mfsk_core/ft4/
mod.rs

1//! # `ft4` — FT4 decoder and synthesiser
2//!
3//! FT4 shares the LDPC(174, 91) code and WSJT 77-bit message payload with FT8;
4//! only the modulation parameters (4-GFSK at 20.833 baud), frame layout (four
5//! 4-symbol Costas arrays at symbols 0 / 33 / 66 / 99) and DSP ratios differ.
6//! All heavy lifting is delegated to generic code in [`crate::core`]; this
7//! module mainly wires the trait impls and provides decode / synth entry
8//! points.
9//!
10//! ## Quick example
11//!
12//! ```no_run
13//! use mfsk_core::ft4::decode::decode_frame;
14//! use mfsk_core::msg::wsjt77::unpack77;
15//!
16//! # let audio: Vec<i16> = vec![];
17//! // `audio` is 90_000 i16 samples at 12 kHz (7.5 s slot).
18//! for r in decode_frame(&audio, 100.0, 3_000.0, 1.0, /* max_cand */ 100) {
19//!     if let Some(text) = unpack77(&r.message77) {
20//!         println!("{:7.1} Hz  dt={:+.2} s  SNR={:+.0} dB  {}",
21//!                  r.freq_hz, r.dt_sec, r.snr_db, text);
22//!     }
23//! }
24//! ```
25
26use crate::core::{FrameLayout, ModulationParams, Protocol, ProtocolId, SyncBlock, SyncMode};
27use crate::fec::Ldpc174_91;
28use crate::msg::Wsjt77Message;
29
30pub mod decode;
31pub mod encode;
32
33/// FT4 protocol marker: 4-GFSK, 103 symbols over 7.5 s slot, 20.833 Hz tone
34/// spacing, four different Costas-4 arrays, LDPC(174,91) FEC, WSJT 77-bit
35/// message payload.
36#[derive(Copy, Clone, Debug, Default)]
37pub struct Ft4;
38
39impl ModulationParams for Ft4 {
40    const NTONES: u32 = 4;
41    const BITS_PER_SYMBOL: u32 = 2;
42    const NSPS: u32 = 576; // 48 ms @ 12 kHz
43    const SYMBOL_DT: f32 = 0.048;
44    const TONE_SPACING_HZ: f32 = 20.833;
45    const GRAY_MAP: &'static [u8] = &[0, 1, 3, 2];
46    const GFSK_BT: f32 = 1.0;
47    const GFSK_HMOD: f32 = 1.0;
48    const NFFT_PER_SYMBOL_FACTOR: u32 = 4; // NFFT1 = 4 × NSPS = 2304
49    const NSTEP_PER_SYMBOL: u32 = 2; // half-symbol coarse-sync step (24 ms)
50    const NDOWN: u32 = 18; // 12 000 / 18 ≈ 666.7 Hz baseband
51    // LLR_SCALE tuning (2.0 / 2.83 / 3.5) was measured to give identical
52    // threshold curves — BP already converges within that range. Keeping
53    // the WSJT-X default.
54}
55
56impl FrameLayout for Ft4 {
57    const N_DATA: u32 = 87;
58    const N_SYNC: u32 = 16; // 4 × 4-symbol Costas
59    const N_SYMBOLS: u32 = 103; // active channel symbols (excludes 2 ramp symbols)
60    const N_RAMP: u32 = 2; // 1 each side, NN2 = 105
61    const SYNC_MODE: SyncMode = SyncMode::Block(&FT4_SYNC_BLOCKS);
62    const T_SLOT_S: f32 = 7.5;
63    const TX_START_OFFSET_S: f32 = 0.5;
64}
65
66impl Protocol for Ft4 {
67    type Fec = Ldpc174_91;
68    type Msg = Wsjt77Message;
69    const ID: ProtocolId = ProtocolId::Ft4;
70}
71
72/// FT4's four Costas arrays — each a distinct permutation of `[0,1,2,3]`.
73const FT4_COSTAS_A: [u8; 4] = [0, 1, 3, 2];
74const FT4_COSTAS_B: [u8; 4] = [1, 0, 2, 3];
75const FT4_COSTAS_C: [u8; 4] = [2, 3, 1, 0];
76const FT4_COSTAS_D: [u8; 4] = [3, 2, 0, 1];
77
78const FT4_SYNC_BLOCKS: [SyncBlock; 4] = [
79    SyncBlock {
80        start_symbol: 0,
81        pattern: &FT4_COSTAS_A,
82    },
83    SyncBlock {
84        start_symbol: 33,
85        pattern: &FT4_COSTAS_B,
86    },
87    SyncBlock {
88        start_symbol: 66,
89        pattern: &FT4_COSTAS_C,
90    },
91    SyncBlock {
92        start_symbol: 99,
93        pattern: &FT4_COSTAS_D,
94    },
95];
mfsk_core/ft4/mod.rs

mfsk_core/ft4/
mod.rs
