Lossy or lossless: which audio format should I keep my library in?

For long-term archiving choose FLAC (lossless, ~50% the size of WAV). For everyday listening on phones and streaming use MP3 320 kbps or Opus 192 kbps (transparent quality, no audible difference at normal volumes). Avoid converting lossy → lossy → lossy: every step compounds artifacts.

Lossy or lossless: which audio format should I keep my library in?

For long-term archiving choose FLAC (lossless, ~50% the size of WAV). For everyday listening on phones and streaming use MP3 320 kbps or Opus 192 kbps (transparent quality, no audible difference at normal volumes). Avoid converting lossy → lossy → lossy: every step compounds artifacts.

Guide

Opus Audio Format: The Complete Technical Guide

PC By Pablo Cirre • Updated May 7, 2026

Opus Audio Format: The Complete Technical Guide

Opus is an open, royalty-free audio codec standardized by the IETF in RFC 6716 (2012) and developed by Xiph.Org and Mozilla. It uniquely combines two completely different coding technologies — SILK (from Skype, optimized for speech) and CELT (from Xiph, optimized for music and general audio) — into a single hybrid codec that outperforms every other codec across the full bitrate range from 6 kbps to 510 kbps. Opus is mandatory in WebRTC, the standard for modern VoIP, and is supported in all major browsers without a plugin.

Design Goals and Standardization

Opus was designed to solve a problem no previous codec addressed: providing excellent quality for both speech and music across the complete range of bitrates needed for internet communications. Prior to Opus:

Narrowband speech codecs (G.711, G.729) were excellent for voice but useless for music
Wideband speech codecs (G.722, AMR-WB) improved voice quality but still couldn't handle music
Music codecs (MP3, AAC, Vorbis) required 64+ kbps for acceptable quality and added latency

Opus merges SILK (used in Skype since 2009) and CELT (a low-latency music codec) into an adaptive system that automatically selects the right technology for each 20 ms audio frame.

SILK and CELT: The Two Engines

SILK — Speech Coding Engine

SILK (Skype Internet Low-bitrate Codec) is an LPAS (Linear Prediction with Adaptive Spectral) codec:

LPC Analysis: Computes a Linear Predictive Coding model of the speech signal, separating the vocal tract filter from the excitation signal
Long-Term Prediction (LTP): Models the pitch period. Speech is quasi-periodic; predicting from one pitch period back dramatically reduces residual energy.
NLSF (Normalized Line Spectral Frequencies): The LPC filter is parameterized as NLSFs for quantization efficiency
Noise shaping: Perceptually shapes the quantization noise
Entropy coding: Arithmetic coding on the residual

SILK operates at 8 kHz (NB), 12 kHz (MB), or 16 kHz (WB) internal sample rates, handling 6–40 kbps.

CELT — Music Coding Engine

CELT (Constrained Energy Lapped Transform) is an MDCT-based codec:

MDCT: 120-sample or 240-sample blocks (2.5 ms or 5 ms) — much shorter than AAC's 1024-sample blocks, giving lower latency
Band energy quantization: Energy for each of 21 frequency bands (covering 0–20 kHz) is quantized first
Pyramid Vector Quantization (PVQ): Spectral coefficients within each band are quantized using a spherical code on a unit sphere. This separates shape from energy, improving efficiency.
Conditional entropy coding: Residuals after PVQ are coded with Laplacian models
Range coder: Arithmetic-like entropy coding for the entire frame

CELT operates at 8, 12, 16, 24, or 48 kHz, handling 32–510 kbps.

Hybrid Mode

In the bitrate range of 10–32 kbps for voice content, Opus uses a hybrid: SILK handles the lower-frequency speech components (0–8 kHz) while CELT covers the upper band (8–20 kHz). This allows wideband speech at very low bitrates where neither alone is adequate.

Opus Frame Structure

An Opus packet consists of:

Table of Contents (TOC) byte (1 byte):

Configuration bits [6:3]: selects mode (SILK/Hybrid/CELT), bandwidth, frame size
Stereo bit [2]: mono (0) or stereo (1)
Frame count code [1:0]: how many frames in the packet (1, 2, or code 3 for variable)

Frame payloads: One or more encoded frames. Multiple frames per packet (up to 120 ms total) reduce network overhead at the cost of latency.

