从 wav 文件头读取采样率

Question

我正在编写要读取波形文件的代码。

我使用this document 作为我的指导方针。

标头第22字节为波形文件的通道数，第24字节为采样率。

我正在使用 Ableton 输出为 2 通道 16 位 44100hz 的测试文件。我已经确认测试波形文件的格式是大胆的，以确保它确实是 44100hz 采样率。

当我读入波形文件时，我得到了 -21436 的采样率值。我很确定我的读取小端整数的代码是正确的。而且我确信我的测试 wavfile 是正确的。所以现在我不知道为什么读取的采样率不正确......

我的int读取代码如下。

int ReadInt(char* bytes , int start) { return (bytes[start+3] << 24) + (bytes[start+2] << 16) + (bytes[start+1] << 8) + bytes[start]; }

读取wave文件的函数如下...

WavFile::WavFile(std::string filename)
{
    std::ifstream ifs;      
    ifs.open( filename, std::ios::binary | std::ios::in );

    LogStream(LOG_DEBUG) << "WavFile::WavFile - - - - BEGIN READING WAV - - - -";

    if(ifs.fail())
        throw std::invalid_argument("WavFile::WavFile : Failed to open wavFile "+filename);

    char hbytes[HEADER_SIZE];

    ifs.read(hbytes , HEADER_SIZE);

    // check that this is actually a wave file
    bool valid_riff = hbytes[0]=='R' && hbytes[1]=='I' && hbytes[2]=='F' && hbytes[3]=='F';
    bool valid_wave = hbytes[8]=='W' && hbytes[9]=='A' && hbytes[10]=='V' && hbytes[11]=='E';
    bool valid_ftm = (hbytes[12]=='f' && hbytes[13]=='m' && hbytes[14]=='t' && hbytes[15]==' ');
    bool valid_data = (hbytes[36]=='d' && hbytes[37]=='a' && hbytes[38]=='t' && hbytes[39]=='a');

    LogStream(LOG_DEBUG) << "WavFile::WavFile - valid_riff="<<valid_riff<<" valid_wave="<<valid_wave<<" valid_ftm="<<valid_ftm<<" valid_data="<<valid_data;
    if(!(valid_data && valid_ftm && valid_riff))
        throw std::invalid_argument("WavFile::WavFile : Invalid argument - unable to open wavfile "+filename);

    int audioFormat = ReadShort(hbytes , 20);
    int SubChunk1Size = ReadInt(hbytes , 16);

    if(audioFormat != 1 || SubChunk1Size != 16)
        throw std::invalid_argument("WavFile::WavFile : Only uncompressed PCM wave format supported."+filename);    

    int subChunk2size = ReadInt(hbytes , 40);
    m_header.num_channels = ReadShort(hbytes , 22);
    m_header.sample_rate = ReadInt(hbytes , 24);
    m_header.bits_per_sample = ReadShort(hbytes , 34);

    LogStream(LOG_DEBUG) << "WavFile::WavFile num_channels="<<m_header.num_channels << " sample_rate="<<m_header.sample_rate<<" bits_per_sample="<<m_header.bits_per_sample;

    m_pcm_data.resize( subChunk2size / sizeof(int16_t) );

    LogStream(LOG_DEBUG) << "WavFile::WavFile - subChunk2size = "<<subChunk2size;
    LogStream(LOG_DEBUG) << "WavFile::WavFile - m_pcm_data.size() = "<<m_pcm_data.size();
    ifs.read((char*)m_pcm_data.data() , subChunk2size);

    LogStream(LOG_DEBUG) << "WavFile: ifstream failbit="<<ifs.fail()<<" badbit="<<ifs.bad()<<" goodbit="<<ifs.good();

    ifs.close();

    LogStream(LOG_DEBUG) << "WavFile::WavFile - - - - END READING WAV - - - -\n";
    LogStream(LOG_DEBUG) << "WavFile::WavFile";
}

Answer 1

44100 具有十六进制值 44ac（无符号 int16），-21436 也具有十六进制值 44ac（有符号 int16） - 问题在于编译器在移位之前将每个有符号字符隐式转换为有符号整数的方式。您可以通过以下方式进行转换（输出 44100）来避免这种情况：

int main()
{
    char bytes[4] = { 0x44, 0xac, 0x00, 0x00 };

    printf("%i\n", (((unsigned char)bytes[3]) << 24) | (((unsigned char)bytes[2]) << 16) | (((unsigned char)bytes[1]) << 8) | ((unsigned char)bytes[0]));
    return 0;
}

或简单地读取为无符号字节 - 这将避免其他字段出现同样的问题：

int main()
{
    unsigned char bytes[4] = { 0x44, 0xac, 0x00, 0x00 };

    printf("%i\n", (bytes[3] << 24) | (bytes[2] << 16) | (bytes[1] << 8) | bytes[0]);
    return 0;
}