1 files changed, 66 insertions, 0 deletions
diff --git a/libs/android-transcoder/src/main/java/net/ypresto/androidtranscoder/engine/AudioRemixer.java b/libs/android-transcoder/src/main/java/net/ypresto/androidtranscoder/engine/AudioRemixer.java
new file mode 100644
index 000000000..5255a2219
--- /dev/null
+++ b/libs/android-transcoder/src/main/java/net/ypresto/androidtranscoder/engine/AudioRemixer.java
@@ -0,0 +1,66 @@
+package net.ypresto.androidtranscoder.engine;
+
+import java.nio.ShortBuffer;
+
+public interface AudioRemixer {
+    void remix(final ShortBuffer inSBuff, final ShortBuffer outSBuff);
+
+    AudioRemixer DOWNMIX = new AudioRemixer() {
+        private static final int SIGNED_SHORT_LIMIT = 32768;
+        private static final int UNSIGNED_SHORT_MAX = 65535;
+
+        @Override
+        public void remix(final ShortBuffer inSBuff, final ShortBuffer outSBuff) {
+            // Down-mix stereo to mono
+            // Viktor Toth's algorithm -
+            // See: http://www.vttoth.com/CMS/index.php/technical-notes/68
+            //      http://stackoverflow.com/a/25102339
+            final int inRemaining = inSBuff.remaining() / 2;
+            final int outSpace = outSBuff.remaining();
+
+            final int samplesToBeProcessed = Math.min(inRemaining, outSpace);
+            for (int i = 0; i < samplesToBeProcessed; ++i) {
+                // Convert to unsigned
+                final int a = inSBuff.get() + SIGNED_SHORT_LIMIT;
+                final int b = inSBuff.get() + SIGNED_SHORT_LIMIT;
+                int m;
+                // Pick the equation
+                if ((a < SIGNED_SHORT_LIMIT) || (b < SIGNED_SHORT_LIMIT)) {
+                    // Viktor's first equation when both sources are "quiet"
+                    // (i.e. less than middle of the dynamic range)
+                    m = a * b / SIGNED_SHORT_LIMIT;
+                } else {
+                    // Viktor's second equation when one or both sources are loud
+                    m = 2 * (a + b) - (a * b) / SIGNED_SHORT_LIMIT - UNSIGNED_SHORT_MAX;
+                }
+                // Convert output back to signed short
+                if (m == UNSIGNED_SHORT_MAX + 1) m = UNSIGNED_SHORT_MAX;
+                outSBuff.put((short) (m - SIGNED_SHORT_LIMIT));
+            }
+        }
+    };
+
+    AudioRemixer UPMIX = new AudioRemixer() {
+        @Override
+        public void remix(final ShortBuffer inSBuff, final ShortBuffer outSBuff) {
+            // Up-mix mono to stereo
+            final int inRemaining = inSBuff.remaining();
+            final int outSpace = outSBuff.remaining() / 2;
+
+            final int samplesToBeProcessed = Math.min(inRemaining, outSpace);
+            for (int i = 0; i < samplesToBeProcessed; ++i) {
+                final short inSample = inSBuff.get();
+                outSBuff.put(inSample);
+                outSBuff.put(inSample);
+            }
+        }
+    };
+
+    AudioRemixer PASSTHROUGH = new AudioRemixer() {
+        @Override
+        public void remix(final ShortBuffer inSBuff, final ShortBuffer outSBuff) {
+            // Passthrough
+            outSBuff.put(inSBuff);
+        }
+    };
+}