//////////////////////////////////////////////////////////////////////////
// private methods
//////////////////////////////////////////////////////////////////////////
private void Read()
{
// Since we've likely just looked for the ID3v1 tag, start at the end of the
// file where we're least likely to have to have to move the disk head.
long last = file.LastFrameOffset;
if (last < 0)
{
TagLibDebugger.Debug("Mpeg.Properties.Read() -- Could not find a valid last MPEG frame in the stream.");
return;
}
file.Seek(last);
MpegHeader last_header = new MpegHeader(file.ReadBlock(4));
long first = file.FirstFrameOffset;
if (first < 0)
{
TagLibDebugger.Debug("Mpeg.Properties.Read() -- Could not find a valid first MPEG frame in the stream.");
return;
}
if (!last_header.IsValid)
{
long pos = last;
while (pos > first)
{
pos = file.PreviousFrameOffset(pos);
if (pos < 0)
{
break;
}
file.Seek(pos);
MpegHeader header = new MpegHeader(file.ReadBlock(4));
if (header.IsValid)
{
last_header = header;
last = pos;
break;
}
}
}
// Now jump back to the front of the file and read what we need from there.
file.Seek(first);
MpegHeader first_header = new MpegHeader(file.ReadBlock(4));
if (!first_header.IsValid || !last_header.IsValid)
{
TagLibDebugger.Debug("Mpeg.Properties.Read() -- Page headers were invalid.");
return;
}
// Check for a Xing header that will help us in gathering information about a
// VBR stream.
int xing_header_offset = MpegXingHeader.XingHeaderOffset(first_header.Version,
first_header.ChannelMode);
file.Seek(first + xing_header_offset);
MpegXingHeader xing_header = new MpegXingHeader(file.ReadBlock(16));
// Read the length and the bitrate from the Xing header.
if (xing_header.IsValid && first_header.SampleRate > 0 && xing_header.TotalFrames > 0)
{
int [] block_size = { 0, 384, 1152, 1152 };
double time_per_frame = block_size [first_header.Layer];
time_per_frame = first_header.SampleRate > 0 ? time_per_frame / first_header.SampleRate : 0;
duration = new TimeSpan((int)(time_per_frame * xing_header.TotalFrames) * TimeSpan.TicksPerSecond);
bitrate = (int)(duration > TimeSpan.Zero ? ((xing_header.TotalSize * 8L) / duration.TotalSeconds) / 1000 : 0);
}
// Since there was no valid Xing header found, we hope that we're in a constant
// bitrate file.
// TODO: Make this more robust with audio property detection for VBR without a
// Xing header.
else if (first_header.FrameLength > 0 && first_header.Bitrate > 0)
{
int frames = (int)((last - first) / first_header.FrameLength + 1);
duration = TimeSpan.FromSeconds((double)(first_header.FrameLength * frames) / (double)(first_header.Bitrate * 125) + 0.5);
bitrate = first_header.Bitrate;
}
sample_rate = first_header.SampleRate;
channels = first_header.ChannelMode == MpegChannelMode.SingleChannel ? 1 : 2;
version = first_header.Version;
layer = first_header.Layer;
channel_mode = first_header.ChannelMode;
is_copyrighted = first_header.IsCopyrighted;
is_original = first_header.IsOriginal;
}
