Add optional USB DAC routing for high-res playback

app/src/main/java/github/paroj/dsub2000/service/AudioPlayer.java

@@ -5,6 +5,7 @@
 package github.paroj.dsub2000.service;
 
 import android.content.Context;
+import android.media.AudioDeviceInfo;
 import android.media.PlaybackParams;
 
 import androidx.annotation.RequiresApi;
@@ -75,6 +76,17 @@ public interface AudioPlayer {
      */
     void setNextMediaPlayer(AudioPlayer next);
 
+    /**
+     * Route playback to the given output device (e.g. a connected USB DAC). Must
+     * be called before {@link #prepareAsync()} to take effect on the next track.
+     * Default no-op for backends that don't support routing.
+     *
+     * @return true if the call was applied; false if unsupported on this backend
+     *         or on this Android version.
+     */
+    @RequiresApi(28)
+    default boolean setPreferredDevice(AudioDeviceInfo device) { return false; }
+
     interface OnPreparedListener {
         void onPrepared(AudioPlayer player);
     }

app/src/main/java/github/paroj/dsub2000/service/DownloadService.java

@@ -80,6 +80,7 @@
 import android.content.Intent;
 import android.content.IntentFilter;
 import android.content.SharedPreferences;
+import android.media.AudioDeviceInfo;
 import android.media.AudioManager;
 import android.media.MediaPlayer;
 // AudioPlayer abstraction: legacy MediaPlayer is used for local files via
@@ -2042,6 +2043,24 @@ private synchronized void doPlay(final DownloadFile downloadFile, final int posi
 			}
 
 			mediaPlayer.setDataSource(dataSource);
+
+			// Optional USB DAC routing (issue #141). Apply before prepareAsync so
+			// the player picks up the device on the upcoming prepare. Hot-plug
+			// mid-track is not handled here; the next track will pick up the new
+			// device. TODO: register AudioDeviceCallback to re-route mid-track.
+			if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
+				AudioDeviceInfo usbDevice = UsbDacHelper.findUsbAudioDevice(this);
+				if (usbDevice != null) {
+					mediaPlayer.setPreferredDevice(usbDevice);
+					if (!downloadFile.isStream()) {
+						Integer rate = UsbDacHelper.readSampleRate(dataSource);
+						if (rate != null) {
+							Log.i(TAG, "USB DAC routing enabled; source sample rate " + rate + " Hz");
+						}
+					}
+				}
+			}
+
 			setPlayerState(PREPARING);
 
 			mediaPlayer.setOnBufferingUpdateListener(new AudioPlayer.OnBufferingUpdateListener() {

app/src/main/java/github/paroj/dsub2000/service/ExoPlayerAudio.java

@@ -5,10 +5,12 @@
 package github.paroj.dsub2000.service;
 
 import android.content.Context;
+import android.media.AudioDeviceInfo;
 import android.media.AudioManager;
 import android.media.PlaybackParams;
 import android.os.Handler;
 import android.os.Looper;
+import android.util.Log;
 
 import androidx.annotation.OptIn;
 import androidx.annotation.RequiresApi;
@@ -33,6 +35,8 @@
 @OptIn(markerClass = UnstableApi.class)
 public class ExoPlayerAudio implements AudioPlayer {
 
+    private static final String TAG = ExoPlayerAudio.class.getSimpleName();
+
     private final ExoPlayer player;
     private final Handler mainHandler;
 
@@ -255,4 +259,15 @@ public void setPlaybackParams(PlaybackParams params) {
     public void setNextMediaPlayer(AudioPlayer next) {
         // Live HTTP streams have no end of file; gapless does not apply. No-op.
     }
+
+    @Override @RequiresApi(28)
+    public boolean setPreferredDevice(AudioDeviceInfo device) {
+        try {
+            onPlayerThread(() -> player.setPreferredAudioDevice(device));
+            return true;
+        } catch (Throwable t) {
+            Log.w(TAG, "setPreferredAudioDevice failed", t);
+            return false;
+        }
+    }
 }

app/src/main/java/github/paroj/dsub2000/service/MediaPlayerAudio.java

@@ -5,8 +5,10 @@
 package github.paroj.dsub2000.service;
 
 import android.content.Context;
+import android.media.AudioDeviceInfo;
 import android.media.MediaPlayer;
 import android.media.PlaybackParams;
+import android.util.Log;
 
 import androidx.annotation.RequiresApi;
 
@@ -20,6 +22,8 @@
  */
 public class MediaPlayerAudio implements AudioPlayer {
 
+    private static final String TAG = MediaPlayerAudio.class.getSimpleName();
+
     private final MediaPlayer mp = new MediaPlayer();
 
     /**
@@ -91,6 +95,16 @@ public void setOnBufferingUpdateListener(final OnBufferingUpdateListener listene
     @Override @RequiresApi(23)
     public void setPlaybackParams(PlaybackParams params) { mp.setPlaybackParams(params); }
 
