Add notch filter to BlockBiquad

Author: jepler

shared-bindings/synthio/BlockBiquad.c

@@ -24,11 +24,13 @@
 MAKE_ENUM_VALUE(synthio_filter_mode_type, mode, LOW_PASS, SYNTHIO_LOW_PASS);
 MAKE_ENUM_VALUE(synthio_filter_mode_type, mode, HIGH_PASS, SYNTHIO_HIGH_PASS);
 MAKE_ENUM_VALUE(synthio_filter_mode_type, mode, BAND_PASS, SYNTHIO_BAND_PASS);
+MAKE_ENUM_VALUE(synthio_filter_mode_type, mode, NOTCH, SYNTHIO_NOTCH);
 
 MAKE_ENUM_MAP(synthio_filter_mode) {
     MAKE_ENUM_MAP_ENTRY(mode, LOW_PASS),
     MAKE_ENUM_MAP_ENTRY(mode, HIGH_PASS),
     MAKE_ENUM_MAP_ENTRY(mode, BAND_PASS),
+    MAKE_ENUM_MAP_ENTRY(mode, NOTCH),
 };
 
 static MP_DEFINE_CONST_DICT(synthio_filter_mode_locals_dict, synthio_filter_mode_locals_table);

shared-bindings/synthio/BlockBiquad.h

@@ -13,7 +13,7 @@ extern const mp_obj_type_t synthio_filter_mode_type;
 typedef struct synthio_block_biquad synthio_block_biquad_t;
 
 typedef enum {
-    SYNTHIO_LOW_PASS, SYNTHIO_HIGH_PASS, SYNTHIO_BAND_PASS
+    SYNTHIO_LOW_PASS, SYNTHIO_HIGH_PASS, SYNTHIO_BAND_PASS, SYNTHIO_NOTCH
 } synthio_filter_mode;
 
 

shared-module/synthio/BlockBiquad.c

@@ -95,6 +95,12 @@ void common_hal_synthio_block_biquad_tick(mp_obj_t self_in, biquad_filter_state
             b0 = alpha;
             b1 = 0;
             b2 = -b0;
+            break;
+
+        case SYNTHIO_NOTCH:
+            b0 = 1;
+            b1 = -2 * sc.c;
+            b2 = 1;
     }
 
     mp_float_t recip_a0 = 1 / a0;

tests/circuitpython-manual/synthio/note/blockfilter.py

@@ -39,6 +39,7 @@ def synthesize(synth):
         synthio.BlockBiquad(synthio.FilterMode.LOW_PASS, freq_sweep),
         synthio.BlockBiquad(synthio.FilterMode.HIGH_PASS, freq_sweep),
         synthio.BlockBiquad(synthio.FilterMode.BAND_PASS, freq_sweep, Q=8),
+        synthio.BlockBiquad(synthio.FilterMode.NOTCH, freq_sweep, Q=8),
     ):
         n = synthio.Note(
             frequency=synthio.midi_to_hz(72),