/// <summary>
/// 解析ID3V2标签固定的10个字节长度的头部
/// </summary>
/// <returns>
/// 成功返回不包含标签头的tag的大小
/// 失败返回-1
/// </returns>
///
/// <remarks>
/// ID3v2/file identifier "ID3"
/// ID3v2 version 04 00
/// ID3v2 flags abcd0000
/// ID3v2 size 4 * 0xxxxxxx
///
/// Version or revision will never be $FF.
/// An ID3v2 tag can be detected with the following pattern:
/// 49 44 33 yy yy xx zz zz zz zz
/// Where yy is less than 0xFF, xx is the 'flags' byte and zz is less than 0x80.
///
/// 参考：
/// http://id3.org/id3v2.4.0-structure
/// mplayer, demux_audio.c, id3v2_tag_size
/// </remarks>
public static int ParseID3V2FixedTagHead(IGMusicStream stream, byte id3v2_tag_major_version)
{
if (id3v2_tag_major_version == 0xFF)
{
logger.ErrorFormat("invalide id3v2_tag_major_version:{0}", id3v2_tag_major_version);
return(-1);
}
byte revision;
if (!stream.ReadByte(out revision) || revision == 0xFF)
{
logger.ErrorFormat("invalide revision:{0}", revision);
return(-1);
}
byte flag;
if (!stream.ReadByte(out flag))
{
logger.ErrorFormat("read flag failed");
return(-1);
}
/* 计算标标签帧的大小 */
int size = 0;
for (int i = 0; i < 4; i++)
{
byte size_byte;
if (!stream.ReadByte(out size_byte))
{
logger.ErrorFormat("read size byte failed");
return(-1);
}
if (size_byte > 0x80)
{
logger.ErrorFormat("invalide size byte");
return(-1);
}
size = size << 7 | size_byte;
}
return(size);
}
/// <summary>
/// 找到第一帧的帧头部存储的流媒体信息，包含比特率，采样率等等
/// MP3每一帧的帧头信息都是一样的，只要找到第一帧的帧头信息就知道了整个文件的比特率，采样率等流媒体信息
/// 参考：
/// http://id3.org/mp3Frame
/// https://tools.ietf.org/html/rfc5219#page-3
/// https://www.cnblogs.com/ranson7zop/p/7655474.html
/// mplayer, mp3_hdr.c, mp_get_mp3_header
/// </summary>
public static bool FindFirstFrameHeader(IGMusicStream stream, byte id3v2_tag_major_version)
{
bool sync_detected = false;
uint int32_frame_head = 0;
Layers layer = Layers.Reserve;
MPEGVersion mpeg_ver = MPEGVersion.Reserve;
int sampling_frequency = 0;
int bit_rate_kbps = 0;
int channels = 0;
long frame_size = 0;
int spf = 0; // sample per frame, 每一个音频帧中，采样的个数，是一个恒定值
uint padding = 0; // 填充
#region 检测MP3第一帧头部同步信息（sync）,如果是同步信息，那么就说明是编码帧的帧头
/*
* MP3编码的每一帧的帧头的有个固定的11位的同步信息，从高位开始计算，每一位都是1
* 这里通过“与”运算符判断帧头的前11位是否都是1（与上0xFFE00000（二进制是11111111111000000000000000000000））
* 这11位的数据叫做同步信息（sync）
*/
byte[] frame_head = new byte[4];
while (true)
{
byte cur_byte;
stream.ReadByte(out cur_byte);
AppendByte(frame_head, cur_byte); /* 每次读取一个字节，并且追加到frame_head里 */
int32_frame_head = (uint)(frame_head[0] << 24 | frame_head[1] << 16 | frame_head[2] << 8 | frame_head[3]);
if ((int32_frame_head & 0xFFE00000) == 0xFFE00000)
{
sync_detected = true;
break;
}
}
#endregion
if (!sync_detected)
{
logger.ErrorFormat("MP3 frame head , sync info not found");
return(false);
}
#region Layer
/*
|-----------------|
| 0 0 | Reserved |
|-----------------|
| 0 1 | Layer III |
|-----------------|
| 1 0 | Layer II |
|-----------------|
| 1 1 | Layer I |
|-----------------|
*/
bool bit18 = Utils.GetBit(int32_frame_head, 18);
bool bit17 = Utils.GetBit(int32_frame_head, 17);
if (!bit18 && !bit17)
{
logger.ErrorFormat("not layer-1/2/3");
return(false);
}
else if (!bit18 && bit17)
{
layer = Layers.LayerIII;
}
else if (bit18 && !bit17)
{
layer = Layers.LayerII;
}
else if (bit18 && bit17)
{
layer = Layers.LayerI;
}
#endregion
#region mpeg版本
/*
|-----------------|
| 0 0 | MPEG 2.5 |
|-----------------|
| 0 1 | Reserve |
|-----------------|
| 1 0 | MPEG 2 |
|-----------------|
| 1 1 | MPEG 1 |
|-----------------|
*/
bool bit20 = Utils.GetBit(int32_frame_head, 20);
bool bit19 = Utils.GetBit(int32_frame_head, 19);
if (!bit20 && bit19)
{
logger.ErrorFormat("undefinde mpeg version");
return(false);
}
else if (!bit20 && !bit19)
{
mpeg_ver = MPEGVersion.Mpeg2_5;
}
else if (bit20 && !bit19)
{
mpeg_ver = MPEGVersion.Mpeg2;
}
else if (bit20 && bit19)
{
