Codec Modes

Opus contains two independent audio codecs that it combines to cover every use case from 6 kbps narrowband phone calls to 512 kbps transparent music archival. Understanding the three modes (SILK-only, CELT-only, and hybrid) helps you pick the right encoder settings.

The Three Modes

SILK-Only

SILK is a wideband speech codec (LP-based, similar in heritage to iSAC). It operates at internal rates of 8, 12, or 16 kHz and is optimised for speech at low-to-medium bitrates.

Best for: speech, voice calls, VoIP at 6-32 kbps
Frame sizes: 10, 20, 40, or 60 ms
Bandwidth: narrowband (NB), medium-band (MB), or wideband (WB)

enc = ruopus.OpusEncoder(
    1,
    bitrate=16_000,
    application=ruopus.Application.Voip,
    signal=ruopus.Signal.Voice,
)
packet = enc.encode_silk(frame)    # force SILK-only for this frame

CELT-Only

CELT is an MDCT-based codec (similar in heritage to Vorbis). It operates at the full 48 kHz sample rate and excels at music, wideband audio, and low-latency applications.

Best for: music, game audio, low-delay voice at 32+ kbps
Frame sizes: 2.5, 5, 10, or 20 ms
Bandwidth: narrowband through fullband

enc = ruopus.OpusEncoder(
    2,
    bitrate=96_000,
    application=ruopus.Application.Audio,
    signal=ruopus.Signal.Music,
)
packet = enc.encode(frame)    # encode() always uses CELT

Hybrid

Hybrid coding uses SILK for the low band (below 8 kHz) and CELT for the high band above it. This gives SILK’s speech quality at low bitrates while preserving fullband presence.

Best for: super-wideband or fullband speech at 16-32 kbps
Frame sizes: 10 or 20 ms
Bandwidth: super-wideband (SWB) or fullband (FB)

packet = enc.encode_hybrid(frame)    # force hybrid for this frame

Automatic Mode Selection

encode_auto() is the right choice for most applications. The encoder runs a signal classifier on every frame and selects SILK, hybrid, or CELT based on:

The signal hint (Auto, Voice, Music)
The application profile
The target bitrate and forced/maximum bandwidth
Cross-frame hysteresis (to avoid rapid mode flipping)

enc = ruopus.OpusEncoder(2, bitrate=32_000)
# Auto mode: classifier picks SILK/hybrid/CELT per frame
packet = enc.encode_auto(frame)

Checking the Mode of a Decoded Packet

After encoding, inspect the TOC byte to confirm which mode was used:

pkt = ruopus.Packet(packet)
print(pkt.toc.mode)        # Mode.SilkOnly / Mode.Hybrid / Mode.CeltOnly
print(pkt.toc.bandwidth)   # Bandwidth.FullBand, etc.
print(pkt.toc.frame_size)  # FrameSize.Ms20, etc.

See Packet Inspection for the full packet introspection API.

Application Profiles

The Application enum nudges the automatic classifier:

Value	Effect
`Application.Audio`	Balanced default. Suitable for music and general audio.
`Application.Voip`	Biases toward SILK / hybrid; adds noise suppression tuned for speech.
`Application.RestrictedLowDelay`	Forces CELT-only on every frame. Lowest latency (encoder lookahead = 2.5 ms). Never uses SILK.

Signal Hints

The Signal enum biases the speech/music classifier:

Value	Effect
`Signal.Auto`	Run the built-in classifier (default).
`Signal.Voice`	Treat the source as speech; push toward SILK / hybrid modes.
`Signal.Music`	Treat the source as music; push toward CELT.

Bandwidth Control

Bandwidth controls the coded audio bandwidth:

Enum value	Audio BW	Sample rate	Notes
`Bandwidth.NarrowBand`	4 kHz	8 kHz	SILK-only
`Bandwidth.MediumBand`	6 kHz	12 kHz	SILK-only
`Bandwidth.WideBand`	8 kHz	16 kHz	SILK or hybrid
`Bandwidth.SuperWideBand`	12 kHz	24 kHz	Hybrid or CELT
`Bandwidth.FullBand`	20 kHz	48 kHz	CELT or hybrid

Set max_bandwidth to cap automatic selection, or bandwidth to pin it. Call set_auto_bandwidth() to restore automatic selection.