[feat](trx-rs): add FT2 decoder protocol support

Implement a distinct FT2 protocol path in the decoder stack and align\nits timing with the confirmed FT2 framing used by Decodium.\n\nCo-authored-by: OpenAI Codex <codex@openai.com>

Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

external/ft8_lib/ft8/constants.h

@@ -14,6 +14,9 @@ extern "C"
 #define FT4_SYMBOL_PERIOD (0.048f) ///< FT4 symbol duration, defines tone deviation in Hz and symbol rate
 #define FT4_SLOT_TIME     (7.5f)   ///< FT4 slot period
 
+#define FT2_SYMBOL_PERIOD (0.024f) ///< FT2 symbol duration (288 samples @ 12 kHz)
+#define FT2_SLOT_TIME     (3.75f)  ///< FT2 slot period
+
 // Define FT8 symbol counts
 // FT8 message structure:
 //     S D1 S D2 S
@@ -38,6 +41,14 @@ extern "C"
 #define FT4_NUM_SYNC    (4)   ///< Number of sync groups
 #define FT4_SYNC_OFFSET (33)  ///< Offset between sync groups
 
+// FT2 reuses the FT4 channel structure with a shorter slot and symbol period.
+#define FT2_ND          FT4_ND
+#define FT2_NR          FT4_NR
+#define FT2_NN          FT4_NN
+#define FT2_LENGTH_SYNC FT4_LENGTH_SYNC
+#define FT2_NUM_SYNC    FT4_NUM_SYNC
+#define FT2_SYNC_OFFSET FT4_SYNC_OFFSET
+
 // Define LDPC parameters
 #define FTX_LDPC_N       (174)                  ///< Number of bits in the encoded message (payload with LDPC checksum bits)
 #define FTX_LDPC_K       (91)                   ///< Number of payload bits (including CRC)
@@ -52,9 +63,29 @@ extern "C"
 typedef enum
 {
     FTX_PROTOCOL_FT4,
-    FTX_PROTOCOL_FT8
+    FTX_PROTOCOL_FT8,
+    FTX_PROTOCOL_FT2
 } ftx_protocol_t;
 
+static inline float ftx_protocol_symbol_period(ftx_protocol_t protocol)
+{
+    return (protocol == FTX_PROTOCOL_FT8)
+        ? FT8_SYMBOL_PERIOD
+        : ((protocol == FTX_PROTOCOL_FT2) ? FT2_SYMBOL_PERIOD : FT4_SYMBOL_PERIOD);
+}
+
+static inline float ftx_protocol_slot_time(ftx_protocol_t protocol)
+{
+    return (protocol == FTX_PROTOCOL_FT8)
+        ? FT8_SLOT_TIME
+        : ((protocol == FTX_PROTOCOL_FT2) ? FT2_SLOT_TIME : FT4_SLOT_TIME);
+}
+
+static inline int ftx_protocol_uses_ft4_layout(ftx_protocol_t protocol)
+{
+    return (protocol == FTX_PROTOCOL_FT4) || (protocol == FTX_PROTOCOL_FT2);
+}
+
 /// Costas 7x7 tone pattern for synchronization
 extern const uint8_t kFT8_Costas_pattern[7];
 extern const uint8_t kFT4_Costas_pattern[4][4];

external/ft8_lib/ft8/decode.c

@@ -189,8 +189,9 @@ static int ft4_sync_score(const ftx_waterfall_t* wf, const ftx_candidate_t* cand
 
 int ftx_find_candidates(const ftx_waterfall_t* wf, int num_candidates, ftx_candidate_t heap[], int min_score)
 {
-    int (*sync_fun)(const ftx_waterfall_t*, const ftx_candidate_t*) = (wf->protocol == FTX_PROTOCOL_FT4) ? ft4_sync_score : ft8_sync_score;
-    int num_tones = (wf->protocol == FTX_PROTOCOL_FT4) ? 4 : 8;
+    int (*sync_fun)(const ftx_waterfall_t*, const ftx_candidate_t*) =
+        ftx_protocol_uses_ft4_layout(wf->protocol) ? ft4_sync_score : ft8_sync_score;
+    int num_tones = ftx_protocol_uses_ft4_layout(wf->protocol) ? 4 : 8;
 
     int heap_size = 0;
     ftx_candidate_t candidate;
@@ -327,7 +328,7 @@ static void ftx_normalize_logl(float* log174)
 bool ftx_decode_candidate(const ftx_waterfall_t* wf, const ftx_candidate_t* cand, int max_iterations, ftx_message_t* message, ftx_decode_status_t* status)
 {
     float log174[FTX_LDPC_N]; // message bits encoded as likelihood
-    if (wf->protocol == FTX_PROTOCOL_FT4)
+    if (ftx_protocol_uses_ft4_layout(wf->protocol))
     {
         ft4_extract_likelihood(wf, cand, log174);
     }
@@ -366,7 +367,7 @@ bool ftx_decode_candidate(const ftx_waterfall_t* wf, const ftx_candidate_t* cand
     // Reuse CRC value as a hash for the message (TODO: 14 bits only, should perhaps use full 16 or 32 bits?)
     message->hash = status->crc_calculated;
 
-    if (wf->protocol == FTX_PROTOCOL_FT4)
+    if (ftx_protocol_uses_ft4_layout(wf->protocol))
     {
         // '[..] for FT4 only, in order to avoid transmitting a long string of zeros when sending CQ messages,
         // the assembled 77-bit message is bitwise exclusive-OR’ed with [a] pseudorandom sequence before computing the CRC and FEC parity bits'
@@ -589,4 +590,4 @@ static void pack_bits(const uint8_t bit_array[], int num_bits, uint8_t packed[])
             ++byte_idx;
         }
     }
-}
+}

external/ft8_lib/ft8/encode.c

@@ -193,3 +193,8 @@ void ft4_encode(const uint8_t* payload, uint8_t* tones)
         }
     }
 }
+
+void ft2_encode(const uint8_t* payload, uint8_t* tones)
+{
+    ft4_encode(payload, tones);
+}

external/ft8_lib/ft8/encode.h

@@ -34,6 +34,12 @@ void ft8_encode(const uint8_t* payload, uint8_t* tones);
 /// @param[out] tones  - array of FT4_NN (105) bytes to store the generated tones (encoded as 0..3)
 void ft4_encode(const uint8_t* payload, uint8_t* tones);
 
+/// Generate FT2 tone sequence from payload data.
+/// FT2 uses the FT4 framing with a doubled symbol rate.
+/// @param[in] payload - 10 byte array consisting of 77 bit payload
+/// @param[out] tones  - array of FT2_NN (105) bytes to store the generated tones (encoded as 0..3)
+void ft2_encode(const uint8_t* payload, uint8_t* tones);
+
 #ifdef __cplusplus
 }
 #endif