Padding (optional): OPUS_SET_PADDING can add unused bytes to reach a target packet size for traffic analysis resistance.

Bandwidth Modes

Opus automatically selects bandwidth based on bitrate and content, or can be forced:

Mode	Bandwidth	Sample Rate	Bitrate Range
NB (Narrowband)	0–4 kHz	8 kHz	6–20 kbps
MB (Medium Band)	0–6 kHz	12 kHz	7–25 kbps
WB (Wideband)	0–8 kHz	16 kHz	8–40 kbps
SWB (Super-Wideband)	0–12 kHz	24 kHz	12–64 kbps
FB (Fullband)	0–20 kHz	48 kHz	16–510 kbps

Bitrate Reference

Bitrate	Mode	Quality	Use Case
6 kbps	SILK NB	Telephony minimum	Extremely constrained links
8–12 kbps	SILK NB/MB	Intelligible voice	Low-bandwidth VoIP
16–24 kbps	SILK WB	Good voice	WebRTC voice calls
32–48 kbps	Hybrid	Excellent voice + acceptable music	Conference calls
64 kbps	CELT FB	Good music	Podcast streaming
96 kbps	CELT FB	High-quality music	Web audio streaming
128 kbps	CELT FB	Transparent for most	Discord, Spotify (some tiers)
192–256 kbps	CELT FB	Near-transparent	High-fidelity streaming
320+ kbps	CELT FB	Transparent	Near-lossless applications

Container: Ogg Opus

Opus is stored in Ogg container as Ogg Opus (RFC 7845):

OpusHead logical stream header (first page): version, channel count, pre-skip, input sample rate, output gain
OpusTags (second page): same key=value format as Vorbis Comments (TITLE, ARTIST, ALBUM, etc.)
Audio pages: Ogg pages with Opus packets, granule positions in 48 kHz samples

The file extension is .opus (not .ogg). MIME type is audio/ogg; codecs=opus for Ogg-wrapped Opus.

Opus can also be stored in Matroska (.mka, .webm) and MP4 containers.

Latency

Opus achieves algorithmic latency of 22.5 ms (with CELT at 2.5 ms frame size + look-ahead). This is dramatically lower than AAC-LC (~60 ms) and HE-AAC (~120 ms). The practical WebRTC end-to-end latency (including network buffers) is typically 100–200 ms, compared to 200–400 ms for MP3 streaming.

Frame size options:

2.5 ms: Minimum latency (CELT only)
5, 10, 20 ms: Typical VoIP/WebRTC
40, 60 ms: Maximum efficiency per packet (highest compression ratio)
120 ms: Maximum allowed frame size

Encoding Commands

# Encode to Opus at 128 kbps (ffmpeg)
ffmpeg -i input.wav -c:a libopus -b:a 128k output.opus

# Encode with opusenc (reference encoder)
opusenc --bitrate 128 input.wav output.opus

# VBR mode (default, recommended)
opusenc --vbr --bitrate 128 input.wav output.opus

# CVBR (Constrained VBR) for streaming
ffmpeg -i input.wav -c:a libopus -b:a 96k -vbr constrained output.opus

# CBR for strict network requirements
opusenc --cbr --bitrate 64 input.wav output.opus

# Low-latency encoding (10 ms frames)
ffmpeg -i input.wav -c:a libopus -b:a 32k -frame_duration 10 output.opus

# Force SILK mode (voice only, low bitrate)
opusenc --speech --bitrate 16 input.wav output.opus

# Opus in WebM container (for web video)
ffmpeg -i input.mp4 -c:v libvpx-vp9 -c:a libopus -b:a 128k output.webm

# Batch FLAC to Opus
for f in *.flac; do opusenc --bitrate 192 "$f" "${f%.flac}.opus"; done

Browser and Platform Support

Platform	Support	Notes
Chrome	✓ Full	Since Chrome 25 (2013)
Firefox	✓ Full	Since Firefox 15 (2012)
Edge	✓ Full	Since Edge 14 (2016)
Safari	✓ Since 15.4	macOS 12.3, iOS 15.4 (2022)
Android	✓ Full	MediaCodec since Android 5.0
iOS	✓ Since iOS 15.4	AVFoundation support
Windows 10/11	✓	Media Foundation since 2021

Opus vs Competitors

Codec	Quality at 64 kbps	Quality at 128 kbps	Latency	Royalties
Opus	Excellent	Transparent	22.5 ms	Free
AAC-LC	Good	High	~60 ms	Patent-encumbered
HE-AAC v1	Excellent	Good	~120 ms	Patent-encumbered
MP3	Fair	Good	~26 ms	Free (2017)
Vorbis	Good	High	~60 ms	Free

At 128 kbps and below, Opus is the best-quality open codec available. It is the recommended format for any web audio application and for archival where file size matters more than universal legacy device compatibility.

