Opus

Opus is an open, royalty-free lossy audio codec standardised as RFC 6716. It combines Skype’s SILK (speech) and Xiph’s CELT (general audio) and switches between them automatically, so one codec covers voice calls, podcasts and music. It is the default recommendation for lossy audio unless you need MP4/Apple-device compatibility, where AAC is still the safe choice. For web video it is the standard companion to VP9 and AV1 in WebM.

At the same bitrate Opus generally sounds better than AAC-LC, Vorbis and MP3, and it stays usable far lower. Stereo music is near-transparent at around 128 kbit/s VBR. It is lossy: encode from the lossless original, never from another lossy file.

Recommended bitrates

Bitrates are for the whole stream, not per channel. These are the Xiph recommended settings:

Content	Channels	kbit/s
Voice over IP	Mono	10–24
Podcast / audiobook	Mono	24
Podcast / audiobook	Stereo	32
Music streaming	Stereo	64–96
Music archive / storage	Stereo	96–128
Music	5.1	128–256
Music	7.1	256–450

Below about 19 kbit/s the encoder starts folding stereo to mono. Opus covers 6–510 kbit/s. For archiving, keep a lossless copy (FLAC) instead.

Encoding with opusenc

opusenc (from opus-tools) is the reference encoder. Its defaults are already the right ones for stored audio: VBR, complexity 10, 20 ms frames, automatic speech/music detection. It reads WAV, AIFF and FLAC and resamples 44.1 kHz material itself.

opusenc --bitrate 128 input.flac output.opus
opusenc --speech --downmix-mono --bitrate 24 interview.wav interview.opus

--bitrate is in kbit/s, unlike FFmpeg’s bit/s. --speech and --music override the automatic speech/music detection and only matter at low bitrates. --cvbr and --hard-cbr exist for transports with a strict rate limit; leave VBR on for files.

Encoding with FFmpeg

ffmpeg -i input.flac -c:a libopus -b:a 128k output.opus

Everything else defaults sensibly: -vbr on, -application audio, 20 ms frames, complexity 10. The options worth knowing:

Option	Use
-application voip	Speech intelligibility tuning for voice recordings. audio (default) is right for music; lowdelay is only for real-time work.
-vbr constrained	Limits rate swings for streaming. -vbr off is hard CBR.
-frame_duration	Leave at 20. Longer frames only help at very low bitrates; shorter ones cost quality and are for latency.
-mapping_family 1	Surround layouts, see below.
-ac 1	Mono downmix for speech.

-q:a is not supported by libopus; control quality with -b:a. -packet_loss and -fec are for real-time links and do nothing useful in a stored file.

Sample rate

Opus works at 48 kHz internally, and players and FFmpeg decode to 48 kHz (8, 12, 16 and 24 kHz are also accepted as input rates). A 44.1 kHz source is resampled by opusenc or FFmpeg automatically; do not force -ar 44100. The original rate is stored only as informational metadata. Audible bandwidth tops out around 20 kHz regardless of the input rate.

Surround

Opus carries mono and stereo directly (mapping family 0). For 3–8 channels it uses mapping family 1, which defines the channel order and enables surround-specific bit allocation. FFmpeg writes a family 1 header by default but only uses libopus’s surround-aware encoder when you ask for it:

ffmpeg -i surround.mkv -c:a libopus -b:a 256k -mapping_family 1 output.mka

libopus rejects the 5.1(side) layout that many Blu-ray and AC-3 sources use (“Invalid channel layout 5.1(side) for specified mapping family”). Convert it to plain 5.1 first:

ffmpeg -i input.mkv -af aformat=channel_layouts=5.1 -c:a libopus -b:a 256k -mapping_family 1 output.mka

aformat relabels the side channels as back channels; nothing is downmixed. -af applies to every audio output stream, so when a command encodes several audio tracks, target the surround one with -filter:a:N instead (see audio and subtitles). To handle unknown inputs in one command, offer several layouts and let FFmpeg pick the closest: -af aformat=channel_layouts=7.1|5.1|stereo -mapping_family 1. More than 8 channels needs mapping family 255 (unmapped channels), which most players will not render as surround.

opusenc handles 5.1 WAV files directly.

