/// <summary>
/// Construct the MP3 header using the settings of the global flags.
/// <img src="1000px-Mp3filestructure.svg.png">
/// </summary>
/// <param name="gfp">
/// global flags
/// </param>
/// <param name="buffer">
/// header
/// </param>
private void setLameTagFrameHeader(LameGlobalFlags gfp, byte[] buffer)
{
var gfc = gfp.internal_flags;
// MP3 Sync Word
buffer[0] = shiftInBitsValue(buffer[0], 8, 0xff);
buffer[1] = shiftInBitsValue(buffer[1], 3, 7);
buffer[1] = shiftInBitsValue(buffer[1], 1, gfp.out_samplerate < 16000 ? 0 : 1);
// Mp3Version
buffer[1] = shiftInBitsValue(buffer[1], 1, gfp.version);
// 01 == Layer 3
buffer[1] = shiftInBitsValue(buffer[1], 2, 4 - 3);
// Error protection
buffer[1] = shiftInBitsValue(buffer[1], 1, !gfp.error_protection ? 1 : 0);
// Bit rate
buffer[2] = shiftInBitsValue(buffer[2], 4, gfc.bitrate_index);
// Frequency
buffer[2] = shiftInBitsValue(buffer[2], 2, gfc.samplerate_index);
// Pad. Bit
buffer[2] = shiftInBitsValue(buffer[2], 1, 0);
// Priv. Bit
buffer[2] = shiftInBitsValue(buffer[2], 1, gfp.extension);
// Mode
buffer[3] = shiftInBitsValue(buffer[3], 2, (int)gfp.mode);
// Mode extension (Used with Joint Stereo)
buffer[3] = shiftInBitsValue(buffer[3], 2, gfc.mode_ext);
// Copy
buffer[3] = shiftInBitsValue(buffer[3], 1, gfp.copyright);
// Original
buffer[3] = shiftInBitsValue(buffer[3], 1, gfp.original);
// Emphasis
buffer[3] = shiftInBitsValue(buffer[3], 2, gfp.emphasis);
/* the default VBR header. 48 kbps layer III, no padding, no crc */
/* but sampling freq, mode and copyright/copy protection taken */
/* from first valid frame */
buffer[0] = 0xff;
var abyte = unchecked ((byte)(buffer[1] & 0xf1));
int bitrate;
if (1 == gfp.version)
{
bitrate = XING_BITRATE1;
}
else
{
if (gfp.out_samplerate < 16000)
{
bitrate = XING_BITRATE25;
}
else
{
bitrate = XING_BITRATE2;
}
}
if (gfp.VBR == VbrMode.vbr_off)
{
bitrate = gfp.brate;
}
byte bbyte;
if (gfp.free_format)
{
bbyte = 0x00;
}
else
{
bbyte = (byte)(16 * lame.BitrateIndex(bitrate, gfp.version, gfp.out_samplerate));
}
/*
* Use as much of the info from the real frames in the Xing header:
* samplerate, channels, crc, etc...
*/
if (gfp.version == 1)
{
/* MPEG1 */
buffer[1] = (byte)(abyte | 0x0a); // was 0x0b;
abyte = (byte)(buffer[2] & 0x0d); // AF keep also private bit
buffer[2] = (byte)(bbyte | abyte); // 64kbs MPEG1 frame
}
else
{
/* MPEG2 */
buffer[1] = (byte)(abyte | 0x02); // was 0x03;
abyte = (byte)(buffer[2] & 0x0d); // AF keep also private bit
buffer[2] = (byte)(bbyte | abyte); // 64kbs MPEG2 frame
}
}
