melpe-rs

Pure Rust implementation of the MELPe (Mixed Excitation Linear Prediction — enhanced) vocoder, conforming to STANAG 4591 / NATO narrowband voice coding.

600 bps operation with superframe architecture. Zero heap allocation. no_std compatible for embedded/Xtensa targets.

Features

600 bps codec — 540 samples in, 6 bytes out (and back)
Superframe architecture — 3 × 22.5 ms frames packed into a single 67.5 ms superframe
Full analysis chain — pre-emphasis → LPC → LSF → pitch detection → bandpass voicing → quantization → bitstream packing
Full synthesis chain — unpacking → dequantization → mixed excitation → all-pole filter → de-emphasis
Pure core — no heap allocation in production code, all fixed-size arrays
Dual math backend — native f32 methods under std, libm under no_std

Quick start

use melpe::encoder::Encoder;
use melpe::decoder::Decoder;
use melpe::core_types::{SUPERFRAME_SAMPLES, SUPERFRAME_BYTES_600};

let mut enc = Encoder::new();
let mut dec = Decoder::new();

// Audio in: 540 f32 samples at 8 kHz
let samples = [0.0f32; SUPERFRAME_SAMPLES];

// Encode: 540 samples → 6 bytes
let mut bitstream = [0u8; SUPERFRAME_BYTES_600];
enc.encode(&samples, &mut bitstream);

// Decode: 6 bytes → 540 samples
let mut output = [0.0f32; SUPERFRAME_SAMPLES];
dec.decode(&bitstream, &mut output);

Feature flags

Flag	Default	Effect
`std`	✅	Uses native `f32` math (hardware SIMD on x86_64)
`embedded`	—	`#![no_std]`, routes math through `libm` for bare-metal targets

# Desktop / server (default)
cargo add melpe-rs

# Embedded / Xtensa / no_std
cargo add melpe-rs --no-default-features --features embedded

Architecture

samples ──→ Encoder ──→ 6 bytes ──→ Decoder ──→ samples
             │                        │
             ├─ lpc.rs                ├─ bitstream.rs (unpack)
             ├─ pitch.rs             ├─ quantize.rs (dequantize)
             ├─ voicing.rs           ├─ voicing.rs (mixed excitation)
             ├─ quantize.rs          └─ synthesis.rs (all-pole filter)
             └─ bitstream.rs (pack)

Modules

Module	Purpose
`core_types`	Constants, frame geometry, superframe structures
`lpc`	Autocorrelation, Levinson-Durbin, LPC↔LSF conversion, Hamming window
`pitch`	Normalized autocorrelation pitch estimator with parabolic interpolation
`voicing`	5-band bandpass filter bank, per-band voicing strength, mixed excitation
`quantize`	Scalar LSF/pitch/gain quantization, voicing collapse, LSF interpolation
`bitstream`	Bit-level packing/unpacking to 6-byte superframe format
`synthesis`	All-pole filter, de-emphasis, frame-level synthesis processor
`encoder`	Top-level encode: 540 samples → 6 bytes
`decoder`	Top-level decode: 6 bytes → 540 samples
`math`	`no_std` shim — `std` ↔ `libm` dispatch for all floating-point intrinsics

Codec parameters

Parameter	Value
Sample rate	8000 Hz
Frame size	180 samples (22.5 ms)
Superframe	3 frames, 540 samples (67.5 ms)
Bit rate	600 bps
Bits per superframe	41
Bytes per superframe	6 (48 bits, 7 spare)
LPC order	10
LSF count	10
Voicing bands	5 (0–500, 500–1000, 1000–2000, 2000–3000, 3000–4000 Hz)
Pitch range	20–160 samples (50–400 Hz)

Testing

# Full test suite (138 tests: 125 unit + 13 integration)
cargo test

# no_std subset (100 tests — core-only, no heap)
cargo test --lib --no-default-features --features embedded

License

MIT

melpe-rs 0.1.2