Add a more correct absolute difference function to dsp-util.

Differences between signed samples cover the entire unsigned 32-bit
range. "abs" will think any difference exceeding INT32_MAX is negative
which is not corrent. Test which argument is greater and subtract the
lesser from it, outputting unsigned difference.

Change-Id: I73a8e5e418d49ff73d1a7c98eeb4731946dcfe84

2 files changed, 13 insertions, 7 deletions

diff --git a/firmware/export/dsp-util.h b/firmware/export/dsp-util.h
index 76962b527a..b86b2b776e 100644
--- a/firmware/export/dsp-util.h
+++ b/firmware/export/dsp-util.h
@@ -18,8 +18,8 @@
  * KIND, either express or implied.
  *
  ****************************************************************************/
-#ifndef DSP_HELPER_H
-#define DSP_HELPER_H
+#ifndef DSP_UTIL_H
+#define DSP_UTIL_H
 
 /** Clip sample to signed 16 bit range **/
 
@@ -48,4 +48,11 @@ static FORCE_INLINE int32_t clip_sample_16(int32_t sample)
 
 #undef CLIP_SAMPLE_16_DEFINED
 
-#endif /* DSP_HELPER_H */
+/* Absolute difference of signed 32-bit numbers which must be dealt with
+ * in the unsigned 32-bit range */
+static FORCE_INLINE uint32_t ad_s32(int32_t a, int32_t b)
+{
+    return (a >= b) ? (a - b) : (b - a);
+}
+
+#endif /* DSP_UTIL_H */
diff --git a/lib/rbcodec/dsp/tdspeed.c b/lib/rbcodec/dsp/tdspeed.c
index 731be12621..c2f4a3f704 100644
--- a/lib/rbcodec/dsp/tdspeed.c
+++ b/lib/rbcodec/dsp/tdspeed.c
@@ -29,6 +29,7 @@
 #include "system.h"
 #include "tdspeed.h"
 #include "settings.h"
+#include "dsp-util.h"
 
 #define assert(cond)
 
@@ -308,8 +309,7 @@ static int tdspeed_apply(int32_t *buf_out[2], int32_t *buf_in[2],
 
             for (int j = 0; j < st->dst_step; j += INC2, curr += INC2, prev += INC2)
             {
-                int32_t diff = *curr - *prev;
-                delta += abs(diff);
+                delta += ad_s32(*curr, *prev);
 
                 if (delta >= min_delta)
                     goto skip;
@@ -322,8 +322,7 @@ static int tdspeed_apply(int32_t *buf_out[2], int32_t *buf_in[2],
 
                 for (int j = 0; j < st->dst_step; j += INC2, curr += INC2, prev += INC2)
                 {
-                    int32_t diff = *curr - *prev;
-                    delta += abs(diff);
+                    delta += ad_s32(*curr, *prev);
 
                     if (delta >= min_delta)
                         goto skip;