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Frequently Asked Questions

Opus achieves lower latency (22.5 ms vs 60 ms for AAC), better quality at low bitrates (64 kbps Opus beats 128 kbps MP3 in most blind tests), is completely royalty-free, and handles both speech and music efficiently. It is mandatory in WebRTC, making it the backbone of all browser-based video conferencing. The main historical limitation was Safari support, which was added in Safari 15.4 (2022).

Opus achieves baixaer latency (22.5 ms vs 60 ms para AAC), better quality at baixa bitrates (64 kbps Opus beats 128 kbps MP3 in most blind tests), is completely royalty-free, e handles both speech e music efficiently. It is mandatory in WebRTC, making it the backbone of all browser-based video conferencing. The main historical limitation was Safari support, which was added in Safari 15.4 (2022).

Opus achieves niedriger latency (22.5 ms vs 60 ms für AAC), better quality at niedrig Bitrates (64 kbps Opus beats 128 kbps MP3 in most blind tests), is completely royalty-free, und handles both speech und music efficiently. It is mandatory in WebRTC, making it the backbone von all browser-based video conferencing. The main historical limitation was Safari support, which was added in Safari 15.4 (2022).

Opus achieves bajaer latency (22.5 ms vs 60 ms para AAC), better quality at baja bitrates (64 kbps Opus beats 128 kbps MP3 in most blind tests), is completely royalty-free, y handles both speech y music efficiently. It is mandatory in WebRTC, making it the backbone de all browser-based video conferencing. The main historical limitation was Safari support, which was added in Safari 15.4 (2022).

For long-term archiving choose FLAC (lossless, ~50% the size of WAV). For everyday listening on phones and streaming use MP3 320 kbps or Opus 192 kbps (transparent quality, no audible difference at normal volumes). Avoid converting lossy → lossy → lossy: every step compounds artifacts.

Opus contains two independent coding engines: SILK (from Skype) for speech using Linear Predictive Coding with pitch tracking, and CELT for music using MDCT with Pyramid Vector Quantization. At low bitrates with voice content, Opus uses SILK. At higher bitrates or for music, it uses CELT. In the 10–32 kbps range for voice, it uses both simultaneously (hybrid mode), with SILK covering the low frequencies and CELT covering the high frequencies.

Opus contém two independent coding engines: SILK (from Skype) para speech usando Linear Predictive Coding com pitch tracking, e CELT para music usando MDCT com Pyramid Vector Quantization. At baixa bitrates com voice content, Opus uses SILK. At altaer bitrates ou para music, it uses CELT. no 10–32 kbps range para voice, it uses both simultaneously (hybrid mode), com SILK covering the baixa frequencies e CELT covering the alta frequencies.

Opus contains two independent coding engines: SILK (from Skype) für speech using Linear Predictive Coding mit pitch tracking, und CELT für music using MDCT mit Pyramid Vector Quantization. At niedrig Bitrates mit voice content, Opus uses SILK. At hocher Bitrates oder für music, it uses CELT. im 10–32 kbps range für voice, it uses both simultaneously (hybrid mode), mit SILK covering the niedrig frequencies und CELT covering the hoch frequencies.

Opus contains two independent coding engines: SILK (from Skype) para speech using Linear Predictive Coding con pitch tracking, y CELT para music using MDCT con Pyramid Vector Quantization. At baja bitrates con voice content, Opus uses SILK. At altaer bitrates o para music, it uses CELT. en el 10–32 kbps range para voice, it uses both simultaneously (hybrid mode), con SILK covering the baja frequencies y CELT covering the alta frequencies.

Spoken-word podcasts: 44.1 kHz mono, 64–96 kbps Opus or 96 kbps MP3. Music podcasts: 44.1 kHz stereo, 128–192 kbps. Going above wastes bandwidth — speech has limited frequency content above 7 kHz, and listeners on data plans appreciate the smaller file. Apple Podcasts and Spotify both accept up to 48 kHz / 320 kbps.

