Changing the encoding quality

The motivating feature of RustyOpus: decode audio and re-encode it at a different quality, e.g. to shrink a file for storage or speech pipelines.

Presets

RustyOpus.Quality provides three presets that tune bitrate, complexity, and VBR/CBR together:

Higher presets produce larger Opus packets with better fidelity. You can override the bitrate with :target_bitrate and any individual setting per call.

Single-packet quality change

RustyOpus.change_quality/5 decodes one packet to PCM and re-encodes it at a new quality:

{:ok, smaller} = RustyOpus.change_quality(packet, 16_000, 1, :low)
{:ok, larger} = RustyOpus.change_quality(packet, 16_000, 1, :high)

# Explicit bitrate target:
{:ok, tuned} = RustyOpus.change_quality(packet, 16_000, 1, :medium, target_bitrate: 12_000)

The default decoded frame size is div(rate, 50) (20 ms); pass :frame_size to change it for non-20 ms packets.

Whole-stream re-encoding

For a stream of frames, decode each packet, then re-encode the PCM frames with the desired settings:

reencoded =
  Enum.map(packets, fn packet ->
    {:ok, pcm} = RustyOpus.decode_packet(packet, 16_000, 1, 320)
    {:ok, out} = RustyOpus.encode_pcm(pcm, 16_000, 1, quality_settings)
    out
  end)

Notes on quality

• Opus is lossy; changing quality always loses some fidelity on re-encode.
• For a given content, higher bitrate ⇒ larger packets, monotonically in practice for real speech; silence frames encode tiny at every bitrate.
• :target_bitrate below the codec minimum (roughly 6 kb/s at 16 kHz) is clamped by the codec.