Android: Switch AudioTrack to the AudioFormat.ENCODING_PCM_16BIT

android-src/KV4PHT/app/src/main/java/com/vagell/kv4pht/radio/RadioAudioService.java

@@ -131,7 +131,7 @@ public class RadioAudioService extends Service {
     // For transmitting audio to ESP32 / radio
     public static final int AUDIO_SAMPLE_RATE = 22050;
     public static final int channelConfig = AudioFormat.CHANNEL_IN_MONO;
-    public static final  int audioFormat = AudioFormat.ENCODING_PCM_8BIT;
+    public static final  int audioFormat = AudioFormat.ENCODING_PCM_16BIT;
     public static final  int minBufferSize = AudioRecord.getMinBufferSize(AUDIO_SAMPLE_RATE, channelConfig, audioFormat) * 4;
     private UsbManager usbManager;
     private UsbDevice esp32Device;
@@ -1258,6 +1258,18 @@ public static UsbSerialPort getUsbSerialPort() {
         return serialPort;
     }
 
+    public static byte[] convert8BitTo16Bit(byte[] pcm8) {
+        byte[] pcm16 = new byte[pcm8.length * 2];  // 2 bytes per 16-bit sample
+        for (int i = 0; i < pcm8.length; i++) {
+            int unsignedSample = pcm8[i] & 0xFF;  // Convert to unsigned
+            short sample16 = (short)((unsignedSample) << 8);  // Scale and shift
+            // Store as little-endian (least significant byte first)
+            pcm16[i * 2] = (byte)(sample16 & 0xFF);          // LSB
+            pcm16[i * 2 + 1] = (byte)((sample16 >> 8) & 0xFF); // MSB
+        }
+        return pcm16;
+    }
+
     private void handleESP32Data(byte[] data) {
         // Log.d("DEBUG", "Got bytes from ESP32: " + Arrays.toString(data));
          /* try {
@@ -1327,10 +1339,11 @@ private void handleESP32Data(byte[] data) {
                 synchronized (audioTrack) {
                     if (afskDemodulator != null) { // Avoid race condition at app start.
                         // Play the audio.
-                        audioTrack.write(data, 0, data.length);
+                        byte[] pcm16 = convert8BitTo16Bit(data);
+                        audioTrack.write(pcm16, 0, pcm16.length);
 
                         // Add the audio samples to the AFSK demodulator.
-                        float[] audioAsFloats = convertPCM8ToFloatArray(data);
+                        float[] audioAsFloats = convertPCM8SignedToFloatArray(data);
                         afskDemodulator.addSamples(audioAsFloats, audioAsFloats.length);
                     }
 
@@ -1351,7 +1364,8 @@ private void handleESP32Data(byte[] data) {
                                 audioTrack.play();
                             }
                             synchronized (audioTrack) {
-                                audioTrack.write(rxBytesPrebuffer, 0, PRE_BUFFER_SIZE);
+                                byte[] pcm16 = convert8BitTo16Bit(rxBytesPrebuffer);
+                                audioTrack.write(pcm16, 0, pcm16.length);
                             }
                         }
 
@@ -1572,17 +1586,14 @@ private void handleParsedCommand(byte cmd, byte[] param) {
         }
     }
 
-    private float[] convertPCM8ToFloatArray(byte[] pcm8Data) {
+    private float[] convertPCM8SignedToFloatArray(byte[] pcm8Data) {
         // Create a float array of the same length as the input byte array
         float[] floatData = new float[pcm8Data.length];
 
         // Iterate through the byte array and convert each sample
         for (int i = 0; i < pcm8Data.length; i++) {
-            // Convert unsigned 8-bit PCM to signed 8-bit value
-            int signedValue = (pcm8Data[i] & 0xFF) - 128;
-
             // Normalize the signed 8-bit value to the range [-1.0, 1.0]
-            floatData[i] = signedValue / 128.0f;
+            floatData[i] = pcm8Data[i] / 128.0f;
         }
 
         return floatData;

microcontroller-src/kv4p_ht_esp32_wroom_32/kv4p_ht_esp32_wroom_32.ino

@@ -555,7 +555,7 @@ void loop() {
 
         // Apply attenuation to the sample
         int16_t sample = (int32_t)remove_dc(((2048 - (buffer16[i] & 0xfff)) << 4)) * attenuation >> 8;
-        buffer8[i] = (sample >> 8) + 128; // Unsigned PCM8
+        buffer8[i] = (sample >> 8); // Signed
       }
 
       Serial.write(buffer8, samplesRead);