commit 15c57330b9d07f62e23ab1b99efe38f4b9717453
author Sam Lantinga <slouken@libsdl.org> Fri Oct 07 17:23:20 2016 -0700
committer Sam Lantinga <slouken@libsdl.org> Fri Oct 07 17:23:20 2016 -0700
tree 95ded1636a6936fa7929b96c95d9209a451d3085
parent e8283769b5ec4e68e390d1c0cc3c84c955cdd359

Fixed bug 2952 - SDL_MixAudioFormat does not support audio format AUDIO_U16LSB/AUDIO_U16MSB

Simon Sandström

As stated in Summary. The switch statement will execute the default case and set a SDL error message: "SDL_MixAudio(): unknown audio format".

There are atleast two more problems here:

1. SDL_MixAudioFormat does not notify the user that an error has occured and that a SDL error message was set. It took me awhile to understand why I couldn't mix down the volume on my AUDIO_U16LSB formatted audio stream.. until I started digging in the SDL source code.

2. The error message is incorrect, it should read: "SDL_MixAudioFormat(): unknown audio format".

src/audio/SDL_mixer.c

diff --git a/src/audio/SDL_mixer.c b/src/audio/SDL_mixer.c
index b49c73d..e021939 100644
--- a/src/audio/SDL_mixer.c
+++ b/src/audio/SDL_mixer.c
@@ -202,6 +202,54 @@
         }
         break;
 
+    case AUDIO_U16LSB:
+        {
+            Uint16 src1, src2;
+            int dst_sample;
+            const int max_audioval = 0xFFFF;
+
+            len /= 2;
+            while (len--) {
+                src1 = ((src[1]) << 8 | src[0]);
+                ADJUST_VOLUME(src1, volume);
+                src2 = ((dst[1]) << 8 | dst[0]);
+                src += 2;
+                dst_sample = src1 + src2;
+                if (dst_sample > max_audioval) {
+                    dst_sample = max_audioval;
+                }
+                dst[0] = dst_sample & 0xFF;
+                dst_sample >>= 8;
+                dst[1] = dst_sample & 0xFF;
+                dst += 2;
+            }
+        }
+        break;
+
+    case AUDIO_U16MSB:
+        {
+            Uint16 src1, src2;
+            int dst_sample;
+            const int max_audioval = 0xFFFF;
+
+            len /= 2;
+            while (len--) {
+                src1 = ((src[0]) << 8 | src[1]);
+                ADJUST_VOLUME(src1, volume);
+                src2 = ((dst[0]) << 8 | dst[1]);
+                src += 2;
+                dst_sample = src1 + src2;
+                if (dst_sample > max_audioval) {
+                    dst_sample = max_audioval;
+                }
+                dst[1] = dst_sample & 0xFF;
+                dst_sample >>= 8;
+                dst[0] = dst_sample & 0xFF;
+                dst += 2;
+            }
+        }
+        break;
+
     case AUDIO_S32LSB:
         {
             const Uint32 *src32 = (Uint32 *) src;
@@ -313,7 +361,7 @@
         break;
 
     default:                   /* If this happens... FIXME! */
-        SDL_SetError("SDL_MixAudio(): unknown audio format");
+        SDL_SetError("SDL_MixAudioFormat(): unknown audio format");
         return;
     }
 }