这听起来很有趣:)
这是我为此编写的示例。它接受输入文件名模式的列表（假设当前目录）和输出文件的名称。它将文件拼接在一起，在一个文件的末尾淡出约 1 秒，然后在下一个文件的约 1 秒内淡入，依此类推。注意：它不会混合约 1 秒的重叠。不想那样做:)
我使用ReadNextSampleFrameWaveFileReader 上的方法将数据读取为 IEEE 浮点样本（每个通道一个浮点数）。这使得单方面应用音量调整变得更加容易，而不必担心实际的输入 PCM 表示。在输出上，它WriteSamples在 writer 上使用来写入调整后的样本。
我第一次尝试使用 NAudio FadeInFadeOutSampleProvider。但是当你有多个音频通道时，我发现了一个奇怪的错误。
因此，代码手动将体积应用于每个样本读取，在每个文件的开头（第一个文件除外）将体积从 0.0 增加到 1.0。然后它直接复制文件的“中间”。然后在文件结束前大约 1 秒（实际上， ( WaveFormat.SampleRate* WaveFormat.Channels) 在文件结束前采样），它将音量从 1.0f 降低到 0.0f。
我测试了一下，使用sox生成一个5秒长的440Hz正弦波文件，采样率=96K，立体声，如下：
sox -n -c 2 -r 96000 -b 24 sine.wav synth 5 sine 440
测试调用如下：
FadeWeaver.FadeWeave("weaved.wav", "sine.wav", "sine.wav", "sine.wav");
这是代码：
public class FadeWeaver
{
static
public
void
FadeWeave( string _outfilename,
params string [] _inpatterns )
{
WaveFileWriter output = null;
WaveFormat waveformat = null;
float [] sample = null;
float volume = 1.0f;
float volumemod = 0.0f;
// Add .wav extension to the output if not specified.
string extension = Path.GetExtension(_outfilename);
if( string.Compare(extension, ".wav", true) != 0 ) _outfilename += ".wav";
// Assume we're using the current directory. Let's get the
// list of filenames.
List<string> filenames = new List<string>();
foreach( string pattern in _inpatterns )
{
filenames.AddRange(Directory.GetFiles(Directory.GetCurrentDirectory(), pattern));
}
try
{
// Alrighty. Let's march over them. We'll index them (rather than
// foreach'ing) so that we can monitor first/last file.
for( int index = 0; index < filenames.Count; ++index )
{
// Grab the file and use an 'audiofilereader' to load it.
string filename = filenames[index];
using( WaveFileReader reader = new WaveFileReader(filename) )
{
// Get our first/last flags.
bool firstfile = (index == 0 );
bool lastfile = (index == filenames.Count - 1);
// If it's the first...
if( firstfile )
{
// Initialize the writer.
waveformat = reader.WaveFormat;
output = new WaveFileWriter(_outfilename, waveformat);
}
else
{
// All files must have a matching format.
if( !reader.WaveFormat.Equals(waveformat) )
{
throw new InvalidOperationException("Different formats");
}
}
long fadeinsamples = 0;
if( !firstfile )
{
// Assume 1 second of fade in, but set it to total size
// if the file is less than one second.
fadeinsamples = waveformat.SampleRate;
if( fadeinsamples > reader.SampleCount ) fadeinsamples = reader.SampleCount;
}
// Initialize volume and read from the start of the file to
// the 'fadeinsamples' count (which may be 0, if it's the first
// file).
volume = 0.0f;
volumemod = 1.0f / (float)fadeinsamples;
int sampleix = 0;
while( sampleix < (long)fadeinsamples )
{
sample = reader.ReadNextSampleFrame();
for( int floatix = 0; floatix < waveformat.Channels; ++floatix )
{
sample[floatix] = sample[floatix] * volume;
}
// Add modifier to volume. We'll make sure it isn't over
// 1.0!
if( (volume = (volume + volumemod)) > 1.0f ) volume = 1.0f;
// Write them to the output and bump the index.
output.WriteSamples(sample, 0, sample.Length);
++sampleix;
}
// Now for the time between fade-in and fade-out.
// Determine when to start.
long fadeoutstartsample = reader.SampleCount;
//if( !lastfile )
{
// We fade out every file except the last. Move the
// sample counter back by one second.
fadeoutstartsample -= waveformat.SampleRate;
if( fadeoutstartsample < sampleix )
{
// We've actually crossed over into our fade-in
// timeframe. We'll have to adjust the actual
// fade-out time accordingly.
fadeoutstartsample = reader.SampleCount - sampleix;
}
}
// Ok, now copy everything between fade-in and fade-out.
// We don't mess with the volume here.
while( sampleix < (int)fadeoutstartsample )
{
sample = reader.ReadNextSampleFrame();
output.WriteSamples(sample, 0, sample.Length);
++sampleix;
}
// Fade out is next. Initialize the volume. Note that
// we use a bit-shorter of a time frame just to make sure
// we hit 0.0f as our ending volume.
long samplesleft = reader.SampleCount - fadeoutstartsample;
volume = 1.0f;
volumemod = 1.0f / ((float)samplesleft * 0.95f);
// And loop over the reamaining samples
while( sampleix < (int)reader.SampleCount )
{
// Grab a sample (one float per channel) and adjust by
// volume.
sample = reader.ReadNextSampleFrame();
for( int floatix = 0; floatix < waveformat.Channels; ++floatix )
{
sample[floatix] = sample[floatix] * volume;
}
// Subtract modifier from volume. We'll make sure it doesn't
// accidentally go below 0.
if( (volume = (volume - volumemod)) < 0.0f ) volume = 0.0f;
// Write them to the output and bump the index.
output.WriteSamples(sample, 0, sample.Length);
++sampleix;
}
}
}
}
catch( Exception _ex )
{
Console.WriteLine("Exception: {0}", _ex.Message);
}
finally
{
if( output != null ) try{ output.Dispose(); } catch(Exception){}
}
}
}