From cf95f18880d2bf68eccacd7a3682044e988db620 Mon Sep 17 00:00:00 2001
From: George Norton <george_norton_uk@hotmail.com>
Date: Thu, 22 Jun 2023 20:46:58 +0100
Subject: [PATCH] Increase the max fix16 size to -32.0..32.0 as it seems it is
 possible to create coefficients which are >16.

---
 firmware/code/fix16.c |  9 +++++----
 firmware/code/fix16.h | 10 +++++-----
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/firmware/code/fix16.c b/firmware/code/fix16.c
index 53d0b98..50f05d9 100644
--- a/firmware/code/fix16.c
+++ b/firmware/code/fix16.c
@@ -61,15 +61,15 @@ fix16_t fix16_mul(fix16_t inArg0, fix16_t inArg1) {
 }
 #else
 fix16_t fix16_from_s16sample(int16_t a) {
-    return a * fix16_one_normalized;
+    return a * fix16_lsb;
 }
 
 int16_t fix16_to_s16sample(fix16_t a) {
     // Handle rounding up front, adding one can cause an overflow/underflow
     if (a < 0) {
-        a -= (fix16_one_normalized >> 1);
+        a -= (fix16_lsb >> 1);
     } else {
-        a += (fix16_one_normalized >> 1);
+        a += (fix16_lsb >> 1);
     }
 
     // Saturate the value if an overflow has occurred
@@ -100,9 +100,10 @@ double fix16_to_dbl(fix16_t a) {
 
 // We work in 64bits then shift the result to get
 // the bit representing 1 back into the correct position.
+// i.e. 1*1 == 1, so 20000000^2 >> 25 = 20000000
 fix16_t fix16_mul(fix16_t inArg0, fix16_t inArg1) {
     const int64_t product = (int64_t)inArg0 * inArg1;
-    fix16_t result = product >> 29;
+    fix16_t result = product >> 25;
     // Handle rounding where we are choppping off low order bits
     // Disabled for now, too much load. We get crackling when adjusting
     // the volume.
diff --git a/firmware/code/fix16.h b/firmware/code/fix16.h
index d1528af..42b0c03 100644
--- a/firmware/code/fix16.h
+++ b/firmware/code/fix16.h
@@ -30,18 +30,18 @@
 #ifdef USE_DOUBLE
 typedef double fix16_t;
 static const fix16_t fix16_zero = 0;
+static const fix16_t fix16_one = 1;
 #else
-// We normalize all values into the range -1..1 with 1 extra bit for overflows
+// We normalize all values into the range -32..32 with 1 extra bit for overflows
 // and one bit for the sign. We allow fixed point values to overflow, but they
 // are clipped at the point they are written back to a s16sample.
 //
 // The reason for normalizing the samples is because the filter coefficients are
-// small (usually in the range -1..1), by normalizing everything the coefficients
+// small (usually well within in the range -32..32), by normalizing everything the coefficients
 // get lots of additional bits of precision.
 typedef int32_t fix16_t;
-static const fix16_t fix16_overflow = 0x80000000;
-static const fix16_t fix16_one_normalized = 0x00008000;
-static const fix16_t fix16_one = 0x20000000;
+static const fix16_t fix16_lsb = 0x8000;
+static const fix16_t fix16_one = 0x002000000;
 static const fix16_t fix16_zero = 0x00000000;
 #endif