根据RFC文档上的说法，只要知道文件的长度，播放的比特率（bitRate），采样率（Simpling Frequency Rate）以及 填充位数(Padding bits)，以及“恒定每26ms能播放一数据帧” 的约定，就可以计算出播放时长
okay，要做的就是，了解mp3帧格式，获取比特率，采样率 以及 填充位数
在mp3文件的末尾，恒存在一个长度为128字节的ID3 Tag Version 1的标签，用以描述文件。如果愿意，还可以添加一个ID3 Tag Version 2到文件头，长度不固定，不过在其头部，会有10字节的描述头，里头标识出这个TAG结构的总长（含10字节），然后，接下来夹着的一直到TAG V1的部分，就是全部的数据帧。
假定这些帧是采取CBR格式，即固定帧长存储，则每一帧的帧头描述都是一样的，基于此，便可计算出所需要的数据
做一个类，来干这件事情：
using System;using System.IO;using System.Text;namespace IndieCommon
{
public abstract class Mp3Tag
{ // 每秒固定可以播放的帧数 const double playFramesPerSec = 38.461538461538461538461538461538; /**//// <summary> /// 获取MP3帧头包含的信息 /// </summary> /// <param name="FrameHeader">帧头四字节数据</param>
/// <returns>帧信息结构</returns> static Mp3Frame getFrameInfo(byte[] FrameHeader)#region static Mp3Frame getFrameInfo(byte[] FrameHeader) internal static Mp3Frame getFrameInfo(byte[] FrameHeader) { // 比特率检索字典 int[,] bitrateArray = new int[,] { { 0,0,0,0,0,0}, {32,32,32,32,32,8}, {64,48,40,64,48,16}, {64,48,40,64,48,16}, {96,56,48,96,56,24}, {128,64,56,128,64,32}, {160,80,64,160,80,64}, {192,96,80,192,96,80}, {224,112,96,224,112,52}, {256,128,112,256,128,64}, {288,160,128,288,160,128}, {320,192,160,320,192,160}, {352,320,192,352,320,112}, {384,256,224,384,256,128}, {448,384,320,448,384,320}, {0,0,0,0,0,0} }; // 采样率检索字典 int[,] simpArray = new int[,] { {44100,22050,11025}, {48000,24000,12000}, {32000,16000,8000}, {0,0,0} }; // 取帧信息 Mp3Frame frame = new Mp3Frame(); // MPEG版本 switch ((FrameHeader[1] & 0x18) >> 3) { case 3: // MPEG version 1 frame.version = 1; break; case 2: // MPEG version 2 frame.version = 2; break; case 0: // MPEG version 2.5 frame.version = 3; break; case 1: // Reserve frame.version = 0; break; } // 层级 switch ((FrameHeader[1] & 0x6) >> 1) { case 1: // Layer 3 frame.layer = 3; break; case 2: // Layer 2 frame.layer = 2; break; case 3: // Layer 1 frame.layer = 1; break; case 0: // reserve frame.layer = 0; break; } // 是否有CRC保护 frame.protect = FrameHeader[1] & 0x1; // 比特率索引 int j = ((FrameHeader[2] & 0xf0) >> 4)+1; int i = 0; switch (frame.version) { // 通过版本 + 层级 获取 BIT率 case 1: switch (frame.layer) { case 1: i = 0; break; case 2: i = 1; break; case 3: i = 2; break; } break; case 2: case 3: switch (frame.layer) { case 1: i = 3; break; case 2: i = 4; break; case 3: i = 5; break; } break; } frame.bitrate = bitrateArray[j,i]; // 获取采样率 j = ((FrameHeader[2] & 0xc) >> 2); switch (frame.version) { case 1: i = 0; break; case 2: i = 1; break; case 3: i = 2; break; } frame.simplingRate = simpArray[j, i]; // 填充位 frame.paddingBits = ((FrameHeader[2] & 0x2) >> 1); // 声道，1为立体声，0为单声道 frame.channel = (((FrameHeader[3] & 0xc0) >> 6) < 3 ? 1: 0 ); return frame; } #endregion /**//// <summary> /// 由帧头获取Mp3时长信息 /// </summary> /// <param name="paramMp3">Mp3结构</param> /// <param name="FrameHeader">帧头</param> /// <param name="ID3V2_frame_size">ID3 Tag2长度</param> static void getTimeInfo(ref MP3 paramMP3,byte[] FrameHeader,int ID3V2_frame_size)#region static void getTimeInfo(ref MP3 paramMP3,byte[] FrameHeader,int ID3V2_frame_size) internal static void getTimeInfo(ref MP3 paramMP3, byte[] FrameHeader, int ID3V2_frame_size) { // 文件长度 FileInfo fi = new FileInfo(paramMP3.fileComplete); paramMP3.FileLength = fi.Length; // 获取帧信息 Mp3Frame frameInfo = getFrameInfo(FrameHeader); // 获取帧长(仅对CBR起作用） int frameSize = frameInfo.CalcFrameSize(); // 帧数 // 帧总长度 = 文件长度 - ID3 TagV2 - ID3 TagV1(固长128) long frameCount = (paramMP3.FileLength - ID3V2_frame_size - 128) / frameSize; // 时长 double secs = (double)frameCount / playFramesPerSec; paramMP3.hours = (int)secs / 3600; paramMP3.minutes = ((int)secs % 3600) / 60; paramMP3.seconds = ((int)secs % 3600) % 60; } #endregion /**//// <summary> /// 读取MP3信息 /// </summary> /// <param name="paramMP3">Mp3描述结构</param> static void readMP3Tag(ref MP3 paramMP3)#region static void readMP3Tag(ref MP3 paramMP3) public static void readMP3Tag(ref MP3 paramMP3) { // 文件对象 FileStream oFileStream; oFileStream = new FileStream(paramMP3.fileComplete, FileMode.Open); // 是否有ID3 Tag V2 byte[] header = new byte[10]; oFileStream.Seek(0, SeekOrigin.Begin); oFileStream.Read(header, 0, 10); // 标识头长度的字串 Encoding instEncoding = new ASCIIEncoding(); string id3Tag = instEncoding.GetString(header); // 是否有ID3 Tag V2 if (id3Tag.Substring(0, 3) == "ID3") { // ID3Tag V2帧长度 int ID3V2_frame_size = (int)(header[6] & 0x7F) * 0x200000 | (int)(header[7] & 0x7F) * 0x400 | (int)(header[8] & 0x7F) * 0x80 | (int)(header[9] & 0x7F); // 定位到某一帧头 byte[] FrameHeader = new byte[4]; oFileStream.Seek(ID3V2_frame_size + 10, SeekOrigin.Begin); oFileStream.Read(FrameHeader, 0, 4); oFileStream.Close(); // 获取时长 getTimeInfo(ref paramMP3, FrameHeader, ID3V2_frame_size); } // 直读Frame // 读取ID3 Tag V1 else { // 直接获取帧头 byte[] FrameHeader = new byte[4]; oFileStream.Seek(0, SeekOrigin.Begin); oFileStream.Read(FrameHeader, 0, 4); oFileStream.Close(); // 获取时长 getTimeInfo(ref paramMP3, FrameHeader, 0); } // Read the 128 byte ID3 tag into a byte array //byte[] bBuffer = new byte[128]; //oFileStream.Seek(-128, SeekOrigin.End); //oFileStream.Read(bBuffer, 0, 128); //oFileStream.Close(); /**///// Convert the Byte Array to a String //id3Tag = instEncoding.GetString(bBuffer); /**///// If there is an attched ID3 v1.x TAG then read it //if (id3Tag.Substring(0, 3) == "TAG") //{ // paramMP3.id3Title = id3Tag.Substring(3, 30).Trim(); // paramMP3.id3Artist = id3Tag.Substring(33, 30).Trim(); // paramMP3.id3Album = id3Tag.Substring(63, 30).Trim(); // paramMP3.id3Year = id3Tag.Substring(93, 4).Trim(); // paramMP3.id3Comment = id3Tag.Substring(97, 28).Trim(); // // Get the track number if TAG conforms to ID3 v1.1 // if (id3Tag[125] == 0) // paramMP3.id3TrackNumber = bBuffer[126]; // else // paramMP3.id3TrackNumber = 0; // paramMP3.id3Genre = bBuffer[127]; // paramMP3.hasID3Tag = true; // // ********* IF USED IN ANGER: ENSURE to test for non-numeric year //} //else //{ // // ID3 Tag not found so create an empty TAG in case the user saces later // paramMP3.id3Title = ""; // paramMP3.id3Artist = ""; // paramMP3.id3Album = ""; // paramMP3.id3Year = ""; // paramMP3.id3Comment = ""; // paramMP3.id3TrackNumber = 0; // paramMP3.id3Genre = 0; // paramMP3.hasID3Tag = false; //} } #endregion /**//// <summary> /// 更新tag信息 /// </summary> /// <param name="paramMP3"></param> static void updateMP3Tag(ref MP3 paramMP3)#region static void updateMP3Tag(ref MP3 paramMP3) public static void updateMP3Tag(ref MP3 paramMP3) { // Trim any whitespace paramMP3.id3Title = paramMP3.id3Title.Trim(); paramMP3.id3Artist = paramMP3.id3Artist.Trim(); paramMP3.id3Album = paramMP3.id3Album.Trim(); paramMP3.id3Year = paramMP3.id3Year.Trim(); paramMP3.id3Comment = paramMP3.id3Comment.Trim(); // Ensure all properties are correct size if (paramMP3.id3Title.Length > 30) paramMP3.id3Title = paramMP3.id3Title.Substring(0, 30); if (paramMP3.id3Artist.Length > 30) paramMP3.id3Artist = paramMP3.id3Artist.Substring(0, 30); if (paramMP3.id3Album.Length > 30) paramMP3.id3Album = paramMP3.id3Album.Substring(0, 30); if (paramMP3.id3Year.Length > 