Audio Resampler Module

Sample rate conversion using libsoxr (SoX Resampler Library).

Design Goals

High Quality: Professional-grade resampling via libsoxr
Streaming: Process audio as continuous stream, not files
Channel Conversion: Support mono↔stereo conversion
io.Reader Interface: Drop-in replacement for audio sources

Supported Conversions

Sample Rate

Any integer sample rate to any other integer sample rate:

8000 Hz ↔ 16000 Hz ↔ 24000 Hz ↔ 48000 Hz
Non-standard rates supported

Channel Conversion

From	To	Method
Mono	Stereo	Duplicate
Stereo	Mono	Average (L+R)/2

Quality Levels

libsoxr supports multiple quality presets:

Level	Name	Description
0	Quick	Low quality, fast
1	Low	Better than quick
2	Medium	Balance of quality/speed
3	High	Good quality (default)
4	Very High	Best quality

Note: Current implementation uses High quality by default.

Algorithm

libsoxr uses polyphase filter banks with configurable:

Passband rolloff
Stop-band attenuation
Linear/minimum phase

The High quality preset provides:

Passband: 0-0.91 Nyquist
Stop-band attenuation: -100 dB
Linear phase

Common Resampling Scenarios

Speech API to Local Playback

flowchart LR
    A["API Output<br/>(24kHz)"] --> B[Resample]
    B --> C["Mixer Input<br/>(16kHz)"]

Local Capture to Speech API

flowchart LR
    A["Microphone<br/>(48kHz)"] --> B[Resample]
    B --> C["API Input<br/>(16kHz)"]

Multi-source Mixing

flowchart LR
    s1["Source 1<br/>(24kHz)"] --> r1[Resample]
    r1 --> m1["16kHz"]
    
    s2["Source 2<br/>(16kHz)"] --> m2["16kHz"]
    
    s3["Source 3<br/>(48kHz)"] --> r3[Resample]
    r3 --> m3["16kHz"]
    
    m1 --> mixer[Mixer]
    m2 --> mixer
    m3 --> mixer

Performance Characteristics

Approximate cycles per sample (on modern CPU):

Quick: ~10
High: ~50-100
Very High: ~200-500

For real-time 16kHz mono:

16000 samples/sec × 100 cycles ≈ 1.6M cycles/sec
Negligible CPU load on modern hardware

Memory Usage

libsoxr maintains internal buffers for filter state:

~10-50KB per resampler instance
More for higher quality settings

Examples

See parent audio/ documentation for usage examples.

audio/pcm/ - Format definitions, use with resampler
audio/codec/ - Often resample before/after encoding

Giztoy Documentation