audio: Improvements in channel conversion code.

src/audio/SDL_audiocvt.c

@@ -31,7 +31,7 @@
 
 /* Effectively mix right and left channels into a single channel */
 static void SDLCALL
-SDL_ConvertMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
+SDL_ConvertStereoToMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 {
     float *dst = (float *) cvt->buf;
     const float *src = dst;
@@ -51,20 +51,22 @@ SDL_ConvertMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 }
 
 
-/* Discard top 4 channels */
+/* Convert from 5.1 to stereo. Average left and right, discard subwoofer. */
 static void SDLCALL
-SDL_ConvertStrip(SDL_AudioCVT * cvt, SDL_AudioFormat format)
+SDL_Convert51ToStereo(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 {
     float *dst = (float *) cvt->buf;
     const float *src = dst;
     int i;
 
-    LOG_DEBUG_CONVERT("6 channels", "stereo");
+    LOG_DEBUG_CONVERT("5.1", "stereo");
     SDL_assert(format == AUDIO_F32SYS);
 
+    /* this assumes FL+FR+FC+subwoof+BL+BR layout. */
     for (i = cvt->len_cvt / (sizeof (float) * 6); i; --i, src += 6, dst += 2) {
-        dst[0] = src[0];
-        dst[1] = src[1];
+        const double front_center = (double) src[2];
+        dst[0] = (float) ((src[0] + front_center + src[4]) / 3.0);  /* left */
+        dst[1] = (float) ((src[1] + front_center + src[5]) / 3.0);  /* right */
     }
 
     cvt->len_cvt /= 3;
@@ -74,22 +76,25 @@ SDL_ConvertStrip(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 }
 
 
-/* Discard top 2 channels of 6 */
+/* Convert from 5.1 to quad */
 static void SDLCALL
-SDL_ConvertStrip_2(SDL_AudioCVT * cvt, SDL_AudioFormat format)
+SDL_Convert51ToQuad(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 {
     float *dst = (float *) cvt->buf;
     const float *src = dst;
     int i;
 
-    LOG_DEBUG_CONVERT("6 channels", "quad");
+    LOG_DEBUG_CONVERT("5.1", "quad");
     SDL_assert(format == AUDIO_F32SYS);
 
+    /* assumes quad is FL+FR+BL+BR layout and 5.1 is FL+FR+FC+subwoof+BL+BR */
     for (i = cvt->len_cvt / (sizeof (float) * 6); i; --i, src += 6, dst += 4) {
-        dst[0] = src[0];
-        dst[1] = src[1];
-        dst[2] = src[2];
-        dst[3] = src[3];
+        /* FIXME: this is a good candidate for SIMD. */
+        const double front_center = (double) src[2];
+        dst[0] = (float) ((src[0] + front_center) * 0.5);  /* FL */
+        dst[1] = (float) ((src[1] + front_center) * 0.5);  /* FR */
+        dst[2] = (float) ((src[4] + front_center) * 0.5);  /* BL */
+        dst[3] = (float) ((src[5] + front_center) * 0.5);  /* BR */
     }
 
     cvt->len_cvt /= 6;
@@ -99,9 +104,10 @@ SDL_ConvertStrip_2(SDL_AudioCVT * cvt, SDL_AudioFormat format)
     }
 }
 
+
 /* Duplicate a mono channel to both stereo channels */
 static void SDLCALL
-SDL_ConvertStereo(SDL_AudioCVT * cvt, SDL_AudioFormat format)
+SDL_ConvertMonoToStereo(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 {
     const float *src = (const float *) (cvt->buf + cvt->len_cvt);
     float *dst = (float *) (cvt->buf + cvt->len_cvt * 2);
@@ -125,7 +131,7 @@ SDL_ConvertStereo(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 
 /* Duplicate a stereo channel to a pseudo-5.1 stream */
 static void SDLCALL
-SDL_ConvertSurround(SDL_AudioCVT * cvt, SDL_AudioFormat format)
+SDL_ConvertStereoTo51(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 {
     int i;
     float lf, rf, ce;
@@ -140,12 +146,13 @@ SDL_ConvertSurround(SDL_AudioCVT * cvt, SDL_AudioFormat format)
         src -= 2;
         lf = src[0];
         rf = src[1];
-        ce = (lf * 0.5f) + (rf * 0.5f);
-        dst[0] = src[0];
-        dst[1] = src[1];
-        dst[2] = lf - ce;
-        dst[3] = rf - ce;
-        dst[4] = dst[5] = ce;
+        ce = (lf + rf) * 0.5f;
+        dst[0] = lf + (lf - ce);  /* FL */
+        dst[1] = rf + (rf - ce);  /* FR */
+        dst[2] = ce;  /* FC */
+        dst[3] = ce;  /* !!! FIXME: wrong! This is the subwoofer. */
+        dst[4] = lf;  /* BL */
+        dst[5] = rf;  /* BR */
     }
 
     cvt->len_cvt *= 3;
@@ -157,11 +164,11 @@ SDL_ConvertSurround(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 
 /* Duplicate a stereo channel to a pseudo-4.0 stream */
 static void SDLCALL
-SDL_ConvertSurround_4(SDL_AudioCVT * cvt, SDL_AudioFormat format)
+SDL_ConvertStereoToQuad(SDL_AudioCVT * cvt, SDL_AudioFormat format)
 {
     const float *src = (const float *) (cvt->buf + cvt->len_cvt);
     float *dst = (float *) (cvt->buf + cvt->len_cvt * 2);
-    float lf, rf, ce;
+    float lf, rf;
     int i;
 
