From e6ece89049260f9be8d60c1ed7e397097e602183 Mon Sep 17 00:00:00 2001
From: George Norton <george.norton@mediakind.com>
Date: Fri, 31 Mar 2023 13:15:43 +0100
Subject: [PATCH] Fix an overflow in the fix16_t usage which can lead to audio
 distortion.

---
 firmware/code/fix16.c | 32 ++++++++++++++++++--------------
 firmware/code/run.c   |  2 +-
 2 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/firmware/code/fix16.c b/firmware/code/fix16.c
index d10fc76..b6eea54 100644
--- a/firmware/code/fix16.c
+++ b/firmware/code/fix16.c
@@ -22,7 +22,7 @@
 #include <stdio.h>
 #include <inttypes.h>
 #include <math.h>
-
+#include <limits.h>
 #include "fix16.h"
 
 fix16_t fix16_from_int(int16_t a) {
@@ -30,9 +30,23 @@ fix16_t fix16_from_int(int16_t a) {
 }
 
 int16_t fix16_to_int(fix16_t a) {
-    if (a >= 0)
-        return (a + (fix16_one >> 1)) / fix16_one;
-    return (a - (fix16_one >> 1)) / fix16_one;
+    // Handle rounding up front, adding one can cause an overflow/underflow
+    a+=(fix16_one >> 1);
+
+    // Saturate the value if an overflow has occurred
+    uint32_t upper = (a >> 30);
+    if (a < 0) {
+        if (~upper)
+        {
+            return SHRT_MIN;
+        }
+    } else {
+        if (upper)
+        {
+            return SHRT_MAX;
+        }
+    }
+    return (a >> 15);
 }
 
 fix16_t fix16_from_dbl(double a) {
@@ -49,16 +63,6 @@ double fix16_to_dbl(fix16_t a) {
 fix16_t fix16_mul(fix16_t inArg0, fix16_t inArg1) {
     int64_t product = (int64_t)inArg0 * inArg1;
 
-    uint32_t upper = (product >> 47);
-    if (product < 0) {
-        if (~upper)
-            return fix16_overflow;
-        product--;
-    } else {
-        if (upper)
-            return fix16_overflow;
-    }
-
     fix16_t result = product >> 15;
     result += (product & 0x4000) >> 14;
 
diff --git a/firmware/code/run.c b/firmware/code/run.c
index 929a587..9ad3cb3 100644
--- a/firmware/code/run.c
+++ b/firmware/code/run.c
@@ -91,7 +91,7 @@ static void _as_audio_packet(struct usb_endpoint *ep) {
     int samples = usb_buffer->data_len / 2;
 
     for (int i = 0; i < samples; i++)
-        out[i] = in[i] / 4; // fixes digital distortion bug
+        out[i] = in[i];
 
     multicore_fifo_push_blocking(CORE0_READY);
     multicore_fifo_push_blocking(samples);