mpeg_ver = MPEGVersion.Mpeg1;
}
#endregion
#region 采样率
/*
|--------------------------------|
| bits | MPEG1 | MPEG2 | MPEG2.5 |
|--------------------------------|
| 0 0 | 44100 | 22050 | 11025 |
|--------------------------------|
| 0 1 | 48000 | 24000 | 12000 |
|--------------------------------|
| 1 0 | 32000 | 16000 | 8000 |
|--------------------------------|
*/
bool bit11 = Utils.GetBit(int32_frame_head, 11);
bool bit10 = Utils.GetBit(int32_frame_head, 10);
int[] sample_frequency_array = SamplingFrequencyMap[mpeg_ver];
if (bit11 && bit10)
{
logger.ErrorFormat("undefinde sample_frequency");
return(false);
}
else if (!bit11 && !bit10)
{
sampling_frequency = sample_frequency_array[0];
}
else if (!bit11 && bit10)
{
sampling_frequency = sample_frequency_array[1];
}
else if (bit11 && !bit10)
{
sampling_frequency = sample_frequency_array[2];
}
#endregion
#region 比特率
/*
|------------------------------------------------------------------|
| | MPEG1 | MPEG2/2.5(LSF) |
| bits |-----------------------------|-----------------------------|
| | LayerI | LayerII | LayerIII | LayerI | LayerII & LayerIII |
|------------------------------------------------------------------|
| 0000 | Free |
|------------------------------------------------------------------|
| 0001 | 32 | 32 | 32 | 32 | 8 |
|------------------------------------------------------------------|
| 0010 | 64 | 48 | 40 | 48 | 16 |
|------------------------------------------------------------------|
| 0011 | 96 | 56 | 48 | 56 | 24 |
|------------------------------------------------------------------|
| 0100 | 128 | 64 | 56 | 64 | 32 |
|------------------------------------------------------------------|
| 0101 | 160 | 80 | 64 | 80 | 40 |
|------------------------------------------------------------------|
| 0110 | 192 | 96 | 80 | 96 | 48 |
|------------------------------------------------------------------|
| 0111 | 224 | 112 | 96 | 112 | 56 |
|------------------------------------------------------------------|
| 1000 | 256 | 128 | 112 | 128 | 64 |
|------------------------------------------------------------------|
| 1001 | 288 | 160 | 128 | 144 | 80 |
|------------------------------------------------------------------|
| 1010 | 320 | 192 | 160 | 160 | 96 |
|------------------------------------------------------------------|
| 1011 | 352 | 224 | 192 | 176 | 112 |
|------------------------------------------------------------------|
| 1100 | 384 | 256 | 224 | 192 | 128 |
|------------------------------------------------------------------|
| 1101 | 416 | 320 | 256 | 224 | 144 |
|------------------------------------------------------------------|
| 1110 | 448 | 384 | 320 | 256 | 160 |
|------------------------------------------------------------------|
| 1111 | Bad |
|------------------------------------------------------------------|
|
*/
bool bit15 = Utils.GetBit(int32_frame_head, 15);
bool bit14 = Utils.GetBit(int32_frame_head, 14);
bool bit13 = Utils.GetBit(int32_frame_head, 13);
bool bit12 = Utils.GetBit(int32_frame_head, 12);
int[] bit_rate_array = BitRateMap[mpeg_ver][(int)layer];
uint bit_rate_index = (int32_frame_head >> 12) & 0xF;
bit_rate_kbps = bit_rate_array[bit_rate_index];
#endregion
#region 通道数
bool bit7 = Utils.GetBit(int32_frame_head, 7);
bool bit6 = Utils.GetBit(int32_frame_head, 6);
channels = (((int32_frame_head >> 6) & 0x3) == 3) ? 1 : 2;
#endregion
#region 计算填充
padding = (int32_frame_head >> 9) & 0x1;
#endregion
#region 计算帧大小
spf = SamplingPerFrame[mpeg_ver][(int)layer];
if (layer == Layers.LayerI)
{
frame_size = spf / 8 * (bit_rate_kbps * 1000) / sampling_frequency + padding * 4;
}
else if (layer == Layers.LayerII || layer == Layers.LayerIII)
{
frame_size = spf / 8 * (bit_rate_kbps * 1000) / sampling_frequency + padding;
}
#endregion
return(true);
}