96 kbps Opus provides high-quality music streaming comparable to 192 kbps AAC or 256 kbps MP3. For transparent quality that satisfies critical listeners, 128 kbps Opus is the standard recommendation. Discord uses 64–96 kbps for voice channels. For archival purposes, 192–256 kbps provides near-lossless quality. VBR mode (default) is recommended over CBR as it dynamically allocates bits where they are needed most.

96 kbps Opus fornece alta-quality music streaming comparable to 192 kbps AAC ou 256 kbps MP3. para transparente quality that satisfies critical listeners, 128 kbps Opus is the padrão recommendation. Discord uses 64–96 kbps para voice channels. para archival purposes, 192–256 kbps fornece near-lossless quality. VBR mode (default) is recommended over CBR as it dynamically allocates bits where they are needed most.

96 kbps Opus bietet hoch-quality music streaming comparable to 192 kbps AAC oder 256 kbps MP3. für transparent quality that satisfies critical listeners, 128 kbps Opus is the Standard recommendation. Discord uses 64–96 kbps für voice channels. für archival purposes, 192–256 kbps bietet near-lossless quality. VBR mode (default) is recommended over CBR as it dynamically allocates bits where they are needed most.

96 kbps Opus proporciona alta-quality music streaming comparable to 192 kbps AAC o 256 kbps MP3. para transparente quality that satisfies critical listeners, 128 kbps Opus is the estándar recommendation. Discord uses 64–96 kbps para voice channels. para archival purposes, 192–256 kbps proporciona near-lossless quality. VBR mode (default) is recommended over CBR as it dynamically allocates bits where they are needed most.

Lossy → lossy compounds quantization noise. Each encode discards the same kind of perceptual information again, multiplying artifacts. Always re-encode from a lossless master if you have one (WAV, FLAC, or the original recording). If only an MP3 is available, keep the bitrate at or above the source — never go up to "improve quality".

Ogg Opus (.opus) is the standard container for Opus audio files, with full metadata support via OpusTags (same format as Vorbis Comments). For web video, Opus in WebM container (.webm) is standard alongside VP8/VP9/AV1 video. For compatibility with software that expects MP4 containers, Opus in MP4 works in modern players. Avoid raw Opus without a container as metadata cannot be stored. Use the .opus file extension, not .ogg, for Ogg-wrapped Opus files.

Ogg Opus (.opus) is the padrão container para Opus audio files, com full metadata support via OpusTags (same formato as Vorbis Comments). para web video, Opus in WebM container (.webm) is padrão alongside VP8/VP9/AV1 video. para compatibilidade com software that expects MP4 containers, Opus in MP4 funciona in moderno players. Avoid raw Opus sem a container as metadata cannot be stored. usar the .opus extensão de arquivo, not .ogg, para Ogg-wrapped Opus files.

Ogg Opus (.opus) is the Standard Container für Opus audio files, mit full metadata support via OpusTags (same Format as Vorbis Comments). für web video, Opus in WebM Container (.webm) is Standard alongside VP8/VP9/AV1 video. für Kompatibilität mit Software that expects MP4 Containers, Opus in MP4 works in modern players. Avoid raw Opus ohne a Container as metadata cannot be stored. verwenden the .opus Datei extension, not .ogg, für Ogg-wrapped Opus files.

Ogg Opus (.opus) is the estándar contenedor para Opus audio files, con full metadata support via OpusTags (same formato as Vorbis Comments). para web video, Opus in WebM contenedor (.webm) is estándar alongside VP8/VP9/AV1 video. para compatibilidad con software that expects MP4 contenedors, Opus in MP4 works in moderno players. Avoid raw Opus sin a contenedor as metadata cannot be stored. usar the .opus archivo extension, not .ogg, para Ogg-wrapped Opus files.

Most modern tools (FFmpeg with <code>-map_metadata 0</code>, foobar2000, dBpoweramp) preserve ID3 tags and embedded cover art. Some quick-and-dirty converters strip them silently. KaijuConverter preserves tags in its <a href="/convert/flac-to-mp3">audio conversions</a>; if metadata is critical to you, verify the output with <code>ffprobe</code> before deleting the original.