#939 Adds MP3 encoder preferences to the User Preferences dialog and provides user control over sample rate and MP3 encoder settings. MP3 preferences are global and affect both recording and streaming. (#1159)

Co-authored-by: Dennis Sheirer <dsheirer@github.com>

src/main/java/io/github/dsheirer/audio/convert/MP3Header.java

@@ -0,0 +1,115 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2022 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ * ****************************************************************************
+ */
+
+package io.github.dsheirer.audio.convert;
+
+/**
+ * Utility class for inspecting/parsing MP3 frames
+ */
+public class MP3Header
+{
+    private MP3Header() {}
+
+    /**
+     * Indicates if the byte array at the offset has a valid MP3 frame sync and is at least 4 bytes long
+     * @param frame array of bytes
+     * @param offset into the array
+     * @return true if the first two offset bytes contain a frame sync and length from offset is at least 4
+     */
+    public static boolean isValid(byte[] frame, int offset)
+    {
+        return frame.length >= (offset + 4) && hasSync(frame, offset);
+    }
+
+    /**
+     * Indicates if the byte array at the offset has a valid MP3 frame sync
+     * @param frame array of bytes
+     * @param offset into the array
+     * @return true if the first two offset bytes contain a frame sync
+     */
+    public static boolean hasSync(byte[] frame, int offset)
+    {
+        return (frame[offset] & 0xFF) == 0xFF && (frame[offset + 1] & 0xE0) == 0xE0;
+    }
+
+    /**
+     * MPEG version for the frame
+     */
+    public static MPEGVersion getMPEGVersion(byte[] frame, int offset)
+    {
+        return MPEGVersion.fromValue((frame[offset + 1] & 0x18) >> 3);
+    }
+
+    /**
+     * MPEG layer for the frame.  Normally layer 3 for audio.
+     */
+    public static MPEGLayer getMPEGLayer(byte[] frame, int offset)
+    {
+        return MPEGLayer.fromValue((frame[offset + 1] & 0x6) >> 1);
+    }
+
+    /**
+     * MP3 bit rate
+     */
+    public static int getBitRate(byte[] frame, int offset)
+    {
+        return MP3BitRate.getLayer3BitRate(getMPEGVersion(frame, offset), (frame[offset + 2] & 0xF0) >> 4);
+    }
+
+    /**
+     * Input audio sample rate.
+     */
+    public static int getSampleRate(byte[] frame, int offset)
+    {
+        return MP3SampleRate.getSampleRate(getMPEGVersion(frame, offset), (frame[offset + 2] & 0x0C) >> 2);
+    }
+
+    /**
+     * Channel mode
+     */
+    public static ChannelMode getChannelMode(byte[] frame, int offset)
+    {
+        return ChannelMode.fromValue((frame[offset + 3] & 0xC0) >> 6);
+    }
+
+    /**
+     * Inspects the frame bytes and provides details about the header.
+     */
+    public static String inspect(byte[] frame, int offset)
+    {
+        StringBuilder sb = new StringBuilder();
+        sb.append("MP3 FRAME");
+
+        if(!isValid(frame, offset))
+        {
+            sb.append(" INVALID");
+            return sb.toString();
+        }
+
+        sb.append(" VER:").append(getMPEGVersion(frame, offset));
+        sb.append(" MODE:").append(getChannelMode(frame, offset));
+        sb.append(" ").append(getMPEGLayer(frame, offset));
+        sb.append(" BR:").append(getBitRate(frame, offset));
+        sb.append(" SR:").append(getSampleRate(frame, offset));
+
+        //TODO: add MONO/STEREO
+        
+        return sb.toString();
+    }
+}