Add selectable discriminator filter type.

[bertos.git] / bertos / net / afsk.c

diff --git a/bertos/net/afsk.c b/bertos/net/afsk.c
index 4ebf54540c383c55b529b48e8bf17e31813d239a..2472861cebba58247ccd6a822659d0cfa52979f4 100644 (file)
--- a/bertos/net/afsk.c
+++ b/bertos/net/afsk.c
@@ -241,23 +241,38 @@ DEFINE_AFSK_ADC_ISR()
 
        /*
         * Frequency discrimination is achieved by simply multiplying
-        * the sample with a delayed sample of (bits per sample) / 2.
-        * Then the signal is lowpass filtered with a first order, 600 Hz
-        * Butterworth filter.
+        * the sample with a delayed sample of (samples per bit) / 2.
+        * Then the signal is lowpass filtered with a first order,
+        * 600 Hz filter. The filter implementation is selectable
+        * through the CONFIG_AFSK_FILTER config variable.
         */
 
        iir_x[0] = iir_x[1];
-       iir_x[1] = ((int8_t)fifo_pop(&delay_fifo) * curr_sample) >> 2;
+
+       #if (CONFIG_AFSK_FILTER == AFSK_BUTTERWORTH) || (CONFIG_AFSK_FILTER == AFSK_CHEBYSHEV)
+               iir_x[1] = ((int8_t)fifo_pop(&delay_fifo) * curr_sample) >> 2;
+       #else
+               #error Filter type not found!
+       #endif
+
        iir_y[0] = iir_y[1];
 
-       /*
-        * This strange sum + shift is an optimization for iir_y[0] * 0.668.
-        * iir * 0.668 ~= (iir * 21) / 32 =
-        * = (iir * 16) / 32 + (iir * 4) / 32 + iir / 32 =
-        * = iir / 2 + iir / 8 + iir / 32 =
-        * = iir >> 1 + iir >> 3 + iir >> 5
-        */
-       iir_y[1] = iir_x[0] + iir_x[1] + (iir_y[0] >> 1) + (iir_y[0] >> 3) + (iir_y[0] >> 5);
+       #if CONFIG_AFSK_FILTER == AFSK_BUTTERWORTH
+               /*
+                * This strange sum + shift is an optimization for iir_y[0] * 0.668.
+                * iir * 0.668 ~= (iir * 21) / 32 =
+                * = (iir * 16) / 32 + (iir * 4) / 32 + iir / 32 =
+                * = iir / 2 + iir / 8 + iir / 32 =
+                * = iir >> 1 + iir >> 3 + iir >> 5
+                */
+               iir_y[1] = iir_x[0] + iir_x[1] + (iir_y[0] >> 1) + (iir_y[0] >> 3) + (iir_y[0] >> 5);
+       #elif CONFIG_AFSK_FILTER == AFSK_CHEBYSHEV
+               /*
+                * This should be (iir_y[0] * 0.438) but
+                * (iir_y[0] >> 1) is a faster approximation :-)
+                */
+               iir_y[1] = iir_x[0] + iir_x[1] + (iir_y[0] >> 1);
+       #endif
 
        /* Save this sampled bit in a delay line */
        sampled_bits <<= 1;