Variable bitrate experiment

hardware/AFSK.c

@@ -331,11 +331,32 @@ void AFSK_adc_isr(Afsk *afsk, int8_t currentSample) {
     // to "smooth out" the variations in the samples.
 
     afsk->iirX[0] = afsk->iirX[1];
+
+    #if FILTER_CUTOFF == 600
         afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 2;
+        // The above is a simplification of:
+        // afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) / 3.558147322;
+    #elif FILTER_CUTOFF == 800
+        afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) >> 2;
+        // The above is a simplification of:
+        // afsk->iirX[1] = ((int8_t)fifo_pop(&afsk->delayFifo) * currentSample) / 2.899043379;
+    #else
+        #error Unsupported bitrate!
+    #endif
 
     afsk->iirY[0] = afsk->iirY[1];
     
-    afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] >> 1); // Chebyshev filter
+    #if FILTER_CUTOFF == 600
+        afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] >> 1);
+        // The above is a simplification of a first-order 600Hz chebyshev filter:
+        // afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] * 0.4379097269);
+    #elif FILTER_CUTOFF == 800
+        afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] / 3);
+        // The above is a simplification of a first-order 800Hz chebyshev filter:
+        // afsk->iirY[1] = afsk->iirX[0] + afsk->iirX[1] + (afsk->iirY[0] * 0.3101172565);
+    #else
+        #error Unsupported bitrate!
+    #endif
 
 
     // We put the sampled bit in a delay-line:

hardware/AFSK.h

@@ -45,17 +45,33 @@ inline static uint8_t sinSample(uint16_t i) {
 #define CONFIG_AFSK_PREAMBLE_LEN 150UL
 #define CONFIG_AFSK_TRAILER_LEN 50UL
 #define SAMPLERATE 9600
-#define BITRATE    1200
+#define BITRATE    1600
-#define SAMPLESPERBIT (SAMPLERATE / BITRATE)
+
-#define BIT_STUFF_LEN 5
+#if BITRATE == 960
+    #define FILTER_CUTOFF 600
+    #define MARK_FREQ  1300
+    #define SPACE_FREQ 2100
+    #define PHASE_BITS   8                          // How much to increment phase counter each sample
+#if BITRATE == 1200
+    #define FILTER_CUTOFF 600
     #define MARK_FREQ  1200
     #define SPACE_FREQ 2200
-#define PHASE_BITS   8                              // How much to increment phase counter each sample
+    #define PHASE_BITS   8
+#elif BITRATE == 1600
+    #define FILTER_CUTOFF 800
+    #define MARK_FREQ  1600
+    #define SPACE_FREQ 2800
+    #define PHASE_BITS   6
+#else
+    #error Unsupported bitrate!
+#endif
+
+#define SAMPLESPERBIT (SAMPLERATE / BITRATE)
+#define BIT_STUFF_LEN 5
 #define PHASE_INC    1                              // Nudge by an eigth of a sample each adjustment
 #define PHASE_MAX    (SAMPLESPERBIT * PHASE_BITS)   // Resolution of our phase counter = 64
 #define PHASE_THRESHOLD  (PHASE_MAX / 2)            // Target transition point of our phase window
 
-
 typedef struct Hdlc
 {
     uint8_t demodulatedBits;