Test Tool Base

Available Freeware programs that may be suitable to use as a base for the Spectrum Analyser / Multiple Channel Oscilloscope Simulator Module.

Note that this could be researched further.

Spectrum
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-goo...
Spectrum is a GStreamer plugin distributed with GStreamer.  This plugin runs an FFT on the audio signal and outputs spectrum data.  License: LGPL

Monoscope
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-goo...
Monoscope is a GStreamer plugin distributed with GStreamer.  This plugin is an audio visualisation element. It creates a coloured curve of the audio signal like on an oscilloscope.    License: LGPL

libvisual
http://sourceforge.net/projects/libvisual/
http://www.localhost.nl/~synap/libvisual-wiki/index.php/Main_Page
Libvisual is a library that comes between applications and audio visualisation plugins and provides every application that wants to draw visualisation a clean and easy to use API.  A set of plugins for this is already available within the GStreamer plugins.  While these plugins are more based on eye candy than being an information tool they still make a good base for programming a GStreamer test tool around.   Programming Language: C.  License: LGPL.

Baudline
http://www.baudline.com/index.html
This appears to be about the best of the linux spectrum analysers and oscilloscope tools.  Version 0.99 is meant to be GPL but the source doesn't seem to be available any longer.  Later versions are free to download but have restrictions including not being allowed to distribute.

XPortShot RT
Home site not working.
XPortShot RT is as GTK based real time oscilloscope simulator, capable of displaying multiple channels of data.  (Free to use -  Shareware ??? but source is available)

xsr
http://www.ugcs.caltech.edu/~john/computer/xsr/xsr.html
xsr is a small program based solely on Xlib (so its fast and small) that reads in audio data from the soundcard and displays it on the root window in an entertaining and neat fashion. it also does fourier analysis and looks really cool. it is fast enough to run in the background without interfering with anything.

FFT Explorer
http://www.arachnoid.com/FFTExplorer/
A free (GPL) Linux (Xwindows GUI-based) utility that performs spectral analysis on real-time data from a sound card or other source, and can analyze the spectra of various sound file types. It can also accept data from many kinds of applications in plain-text form by way of the "named pipe" feature of Linux. Also available is an unbundled, command-line embodiment of Fast Fourier technology that may be more suitable for tutorial applications on small systems.

Wavelet Sound Explorer
http://gandolf.homelinux.org/wavelet/
Wavelet Sound Explorer is a freeware program for Windows that lets you view sound files in the frequency/time domain.

Qucs - Quite Universal Circuit Simulator
http://qucs.sourceforge.net/
Qucs, is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit. After that simulation has finished you can view the simulation results on a presentation page or window. Runs on Linux.  The GUI is based on Qt.  Licensed as GPL.

SignalGen
http://www.arachnoid.com/SignalGen/
SignalGen is a free (GPL) Linux (Xwindows GUI-based) utility that generates relatively low frequency signals with the use of a PC sound card. It is an entirely suitable replacement for a hardware signal generator if one doesn't require signals of a frequency higher than about 50 KHz. SignalGen creates any frequency between a fraction of one Hertz and an upper limit determined by your sound card's capabilities.

Sonic Visualiser v1.5: 
http://www.sonicvisualiser.org/
View and analyse the contents of music audio files. (License GPL)

Freq 5.1   
http://www.hitsquad.com/smm/programs/Freq_5.1
Spectrum analyzer.  Requires Linux with SVGALib or under X-Windows (using SRGP)(License GPL)

svgafft   
http://www.hitsquad.com/smm/programs/svgafft/
SVGALIB based spectrum analyzer for /dev/dsp(License GPL)

Waterfall
http://www.sethoscope.net/waterfall/
Waterfall is a visualization plugin for XMMS. It's yet another spectrum analyzer, but it presents the data better, which makes it more interesting to watch. Most analyzers show a changing 2D bar graph of frequency vs. intensity. This one shows a scrolling 4D plot of frequency, time, intensity, and a configurable fourth variable mapped to hue.(License GPL)

XMMS Fine Spectrum Analyzer
http://linuxappfinder.com/package/xmms-finespectrum

Frequency Analyser
http://www.relisoft.com/freeware/index.htm
While this is a windows program, the C++ source code along with several tutorials including FFT and the physics of sound are available with it.