |
Where AC-3 appears
AC-3 may appear as a raw .ac3 stream or inside containers/disc structures such as MPEG transport streams, VOB/DVD, MP4 in supported ecosystems, MKV and others.
AC-3 vs E-AC-3
E-AC-3 (Dolby Digital Plus) is a newer, more efficient extension/family member with higher channel and bitrate capability. A file labelled “Dolby Digital Plus” should not be described simply as ordinary AC-3.
Common confusion
5.1 is a channel layout, not a codec. AC-3 is one codec that can carry 5.1. DTS, AAC, FLAC and other codecs can also carry multichannel audio.