+    @Override @RequiresApi(28)
+    public boolean setPreferredDevice(AudioDeviceInfo device) {
+        try {
+            return mp.setPreferredDevice(device);
+        } catch (Throwable t) {
+            Log.w(TAG, "setPreferredDevice failed", t);
+            return false;
+        }
+    }
+
     @Override
     public void setNextMediaPlayer(AudioPlayer next) {
         if (next == null) {

app/src/main/java/github/paroj/dsub2000/service/UsbDacHelper.java

@@ -0,0 +1,99 @@
+/*
+ This file is part of DSub2000.
+ Released under GPLv3, see project LICENSE.txt.
+*/
+package github.paroj.dsub2000.service;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import android.media.AudioDeviceInfo;
+import android.media.AudioManager;
+import android.media.MediaExtractor;
+import android.media.MediaFormat;
+import android.os.Build;
+import android.util.Log;
+
+import androidx.annotation.RequiresApi;
+
+import github.paroj.dsub2000.util.Constants;
+import github.paroj.dsub2000.util.Util;
+
+/**
+ * Helpers for the optional "Route to USB DAC" preference (see issue #141).
+ * Locates a connected USB audio output device so callers can hand it to
+ * {@link AudioPlayer#setPreferredDevice(AudioDeviceInfo)} before {@code prepare},
+ * and reads source sample rates for diagnostics so users can confirm what the
+ * DAC is actually receiving.
+ */
+public final class UsbDacHelper {
+
+    private static final String TAG = UsbDacHelper.class.getSimpleName();
+
+    private UsbDacHelper() {}
+
+    /**
+     * @return the first connected USB audio output device, or {@code null} if the
+     *         preference is disabled, the OS is older than Android 9, or no USB
+     *         DAC is plugged in.
+     */
+    public static AudioDeviceInfo findUsbAudioDevice(Context context) {
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.P) {
+            return null;
+        }
+        SharedPreferences prefs = Util.getPreferences(context);
+        if (!prefs.getBoolean(Constants.PREFERENCES_KEY_USB_DAC_EXCLUSIVE_MODE, false)) {
+            return null;
+        }
+        return findUsbAudioDeviceInternal(context);
+    }
+
+    @RequiresApi(Build.VERSION_CODES.P)
+    private static AudioDeviceInfo findUsbAudioDeviceInternal(Context context) {
+        AudioManager am = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
+        if (am == null) {
+            return null;
+        }
+        AudioDeviceInfo[] devices = am.getDevices(AudioManager.GET_DEVICES_OUTPUTS);
+        for (AudioDeviceInfo info : devices) {
+            int type = info.getType();
+            if (type == AudioDeviceInfo.TYPE_USB_DEVICE
+                    || type == AudioDeviceInfo.TYPE_USB_HEADSET
+                    || type == AudioDeviceInfo.TYPE_USB_ACCESSORY) {
+                Log.i(TAG, "Routing playback to USB audio device type=" + type
+                        + " name=" + info.getProductName());
+                return info;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Read the source sample rate of a local media file. Useful for confirming
+     * via {@code adb logcat} that a high-res FLAC actually reaches the player
+     * at its native rate (Android may still resample internally).
+     *
+     * @return sample rate in Hz, or {@code null} on failure / non-local source.
+     */
+    public static Integer readSampleRate(String localPath) {
+        if (localPath == null) {
+            return null;
+        }
+        MediaExtractor extractor = new MediaExtractor();
+        try {
+            extractor.setDataSource(localPath);
+            for (int i = 0; i < extractor.getTrackCount(); i++) {
+                MediaFormat format = extractor.getTrackFormat(i);
+                String mime = format.getString(MediaFormat.KEY_MIME);
+                if (mime != null && mime.startsWith("audio/")
+                        && format.containsKey(MediaFormat.KEY_SAMPLE_RATE)) {
+                    return format.getInteger(MediaFormat.KEY_SAMPLE_RATE);
+                }
+            }
+        } catch (Throwable t) {
+            Log.w(TAG, "Could not read sample rate from " + localPath, t);
+        } finally {
+            extractor.release();
+        }
+        return null;
+    }
+}

app/src/main/java/github/paroj/dsub2000/util/Constants.java

@@ -124,6 +124,7 @@ public final class Constants {
 	public static final String PREFERENCES_KEY_PERSISTENT_NOTIFICATION = "persistentNotification";
 	public static final String PREFERENCES_KEY_MEDIA_STYLE_NOTIFICATION = "mediaStyleNotification";
 	public static final String PREFERENCES_KEY_GAPLESS_PLAYBACK = "gaplessPlayback";
+	public static final String PREFERENCES_KEY_USB_DAC_EXCLUSIVE_MODE = "usbDacExclusiveMode";
 	public static final String PREFERENCES_KEY_REMOVE_PLAYED = "removePlayed";
 	public static final String PREFERENCES_KEY_KEEP_PLAYED_CNT = "keepPlayedCount";
 	public static final String PREFERENCES_KEY_SHUFFLE_MODE = "shuffleMode2";

app/src/main/res/values/strings.xml

@@ -405,6 +405,8 @@
 	<string name="settings.persistent_summary">Show the notification even after pausing.  Press the stop button to clear it away.</string>
 	<string name="settings.gapless_playback">Gapless Playback</string>
 	<string name="settings.gapless_playback_summary">If you are seeing strange bugs during playback, turning this off may help.</string>
+	<string name="settings.usb_dac_exclusive_title">Route to USB DAC</string>
+	<string name="settings.usb_dac_exclusive_summary">Send playback to a connected USB DAC when present. Applies on the next track. Android 9+.</string>
 	<string name="settings.chat_refresh">Chat Refresh Rate (Secs)</string>
 	<string name="settings.chat_enabled">Chat Enabled</string>
 	<string name="settings.chat_enabled_summary">Whether or not to display the chat listing in the pull out drawer</string>

app/src/main/res/xml/settings_playback.xml

@@ -170,5 +170,11 @@
 			android:summary="@string/settings.start_on_headphones_summary"
 			android:key="startOnHeadphones"
 			android:defaultValue="false"/>
+
+		<CheckBoxPreference
+			android:title="@string/settings.usb_dac_exclusive_title"
+			android:summary="@string/settings.usb_dac_exclusive_summary"
+			android:key="usbDacExclusiveMode"
+			android:defaultValue="false"/>
 	</PreferenceCategory>
 </PreferenceScreen>