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OffsetSampleProvider.md

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Using OffsetSampleProvider

OffsetSampleProvider allows you to extract a sub-section of another ISampleProvider. You can skip over the start of the source ISampleProvider with SkipOver and limit how much audio you play from the source with Take. You can also insert leading and trailing silence with DelayBy and LeadOut.

Take is especially useful when working with never-ending ISampleProvider sources such as SignalGenerator.

Let's look at an example. Here, the OffsetSampleProvider uses a SignalGenerator as its source. It inserts 1 second of silence before playing for 5 seconds and then inserts 1 extra second of silence at the end:

// the source ISampleProvider
var sineWave = new SignalGenerator() { 
    Gain = 0.2, 
    Frequency = 500,
    Type = SignalGeneratorType.Sin};
var trimmed = new OffsetSampleProvider(sineWave) {
    DelayBy = TimeSpan.FromSeconds(1),
    Take = TimeSpan.FromSeconds(5),
    LeadOut = TimeSpan.FromSeconds(1)
};

For another example, let's say we have an audio file and we want to skip over the first one minute, and then take a 30 second excerpt and write it to a WAV file:

var source = new AudioFileReader("example.mp3");
var trimmed = new OffsetSampleProvider(source) {
    SkipOver = TimeSpan.FromSeconds(30),
    Take = TimeSpan.FromSeconds(60),
WaveFileWriter.CreateWaveFile16(outputFilePath, trimmed);

Skip and Take Extension Methods

NAudio also offers some helpful extension methods to simplify the above task. Skip and Take are extension methods on ISampleProvider and create an OffsetSampleProvider behind the scenes. So the previous example could be rewritten:

var trimmed = new AudioFileReader("example.mp3")
                   .Skip(TimeSpan.FromSeconds(30))
                   .Take(TimeSpan.FromSeconds(60));
WaveFileWriter.CreateWaveFile16(outputFilePath, trimmed);

Optimizing SkipOver

Note that SkipOver is implemented by simply reading that amount of audio from the source and discarding it. Obviously if the source is a file as in this example, it would be more efficient just to position it to the desired starting point:

var source = new AudioFileReader("example.mp3");
source.CurrentTime = TimeSpan.FromSeconds(30);
var trimmed = source.Take(TimeSpan.FromSeconds(60));
WaveFileWriter.CreateWaveFile16(outputFilePath, trimmed);

Sample Accurate Trimming

As well as the TimeSpan based versions of the SkipOver, DelayBy Take and LeadOut properties, there are sample based ones, for when you need accurate control over exactly how many samples of audio to skip and take. These are called SkipOverSamples, DelayBySamples, TakeSamples and LeadOutSamples. They're calculated automatically for you when you use the TimeSpan based properties, but you can set them directly yourself.