     LOG_DEBUG_CONVERT("stereo", "quad");
@@ -172,11 +179,10 @@ SDL_ConvertSurround_4(SDL_AudioCVT * cvt, SDL_AudioFormat format)
         src -= 2;
         lf = src[0];
         rf = src[1];
-        ce = (lf / 2) + (rf / 2);
-        dst[0] = src[0];
-        dst[1] = src[1];
-        dst[2] = lf - ce;
-        dst[3] = rf - ce;
+        dst[0] = lf;  /* FL */
+        dst[1] = rf;  /* FR */
+        dst[2] = lf;  /* BL */
+        dst[3] = rf;  /* BR */
     }
 
     cvt->len_cvt *= 2;
@@ -536,36 +542,36 @@ SDL_BuildAudioCVT(SDL_AudioCVT * cvt,
     /* Channel conversion */
     if (src_channels != dst_channels) {
         if ((src_channels == 1) && (dst_channels > 1)) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertStereo;
+            cvt->filters[cvt->filter_index++] = SDL_ConvertMonoToStereo;
             cvt->len_mult *= 2;
             src_channels = 2;
             cvt->len_ratio *= 2;
         }
         if ((src_channels == 2) && (dst_channels == 6)) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertSurround;
+            cvt->filters[cvt->filter_index++] = SDL_ConvertStereoTo51;
             src_channels = 6;
             cvt->len_mult *= 3;
             cvt->len_ratio *= 3;
         }
         if ((src_channels == 2) && (dst_channels == 4)) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertSurround_4;
+            cvt->filters[cvt->filter_index++] = SDL_ConvertStereoToQuad;
             src_channels = 4;
             cvt->len_mult *= 2;
             cvt->len_ratio *= 2;
         }
         while ((src_channels * 2) <= dst_channels) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertStereo;
+            cvt->filters[cvt->filter_index++] = SDL_ConvertMonoToStereo;
             cvt->len_mult *= 2;
             src_channels *= 2;
             cvt->len_ratio *= 2;
         }
         if ((src_channels == 6) && (dst_channels <= 2)) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertStrip;
+            cvt->filters[cvt->filter_index++] = SDL_Convert51ToStereo;
             src_channels = 2;
             cvt->len_ratio /= 3;
         }
         if ((src_channels == 6) && (dst_channels == 4)) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertStrip_2;
+            cvt->filters[cvt->filter_index++] = SDL_Convert51ToQuad;
             src_channels = 4;
             cvt->len_ratio /= 2;
         }
@@ -575,7 +581,7 @@ SDL_BuildAudioCVT(SDL_AudioCVT * cvt,
          */
         while (((src_channels % 2) == 0) &&
                ((src_channels / 2) >= dst_channels)) {
-            cvt->filters[cvt->filter_index++] = SDL_ConvertMono;
+            cvt->filters[cvt->filter_index++] = SDL_ConvertStereoToMono;
             src_channels /= 2;
             cvt->len_ratio /= 2;
         }

test/testresample.c

@@ -20,6 +20,7 @@ main(int argc, char **argv)
     Uint32 len = 0;
     Uint8 *data = NULL;
     int cvtfreq = 0;
+    int cvtchans = 0;
     int bitsize = 0;
     int blockalign = 0;
     int avgbytes = 0;
@@ -28,12 +29,13 @@ main(int argc, char **argv)
     /* Enable standard application logging */
     SDL_LogSetPriority(SDL_LOG_CATEGORY_APPLICATION, SDL_LOG_PRIORITY_INFO);
 
-    if (argc != 4) {
-        SDL_Log("USAGE: %s in.wav out.wav newfreq\n", argv[0]);
+    if (argc != 5) {
+        SDL_Log("USAGE: %s in.wav out.wav newfreq newchans\n", argv[0]);
         return 1;
     }
 
     cvtfreq = SDL_atoi(argv[3]);
+    cvtchans = SDL_atoi(argv[4]);
 
     if (SDL_Init(SDL_INIT_AUDIO) == -1) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "SDL_Init() failed: %s\n", SDL_GetError());
@@ -47,7 +49,7 @@ main(int argc, char **argv)
     }
 
     if (SDL_BuildAudioCVT(&cvt, spec.format, spec.channels, spec.freq,
-                          spec.format, spec.channels, cvtfreq) == -1) {
+                          spec.format, cvtchans, cvtfreq) == -1) {
         SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "failed to build CVT: %s\n", SDL_GetError());
         SDL_FreeWAV(data);
         SDL_Quit();
@@ -83,7 +85,7 @@ main(int argc, char **argv)
     }
 
     bitsize = SDL_AUDIO_BITSIZE(spec.format);
-    blockalign = (bitsize / 8) * spec.channels;
+    blockalign = (bitsize / 8) * cvtchans;
     avgbytes = cvtfreq * blockalign;
 
     SDL_WriteLE32(io, 0x46464952);      /* RIFF */
@@ -92,7 +94,7 @@ main(int argc, char **argv)
     SDL_WriteLE32(io, 0x20746D66);      /* fmt */
     SDL_WriteLE32(io, 16);      /* chunk size */
     SDL_WriteLE16(io, 1);       /* uncompressed */
-    SDL_WriteLE16(io, spec.channels);   /* channels */
+    SDL_WriteLE16(io, cvtchans);   /* channels */
     SDL_WriteLE32(io, cvtfreq); /* sample rate */
     SDL_WriteLE32(io, avgbytes);        /* average bytes per second */
     SDL_WriteLE16(io, blockalign);      /* block align */