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  "markdown": "Dolby has several unrelated-looking things under one name: AC-3 and E-AC-3 are lossy surround codecs, TrueHD is a lossless one, AC-4 is a newer lossy one, and Atmos is an immersive-audio system that rides on top of E-AC-3, TrueHD and AC-4. If you work with Blu-rays, streaming rips or home theatre files you will meet all of them. The practical rule: **copy Dolby tracks rather than re-encoding them**, because FFmpeg can only make plain channel-based AC-3, E-AC-3 and (experimentally) TrueHD, with no Atmos.\n\n## The codecs\n\n| Codec | Type | What it is |\n| --- | --- | --- |\n| **AC-3** (Dolby Digital) | Lossy | The original 5.1 codec: DVD, broadcast, Blu-ray core tracks. Up to 640 kbit/s. Every surround device plays it. |\n| **E-AC-3** (Dolby Digital Plus) | Lossy | AC-3's successor: more efficient, 32 kbit/s to 6 Mbit/s, up to 7.1 discrete channels. The main surround codec of streaming services; also how Atmos reaches streaming. Not bitstream-compatible with AC-3, but E-AC-3 decoders also decode AC-3. |\n| **TrueHD** | Lossless | MLP-derived lossless codec used on Blu-ray: 44.1–192 kHz, up to 24-bit, up to 16 channels in the format. Blu-ray limits it to eight channels at 96 kHz or six at 192 kHz. Carries Atmos on disc. |\n| **AC-4** | Lossy | Newer broadcast and streaming codec with multiple presentations (languages, dialogue enhancement) and Atmos support. Not compatible with AC-3 or E-AC-3. |\n\nOn Blu-ray, a TrueHD track is interleaved with an AC-3 version in the same stream for players that cannot decode TrueHD. FFmpeg and eac3to expose the two as separate tracks, so you can keep just the lossless one.\n\n## Atmos is not a codec\n\nAtmos is a production and rendering system. A mix is a channel bed (for example 7.1.2) plus positioned audio objects and metadata, and the player's renderer maps that to whatever speakers or headphones are present. That payload is carried by a normal Dolby codec:\n\n- **TrueHD Atmos** on Blu-ray: the usual lossless 7.1/5.1/stereo presentations, rendered from the full mix, plus an extra substream with the object data.\n- **E-AC-3 with JOC** (Joint Object Coding) on streaming: a lossy 5.1 or 7.1 mix rendered from the full mix, plus side data from which an Atmos decoder reconstructs the objects. Decoders without Atmos support play the channel mix.\n- **AC-4** for broadcast and some streaming.\n\nSo an E-AC-3 or TrueHD file is not necessarily Atmos; check the profile. `ffprobe` reports Atmos streams as \"Dolby Digital Plus + Dolby Atmos\" or \"Dolby TrueHD + Dolby Atmos\" in the `profile` field.\n\n## What FFmpeg can and cannot do\n\n| Task | FFmpeg |\n| --- | --- |\n| Copy any Dolby track into another container | Yes (`-c copy`). Atmos survives. |\n| Encode plain AC-3 or E-AC-3 from PCM | Yes: `ac3` and `eac3`, up to 5.1. A 7.1 input is silently downmixed to 5.1. |\n| Encode TrueHD from PCM | Experimental: `truehd`, up to 5.1, needs `-strict -2`. Bit-exact on decode. |\n| Encode AC-4 | No. |\n| Create Atmos | No. Needs Dolby's Atmos tools and licensed encoders. |\n| Decode Atmos | Decodes the backward-compatible 7.1 or 5.1 mix and ignores the object data, so height and object positioning are lost. |\n\nRe-encoding an Atmos track with FFmpeg to AC-3, E-AC-3, AAC or Opus permanently discards the object data. If you need a compatibility copy, make it additionally and keep the original track.\n\n## Commands\n\nCopy a Dolby track into a new file without touching the audio:\n\n```sh\nffmpeg -i input.mkv -map 0:a:0 -c:a copy output.mka\n```\n\nEncode a 5.1 PCM master to AC-3 or E-AC-3:\n\n```sh\nffmpeg -i input.wav -c:a ac3 -b:a 640k output.ac3\nffmpeg -i input.wav -c:a eac3 -b:a 640k output.eac3\n```\n\nUse 448 kbit/s for 5.1 AC-3 if the target doesn't need the maximum. Stereo is typically 192 kbit/s. The encoder has a lot of metadata options (`-dialnorm`, `-dmix_mode`, `-center_mixlev` and others) that matter for broadcast delivery and rarely otherwise; the defaults are fine for a compatibility track.\n\nEncode lossless TrueHD (5.1 or fewer channels):\n\n```sh\nffmpeg -i input.wav -c:a truehd -strict -2 output.thd\n```\n\nThis is experimental and rarely worth it; if you just want lossless surround, use [FLAC](/codecs/audio/flac/).\n\nAC-3, E-AC-3 and TrueHD go into MP4 (E-AC-3 and AC-3) or Matroska (all). MP4 labels them `ac-3` and `ec-3`. Most TVs and receivers want a bitstream passthrough over HDMI to get Atmos, which means copying the track, not decoding it.\n\n## When to choose what\n\n- **Remuxing a disc or streaming rip:** keep the original track.\n- **A surround compatibility copy for a TV:** E-AC-3 or AC-3 at 448–640 kbit/s. For most people [AAC](/codecs/audio/aac/) or [Opus](/codecs/audio/opus/) are better codecs, but only the Dolby ones pass through to an AV receiver.\n- **Lossless surround you encode yourself:** [FLAC](/codecs/audio/flac/).\n- **Authoring Atmos:** professional Dolby tools; see [spatial and immersive audio](/codecs/audio/spatial-audio/) for the open alternatives.\n"
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