FFmpeg’s native encoder

FFmpeg also has its own opus encoder (CELT-only, mono/stereo, 48 kHz). It is experimental, needs -strict -2, and is not better than libopus; the only reason to reach for it is a build without libopus.

Containers and playback

Opus container mappings

Relationship	Project or format	Evidence and scope	Status
Carried By	Ogg	Ogg Opus uses OpusHead and OpusTags headers with 48 kHz granule-position semantics.Scope: Container Mapping · Sources: RFC 7845: Ogg Encapsulation for the Opus Audio Codec	Normative
Carried By	Matroska	Uses the A_OPUS Codec ID and stores the Opus identification header in CodecPrivate.Scope: Container Mapping · Sources: Matroska codec mappings	Normative
Carried By	WebM	Opus is one of the audio codecs listed by the WebM Project.Scope: Container Mapping · Sources: WebM container guidelines	Documented
Carried By	ISO Base Media File Format	The public Opus binding defines the Opus sample entry and dOps box, but remains an old incomplete draft.Scope: Container Mapping · Sources: Encapsulation of Opus in ISO Base Media File Format, MP4 Registration Authority codec registry	Registered

• Ogg Opus (.opus, RFC 7845) is the standard file format for audio. Safari supports it from 18.4 (macOS 15.4, iOS 18.4).
• WebM / Matroska is the usual choice with video; -c:a copy between them is lossless.
• MP4 can carry Opus, but player support is patchier than for the other two.

Opus is also one of the codecs IAMF/Eclipsa Audio can use; see spatial audio.

Full option reference

Every option of opusenc 0.2 (with libopus 1.6.1 and libopusenc 0.3) and of FFmpeg 9.0.2’s two Opus encoders. Defaults are what you get when the option is omitted.

Complete opusenc 0.2 parameter reference

Rate, signal, and latency

Parameter	Default	Values / units	Effect
--bitrate N	Auto: 64 kbit/s mono, 96 stereo	kbit/s for the whole stream, decimals allowed; 6–256 per channel is the useful range	Target bitrate. The automatic value is 64 kbit/s per stream plus 32 per coupled pair (5.1 gets 320), scaled down for inputs below 44.1 kHz (16 kHz mono gets 33). Capped at 256 kbit/s per channel.
--vbr	On	Flag	Unconstrained VBR: the bitrate follows the content around the target. The right mode for files.
--cvbr	Off	Flag	Constrained VBR: packet sizes vary but stay close to the target, like CBR with a small reservoir. For streaming over constant-rate links.
--hard-cbr	Off	Flag	Every packet the same size, at a quality cost. Only for transports that need it. The last of --vbr, --cvbr and --hard-cbr wins.
--music	Auto-detected	Flag	Treat the input as music, overriding speech/music detection. Mainly matters at low bitrates.
--speech	Auto-detected	Flag	Treat the input as speech, favouring the SILK modes at low bitrates. The last of --music and --speech wins.
--comp N, --complexity N	10	0–10	Encoder effort; 10 is slowest and best. Lower it only for real-time encoding on weak CPUs.
--framesize N	20	2.5, 5, 10, 20, 40, 60 ms	Maximum frame duration. 20 ms suits almost everything; 40 or 60 ms save a little overhead for low-bitrate speech. Frames under 10 ms can only use CELT and cost efficiency.
--expect-loss N	0	0–100 %	Expected packet loss. Makes frames more robust to loss at some quality cost. It does not turn on in-band FEC; add --set-ctl-int 4012=1 for that.
--downmix-mono	Off	Flag	Downmix to mono before encoding.
--downmix-stereo	Off	Flag	Downmix inputs with more than two channels to stereo.
--no-downmix	Off	Flag	Disable the automatic downmix: with more than two channels and --bitrate below 16 kbit/s per channel, opusenc otherwise downmixes to stereo (3–8 channels) or mono (more than 8). Not listed by --help.
--no-phase-inv	Off	Flag	Disable intensity-stereo phase inversion. Slightly lowers stereo quality, but avoids artefacts if the decoded audio is later downmixed to mono.
--max-delay N	1000	0–1000 ms	Maximum Ogg muxing delay: how long packets may wait before a page is written. Lower it for live streaming.

