Codec-Core: Audio Codec Library for VoIP

A simple implementation of G.711 audio codec for VoIP applications. This library provides ITU-T compliant G.711 μ-law and A-law encoding/decoding with lookup table optimizations.

Features

• ITU-T G.711 Compliant: Passes official compliance tests
• Real Audio Tested: Validated with actual speech samples
• Good Quality: ~37 dB SNR with real speech
• Lookup Table Optimized: Fast O(1) encoding/decoding

Implementation

• Lookup Tables: Pre-computed tables for O(1) operations
• Simple APIs: Straightforward encoding/decoding functions

Usage

Quick Start

use codec_core::codecs::g711::G711Codec;
use codec_core::types::{AudioCodec, CodecConfig, CodecType, SampleRate};

// Create a G.711 μ-law codec
let config = CodecConfig::new(CodecType::G711Pcmu)
    .with_sample_rate(SampleRate::Rate8000)
    .with_channels(1);
let mut codec = G711Codec::new_pcmu(config)?;

// Encode audio samples (20ms at 8kHz = 160 samples)
let samples = vec![0i16; 160];
let encoded = codec.encode(&samples)?;

// Decode back to samples
let decoded = codec.decode(&encoded)?;

Testing & Validation

The library includes comprehensive testing including real audio validation:

# Run all codec tests including WAV roundtrip tests
cargo test

# Run only G.711 WAV roundtrip tests (downloads real speech audio)
cargo test wav_roundtrip_test -- --nocapture

The WAV roundtrip tests automatically download real speech samples and validate:

• Signal-to-Noise Ratio (SNR) measurement
• Round-trip audio quality preservation
• Proper encoding/decoding with real audio data
• Output WAV files for manual quality assessment

Error Handling

All codec operations return Result types with detailed error information:

use codec_core::codecs::g711::G711Codec;
use codec_core::types::{CodecConfig, CodecType, SampleRate};
use codec_core::error::CodecError;

// Handle configuration errors
let config = CodecConfig::new(CodecType::G711Pcmu)
    .with_sample_rate(SampleRate::Rate48000) // Invalid for G.711
    .with_channels(1);

match G711Codec::new_pcmu(config) {
    Ok(codec) => println!("Codec created successfully"),
    Err(CodecError::InvalidSampleRate { rate, supported }) => {
        println!("Invalid sample rate {}, supported: {:?}", rate, supported);
    }
    Err(e) => println!("Other error: {}", e),
}

Performance Tips

• Use appropriate frame sizes (160 samples for G.711 at 8kHz/20ms)

Direct G.711 Functions

use codec_core::codecs::g711::{alaw_compress, alaw_expand, ulaw_compress, ulaw_expand};

// Single sample processing
let sample = 1024i16;
let alaw_encoded = alaw_compress(sample);
let alaw_decoded = alaw_expand(alaw_encoded);

let ulaw_encoded = ulaw_compress(sample);
let ulaw_decoded = ulaw_expand(ulaw_encoded);

Frame-Based Processing

use codec_core::codecs::g711::{G711Codec, G711Variant};

let mut codec = G711Codec::new(G711Variant::MuLaw);

// Process 160 samples (20ms at 8kHz)
let input_frame = vec![1000i16; 160]; // Some test samples
let encoded = codec.compress(&input_frame).unwrap();

// Decode back to samples (same count for G.711)
let decoded = codec.expand(&encoded).unwrap();
assert_eq!(input_frame.len(), decoded.len());

Supported Codecs

Quality Metrics

Based on real audio testing with the included WAV roundtrip tests:

• G.711: 37+ dB SNR (excellent quality, industry standard)

Feature Flags

Core Codecs (enabled by default)

• g711: G.711 μ-law/A-law codecs