ComplexSignal Class

A complex discrete-time signal is any complex-valued function of integers. This class is used to keep audio signals represented in complex numbers so they are suitable to be converted to and from the frequency domain in either analytic or Fourier transformed forms.

References:

• Wikipedia, The Free Encyclopedia. Analytics Signal. Available on: http://en.wikipedia.org/wiki/Analytic_signal

If your signal has a length that is a power of two, you can use the following code directly to create your audio signal and obtain its spectrogram:

// Create complex audio signal
ComplexSignal complexSignal = ComplexSignal.FromSignal( signal );

// Do forward Fourier transformation
complexSignal.ForwardFourierTransform( );

// Generate spectrogram
complexSignal.ToBitmap(512,512);

However, if your signal is too lengthy, or if your signal is not yet in a power of two size, you can use a temporal window to slice your signal into smaller cuts, as shown below. In the example, an audio file is being read and its contents are being decoded and stored into a Signal object. Afterwards, an audio window is being used to cut the signal into smaller, power-of-two size signals which can then be transformed into the frequency (Fourier) domain.

string fileName = "audio.wav";

WaveDecoder sourceDecoder = new WaveDecoder(fileName);

// Decode the file and store into a signal
Signal sourceSignal = sourceDecoder.Decode();

// Create Hamming window so that signal will fit into power of 2:           
RaisedCosineWindow window = RaisedCosineWindow.Hamming(1024);

// Splits the source signal by walking each 512 samples, then creating 
// a 1024 sample window. Note that this will result in overlapped windows.
Signal[] windows = sourceSignal.Split(window, 512);

// You might need to import Accord.Math in order to call this:
ComplexSignal[] complex = windows.Apply(ComplexSignal.FromSignal);

// Forward to the Fourier domain
complex.ForwardFourierTransform();

Reference