Input interpretation

Parameter	Default	Values / units	Effect
--raw	Off	Flag	Read headerless PCM instead of detecting WAV, AIFF or FLAC. Any --raw-* option implies it.
--raw-bits N	16	8, 16, 24	Raw sample size; 8-bit is unsigned, 16- and 24-bit are signed.
--raw-rate N	48000	Hz, 100–768000	Raw sample rate. Opus always codes at 48 kHz internally; opusenc resamples any other input rate.
--raw-chan N	2	1–255	Raw channel count. More than eight channels are coded as discrete channels (mapping family 255).
--raw-endianness N	0	0 little, 1 big	Raw byte order.
--ignorelength	Off	Flag	Ignore the WAV data-chunk length, for WAV piped from tools that write a placeholder length.

Tags, pictures, and padding

Tags and pictures from the input file are copied unless you discard them; command-line tags are added to them.

Parameter	Default	Values / units	Effect
--title TEXT	Copied from input	Text	Title.
--artist TEXT	Copied from input	Text; repeatable	Artist.
--album TEXT	Copied from input	Text	Album.
--tracknumber N	Copied from input	Text	Track number.
--genre TEXT	Copied from input	Text; repeatable	Genre.
--date TEXT	Copied from input	YYYY, YYYY-MM or YYYY-MM-DD	Date.
--comment TAG=VALUE	—	NAME=value; repeatable	Add any Vorbis comment.
--picture FILE	Input pictures copied	JPEG, PNG or GIF file, or [TYPE]|[MIME]|[DESC]|[WxHxD]|FILE; repeatable	Attach cover art. The type defaults to 3 (front cover); --help-picture lists the types.
--padding N	512	Bytes	Spare room in the comment header so tags can be edited later without rewriting the file.
--discard-comments	Off	Flag	Don’t copy tags or pictures from the input (implies --discard-pictures). Tags given on the command line are still written.
--discard-pictures	Off	Flag	Don’t copy pictures from the input.

Diagnostics and advanced controls

Parameter	Default	Values / units	Effect
-h, --help	—	Flag	Print help and exit.
-V, --version	—	Flag	Print the opus-tools and libopus versions and exit.
--version-short	—	Flag	Same output as --version in 0.2. Not listed by --help.
--help-picture	—	Flag	Print the --picture syntax and picture types 0–20.
--quiet	Off	Flag	Suppress progress and the final statistics.
--serial N	Random	Integer	Ogg stream serial number.
--save-range FILE	Off	Filename	Write per-frame range-coder check values, for decoder conformance testing.
--set-ctl-int [S:]X=Y	—	X=Y, or S:X=Y to target stream S of a multistream file; repeatable	Pass an integer encoder control, applied after all other settings. Only the requests in the next table are accepted.

Allowed advanced integer requests

--set-ctl-int accepts only these requests. Numbers are from the libopus 1.6.1 headers and libopusenc 0.3; defaults are what opusenc sets, which differ from bare libopus in places. Use the named option where one exists.

