在不使用外部库的情况下确定给定mp3文件长度(以秒为单位)的最简单方法是什么？ (高度赞赏python来源)

解决方案

我们可以使用pymad。它是一个外部库，但不要落入"此处未发明"陷阱。我们不希望使用任何外部库的任何特殊原因？

import mad

mf = mad.MadFile("foo.mp3")
track_length_in_milliseconds = mf.total_time()

在这里发现。

--

如果我们真的不想使用外部库，请在这里查看并查看他的工作方式。警告：这很复杂。

我们可能会计算文件中的帧数。每个帧都有一个起始代码，尽管我无法收集起始代码的确切值，也没有MPEG规范。每个帧都有一定的长度，对于MPEG1层II，大约为40ms。

此方法适用于CBR文件(恒定比特率)，而VBR文件的工作原理则完全不同。

从以下文档中：

对于第I层，我们向我们提交以下公式：

FrameLengthInBytes =(12 * BitRate / SampleRate + Padding)* 4

对于第二层和第三层文件，请使用以下公式：

FrameLengthInBytes = 144 * BitRate / SampleRate +填充

有关MPEG音频帧头的信息

Simple, parse MP3 binary blob to calculate something, in Python

这听起来像是一个很高的要求。我不了解Python，但是这是我曾经尝试编写的另一个程序中重构的一些代码。

注意：它是用C ++语言编写的(很抱歉，这就是我所拥有的)。同样，按原样，它将仅处理恒定比特率的MPEG 1音频第3层文件。那应该涵盖了大多数，但是我不能保证在所有情况下都能正常工作。希望这可以实现我们想要的功能，并希望将其重构为Python比从头开始做起来容易。

// determines the duration, in seconds, of an MP3;
// assumes MPEG 1 (not 2 or 2.5) Audio Layer 3 (not 1 or 2)
// constant bit rate (not variable)

#include <iostream>
#include <fstream>
#include <cstdlib>

using namespace std;

//Bitrates, assuming MPEG 1 Audio Layer 3
const int bitrates[16] = {
         0,  32000,  40000,  48000,  56000,  64000,  80000,   96000,
    112000, 128000, 160000, 192000, 224000, 256000, 320000,       0
  };

//Intel processors are little-endian;
//search Google or see: http://en.wikipedia.org/wiki/Endian
int reverse(int i)
{
    int toReturn = 0;
    toReturn |= ((i & 0x000000FF) << 24);
    toReturn |= ((i & 0x0000FF00) << 8);
    toReturn |= ((i & 0x00FF0000) >> 8);
    toReturn |= ((i & 0xFF000000) >> 24);
    return toReturn;
}

//In short, data in ID3v2 tags are stored as
//"syncsafe integers". This is so the tag info
//isn't mistaken for audio data, and attempted to
//be "played". For more info, have fun Googling it.
int syncsafe(int i)
{
 int toReturn = 0;
 toReturn |= ((i & 0x7F000000) >> 24);
 toReturn |= ((i & 0x007F0000) >>  9);
 toReturn |= ((i & 0x00007F00) <<  6);
 toReturn |= ((i & 0x0000007F) << 21);
 return toReturn;     
}

//How much room does ID3 version 1 tag info
//take up at the end of this file (if any)?
int id3v1size(ifstream& infile)
{
   streampos savePos = infile.tellg(); 

   //get to 128 bytes from file end
   infile.seekg(0, ios::end);
   streampos length = infile.tellg() - (streampos)128;
   infile.seekg(length);

   int size;
   char buffer[3] = {0};
   infile.read(buffer, 3);
   if( buffer[0] == 'T' && buffer[1] == 'A' && buffer[2] == 'G' )
     size = 128; //found tag data
   else
     size = 0; //nothing there

   infile.seekg(savePos);

   return size;

}

//how much room does ID3 version 2 tag info
//take up at the beginning of this file (if any)
int id3v2size(ifstream& infile)
{
   streampos savePos = infile.tellg(); 
   infile.seekg(0, ios::beg);

   char buffer[6] = {0};
   infile.read(buffer, 6);
   if( buffer[0] != 'I' || buffer[1] != 'D' || buffer[2] != '3' )
   {   
       //no tag data
       infile.seekg(savePos);
       return 0;
   }

   int size = 0;
   infile.read(reinterpret_cast<char*>(&size), sizeof(size));
   size = syncsafe(size);

   infile.seekg(savePos);
   //"size" doesn't include the 10 byte ID3v2 header
   return size + 10;
}

int main(int argCount, char* argValues[])
{
  //you'll have to change this
  ifstream infile("C:/Music/Bush - Comedown.mp3", ios::binary);

  if(!infile.is_open())
  {
   infile.close();
   cout << "Error opening file" << endl;
   system("PAUSE");
   return 0;
  }

  //determine beginning and end of primary frame data (not ID3 tags)
  infile.seekg(0, ios::end);
  streampos dataEnd = infile.tellg();

  infile.seekg(0, ios::beg);
  streampos dataBegin = 0;

  dataEnd -= id3v1size(infile);
  dataBegin += id3v2size(infile);

  infile.seekg(dataBegin,ios::beg);

  //determine bitrate based on header for first frame of audio data
  int headerBytes = 0;
  infile.read(reinterpret_cast<char*>(&headerBytes),sizeof(headerBytes));

  headerBytes = reverse(headerBytes);
  int bitrate = bitrates[(int)((headerBytes >> 12) & 0xF)];

  //calculate duration, in seconds
  int duration = (dataEnd - dataBegin)/(bitrate/8);

  infile.close();

  //print duration in minutes : seconds
  cout << duration/60 << ":" << duration%60 << endl;

  system("PAUSE");
  return 0;
}