Spectrogram is a .NET library which makes it easy to create spectrograms from pre-recorded signals or live audio from the sound card. This library supports .NET Framework (4.5) and .NET Core (3.0) and can be installed using NuGet.

The code below converts a WAV file to a spectrograph and saves it as an image. This code analyzed Mozart's Piano Sonata No. 11 in A major to produce the picture above.

// load audio and process FFT
var spec = new Spectrogram.Spectrogram(sampleRate: 8000, fftSize: 2048, step: 700);
float[] values = Spectrogram.Tools.ReadWav("mozart.wav");
spec.AddExtend(values);

// convert FFT to an image and save it
Bitmap bmp = spec.GetBitmap(intensity: 2, freqHigh: 2500);
spec.SaveBitmap(bmp, "mozart.jpg");

This code analyzes audio from HAL's famous quote, "I'm sorry Dave, I'm afraid I can't do that". The output is can be rendered using different colormaps.

// load audio and process FFT
var spec = new Spectrogram.Spectrogram(sampleRate: 15000, fftSize: 4096, step: 400);
float[] values = Spectrogram.Tools.ReadMp3("cant-do-that.mp3");
spec.AddExtend(values);

// convert FFT to an image and save it
Bitmap bmp = spec.GetBitmap(intensity: .2, freqHigh: 1000,
    colormap: Spectrogram.Colormap.grayscaleInverted);
spec.SaveBitmap(bmp, "cant-do-that-grayscale-inverted.jpg");

colormap	sample output
Grayscale Inverted is used in many scientific publications when analyzing things like human voices and bird sounds
Grayscale provides highest contrast output but does not benefit from color vision
Viridis is the default colormap. It was specifically designed to represent 2D data in a way ideally suited for human vision.
vdGreen is the default colormap used for QRSS-VD, a very old software project of mine.

Experimenters with ultra-narrowband radio transmissions often use continuous wave frequency-shifting radio transmitters to send data at very low rates over very long distances using very little power. See What is QRSS? for more information.

The following code produces a QRSS spectrogram from an MP3 file. This program took less than 5 seconds to analyze 30 minutes of audio, producing the image below.

// load audio and process FFT
var spec = new Spectrogram.Spectrogram(sampleRate: 8000, fftSize: 16384, step: 8000);
float[] values = Spectrogram.Tools.ReadMp3("qrss-w4hbk.mp3");
spec.AddExtend(values);

// convert FFT to an image and save it
Bitmap bmp = spec.GetBitmap(intensity: 1.5, freqLow: 1100, freqHigh: 1500,
    showTicks: true, tickSpacingHz: 50, tickSpacingSec: 60);
spec.SaveBitmap(bmp, "qrss.png");

This project comes with a few interactive applications which serve as useful references for some of the ways this Spectrogram library can be used.

If you want to see what this library can do without downloading source code, click-to-run (EXE) demos are available in SpectrogramDemo.zip

A demo program is included which monitors the sound card and continuously creates spectrograms from microphone input. It runs fast enough that the entire bitmap can be recreated on each render. This means brightness and color adjustments can be applied to the whole image, not just new parts.

This demo program was created to demonstrate Spectrogram and ScottPlot working together.

• Argo (website) - closed-source QRSS viewer for Windows
• SpectrumLab (website) - closed-source spectrum analyzer for Windows
• QrssPIG (GitLab) - open-source spectrograph for Raspberry Pi (C++)
• Lopora (GitHub) - open-source spectrograph (Python 3)
• QRSS VD (GitHub) - open source spectrograph (Python 2)

• What is QRSS?
• QRSS and you
• QRSS (slow CW)