Request X	opusenc default	Values Y	Control
4000	2049 (audio)	2048 VoIP, 2049 audio, 2051 low delay	Application.
4002	From --bitrate	bit/s; -1000 auto, -1 maximum	Bitrate. Without S: it is the total for all streams; with S: it sets one stream.
4004	1105 (fullband)	1101–1105 = 4, 6, 8, 12, 20 kHz	Maximum bandwidth the encoder may choose.
4006	1	0/1	VBR.
4008	-1000 (auto)	-1000 or 1101–1105	Force a bandwidth.
4010	10	0–10	Complexity.
4012	0	0 off, 1 on, 2 on without forcing SILK for music	In-band FEC; also set --expect-loss.
4014	0	0–100 %	Expected packet loss.
4016	0	0/1	DTX.
4020	0 (unconstrained)	0/1	Constrained VBR; ignored with hard CBR. Bare libopus defaults to 1.
4022	-1000 (auto)	-1000, 1 mono, 2 stereo	Force the coded channel count of a stream.
4024	-1000 (auto)	-1000, 3001 voice, 3002 music	Signal type.
4036	Input bit depth, clamped to 8–24	8–24	Significant bits in the input.
4042	0	0/1	Disable inter-frame prediction.
4046	0	0/1	Disable intensity-stereo phase inversion.
14000	96000 (2 s)	48 kHz samples, at most 96000	libopusenc decision lookahead.
14002	48000 (1 s)	48 kHz samples	Ogg muxing delay; --max-delay sets the same thing in ms.
14004	512	Bytes	Comment padding.
14006	Random	Integer	Ogg stream serial.
14010	0	1/256 dB, signed 16-bit	Output gain stored in the Opus header and applied by the player.

Complete FFmpeg libopus private options

The private options of the FFmpeg 9.0.2 libopus wrapper.

Parameter	Default	Values / units	Effect
-application	audio	audio, voip, lowdelay	audio for music and general content; voip favours speech intelligibility; lowdelay uses CELT only for minimum latency. Frames under 10 ms force lowdelay.
-frame_duration	20	2.5, 5, 10, 20, 40, 60, 80, 100, 120 ms	Frame length. Other values are rejected, although help shows a continuous 2.5–120 range. 80–120 ms need libopus 1.2 or later.
-packet_loss	0	0–100 %	Expected packet loss. Makes frames more loss-robust and sets how much FEC to send when -fec 1.
-fec	0	0/1	In-band forward error correction (SILK and hybrid modes). Needs -packet_loss above 0.
-vbr	on	off, on, constrained	Hard CBR, VBR or constrained VBR. on is the right choice for files.
-mapping_family	-1	-1, 0, 1, 255	-1: family 0 for mono/stereo; for 3–8 channels a family 1 header, but without libopus’s surround analysis. 1: the surround encoder with inter-channel masking, up to 8 channels; use it for 5.1 and 7.1. 0: mono/stereo only. 255: up to 254 discrete channels.
-dtx	0	0/1	Discontinuous transmission: send almost nothing during silence. For VoIP.
-apply_phase_inv	1	0/1	Intensity-stereo phase inversion. Set 0 if the output will be downmixed to mono. Only present when built against libopus 1.2 or later.

Generic FFmpeg options the wrapper also reads:

Parameter	Default	Values / units	Effect
-b:a	64k per stream + 32k per coupled pair (64k mono, 96k stereo)	500–256000 × channels bit/s	Total bitrate. Unlike opusenc, the default does not drop for low input sample rates.
-compression_level	10	0–10	Complexity.
-cutoff	0 (auto)	4000, 6000, 8000, 12000, 20000 Hz	Maximum coded bandwidth. Other values are ignored with a warning.
-ar	Input rate if supported	8000, 12000, 16000, 24000, 48000	Other input rates, such as 44.1 kHz, are resampled.
-ac, -ch_layout	Input layout	Channel count or layout name	Output channels.
-q:a	—	Not supported	Rejected; use -b:a with -vbr.

FFmpeg native opus: a separate interface

FFmpeg’s own Opus encoder is experimental (-strict -2), CELT-only, and takes 48 kHz mono or stereo. Its default bitrate is 48k mono or 96k stereo. The libopus options above don’t apply; it has two of its own.

Parameter	Default	Values / units	Effect
-opus_delay	360	2.5–360 ms	Maximum encoder delay. Values below 20 ms also shorten the frame size.
-apply_phase_inv	1	0/1	Intensity-stereo phase inversion; set 0 if the output will be downmixed to mono.