4) paramMP3.id3Year = paramMP3.id3Year.Substring(0, 4); if (paramMP3.id3Comment.Length > 28) paramMP3.id3Comment = paramMP3.id3Comment.Substring(0, 28); // Build a new ID3 Tag (128 Bytes) byte[] tagByteArray = new byte[128]; for (int i = 0; i < tagByteArray.Length; i++) tagByteArray[i] = 0; // Initialise array to nulls // Convert the Byte Array to a String Encoding instEncoding = new ASCIIEncoding(); // NB: Encoding is an Abstract class // ************ To DO: Make a shared instance of ASCIIEncoding so we don't keep creating/destroying it // Copy "TAG" to Array byte[] workingByteArray = instEncoding.GetBytes("TAG"); Array.Copy(workingByteArray, 0, tagByteArray, 0, workingByteArray.Length); // Copy Title to Array workingByteArray = instEncoding.GetBytes(paramMP3.id3Title); Array.Copy(workingByteArray, 0, tagByteArray, 3, workingByteArray.Length); // Copy Artist to Array workingByteArray = instEncoding.GetBytes(paramMP3.id3Artist); Array.Copy(workingByteArray, 0, tagByteArray, 33, workingByteArray.Length); // Copy Album to Array workingByteArray = instEncoding.GetBytes(paramMP3.id3Album); Array.Copy(workingByteArray, 0, tagByteArray, 63, workingByteArray.Length); // Copy Year to Array workingByteArray = instEncoding.GetBytes(paramMP3.id3Year); Array.Copy(workingByteArray, 0, tagByteArray, 93, workingByteArray.Length); // Copy Comment to Array workingByteArray = instEncoding.GetBytes(paramMP3.id3Comment); Array.Copy(workingByteArray, 0, tagByteArray, 97, workingByteArray.Length); // Copy Track and Genre to Array tagByteArray[126] = paramMP3.id3TrackNumber; tagByteArray[127] = paramMP3.id3Genre; // SAVE TO DISK: Replace the final 128 Bytes with our new ID3 tag FileStream oFileStream = new FileStream(paramMP3.fileComplete, FileMode.Open); if (paramMP3.hasID3Tag) oFileStream.Seek(-128, SeekOrigin.End); else oFileStream.Seek(0, SeekOrigin.End); oFileStream.Write(tagByteArray, 0, 128); oFileStream.Close(); paramMP3.hasID3Tag = true; } #endregion } /**//// <summary> /// Mp3结构 /// </summary> struct MP3#region struct MP3 public struct MP3 { public string filePath; public string fileFileName; public string fileComplete; public bool hasID3Tag; public string id3Title; public string id3Artist; public string id3Album; public string id3Year; public string id3Comment; public byte id3TrackNumber; public byte id3Genre; public long FileLength; public int hours; public int minutes; public int seconds; // Required struct constructor public MP3(string path, string name) { this.filePath = path; this.fileFileName = name; this.fileComplete = path + "\\" + name; this.hasID3Tag = false; this.id3Title = null; this.id3Artist = null; this.id3Album = null; this.id3Year = null; this.id3Comment = null; this.id3TrackNumber = 0; this.FileLength = 0; this.id3Genre = 0; this.hours = 0; this.minutes = 0; this.seconds = 0; } } #endregion /**//// <summary> /// MP3文件帧结构 /// </summary> struct Mp3Frame#region struct Mp3Frame internal struct Mp3Frame { public int version; // MPEG版本（2+，2，1，0表示保留） public int layer; // 层级(1,2,3,0表示保留) public int protect; // 是否受CRC校验保护(1为保护，0为未保护） public int frameSize; // 帧长度 public int bitrate; // 速率,bps public int simplingRate; // 采样率 public int paddingBits; // 填充位数 public int channel; // 声道模式（1为立体声，0为单声道） /**//// <summary> /// 计算帧长度 /// </summary> public int CalcFrameSize() { // 计算帧长度的公式 this.frameSize = ((this.version == 1 ? 144 : 72) * 1000 * this.bitrate / this.simplingRate) + this.paddingBits; return this.frameSize; } } #endregion
}很显然，这事儿还没干完，先留着，以后再干～～～
