Can't fade out with FadeInOutSampleProvider - naudio

Question

I'm trying to use NAudio's FadeInOutSampleProvider to fade in a sample and fade it out. The fade in works OK, but instead of fading out gradually I get abrupt silence from where the fade-out should begin.
What's the correct way to fade out with FadeInOutSampleProvider?
Here's how I'm trying to do it:
IWaveProvider waveSource; // initialised by reading a WAV file
// The ISampleProvider will be the underlying source for the following operations
ISampleProvider sampleSource = waveSource.ToSampleProvider();
// Create a provider which defines the samples we want to fade in
// (including the full-volume "middle" of the final output)
ISampleProvider fadeInSource = new OffsetSampleProvider(sampleSource);
fadeInSource.TakeSamples = most_of_file; // calculation omitted for brevity
// Create a provider which defines the samples we want to fade out:
// We will play these samples when fadeInSource is finished
ISampleProvider fadeOutSource = new OffsetSampleProvider(sampleSource);
fadeOutSource.SkipOverSamples = fadeInSource.TakeSamples;
// Wrap the truncated sources in FadeInOutSampleProviders
var fadeIn = new FadeInOutSampleProvider(fadeInSource);
fadeIn.BeginFadeIn(500); // half-second fade
var fadeOut = new FadeInOutSampleProvider(fadeOutSource);
fadeOut.BeginFadeOut(500);
// doc-comments suggest the fade-out will begin "after first Read"
I'm expecting fadeOut to initially read non-zero samples from 500ms before the end of the original source, but fade out to zeros by the end of the source.
However, when I play fadeIn to completion, then play fadeOut, I find that the very first Read call to fadeOut fills the buffer with zeros.
Am I doing something wrong? Or is there a bug in NAudio?
Note: I'm handling the sequential playback using a ConcatenatingSampleProvider which I implemented myself — I can't anything similar in NAudio's API. It's pretty trivial, so I've omitted the source here.

Answer 1

The problem is you're trying to reuse sampleSource twice in your graph. So sampleSource has already been read to the end before anything is read from fadeOutSource. Probably for your usage, it would be better for FadeInOutSampleProvider to be able to "schedule" a fade-out after a known number of samples.
An alternative approach is a FadeOutSampleProvider that caches the fade-out duration, and then when it detects the end of its source has been reached, it returns the cached portion faded out. It does mean latency is introduced.