iSpectrum Analyzer Manual

iSpectrum is an easy to use audio spectrum analyzer that allows the user to view live audio in a standard frequency plot, a stereo oscilloscope view and a waterfall display. The user can adjust the display resolution, center frequency and save images to disk.

iSpectrum requires a live audio input device (microphone or line in).




Selecting the Waterfall button switches the display to the Waterfall and audio Spectrum views.

Selecting the Oscilloscope button switches the display to the Oscilloscope view.

Waterfall view, in which a one pixel high line is added to the bottom for each FFT plot that is shown in the FFT view below it. The display scrolls up as each entry is added. The lighter the color the stronger the corrisponding signal. To determine the frequency of any pixel on the waterfall view, enable grid lines by checking the Grid Lines check box. Also you could enable the Frequency Marker and using the left and right arrow keys move the marker over to the signal of interest to get an accurate frequency reading. Using you mouse and click on the FFT plot also causes the frequency marker to move to the point of you choosing.

FFT view displays an FFT plot of data from the audio input device you select using the Input device popup. The audio is always sampled at 44.1Kz and run through an FFT. The size of the FFT is determined by the bandwidth you select using the Bandwidth slider.

The window popup allows you to select what windowing function is applied. (see below)

The Input channel popup allows you to select which audio channel (Left/Right) is used to collect the audio samples that are to be plotted.

When the samples have been run through the FFT it can be plotted as is by selecting Standard from the plot popup. If you would like to see a Normalized plot (a plot in which everything is plotted relative to the highest signal) then select Normalized. When the Normalized plot is selected, the Input Gain slider is disabled.

The Pause button allows to freeze all views.

The Average popup allows you to plot the average of 2, 4, or 6 FFT samples. This can help eliminate a good deal of noise from the displayed plot.

The Input Gain slider can only be used when the plot popup is set to standard and the input device has volume controls. This slider amplifies or attenuates the sampled audio signal. The amount of attenuation or amplification possible is determined by the input device you are using.

The Filter button causes the Filter Drawer and filter center frequency marker to appear.

The Input device popup allows you to select which audio input device data is captured from.

The Output device popup allows you to select which audio output device the data is sent to after passing through the filters if Pass Through is enabled.

The Freq. Marker checkbox controls the display of the frequency marker. The frequency marker can be moved by using the left or right arrow keys. The marker can also be moved by mouse clicking on a point of interest in the FFT view. Midway on marker is displayed the frequency and relative db value.

The Play Through checkbox allows you to send the filtered audio to an output device (speaker etc.)

The Source popup controls the display of the grid line overlay.

The Faster CPU check box allows to reduce the load on your system. This is useful if you have an older/slower system. When you uncheck this option the program only FFTs and plots every third block of samples.

The Bandwidth slider allows you to select the bandwidth resolution of the display, the options are 1, 2, 5, 10, 20 and 40 hz /pixel.

The Center Frequency slider allows you to shift the view so that a selected frequency is at the center of the display.


Filter Drawer Controls

The Enable check box turns the selected filter on or off.

The Filter popup selects the type of filter to apply: Low Pass, High Pass, Band Pass or Notch.

Wikipedia Reference

In signal processing, a digital biquad filter is a second-order recursive linear filter, containing two poles and two zeros. "Biquad" is an abbreviation of "biquadratic", which refers to the fact that in the Z domain, its transfer function is the ratio of two quadratic functions.

High-order recursive filters can be highly sensitive to quantization of their coefficients, and can easily become unstable. This is much less of a problem with first and second-order filters; therefore, higher-order filters are typically implemented as serially-cascaded biquad sections (and a first-order filter if necessary). The two poles of the biquad filter must be inside the unit circle for it to be stable. In general, this is true for all filters i.e. all poles must be inside the unit circle for the filter to be stable.

The Frequency slider adjusts the center frequency of the Notch and Band Pass filters and the cutoff for the High Pass and Low Pass filters.

The Bandwidth slider adjusts the width or "Q" of the filters.


FFT WIndowing Functions

Wikipedia Reference

In signal processing, a window function (also known as an apodization function or tapering function) is a mathematical function that is zero-valued outside of some chosen interval. For instance, a function that is constant inside the interval and zero elsewhere is called a rectangular window, which describes the shape of its graphical representation. When another function or waveform/data-sequence is multiplied by a window function, the product is also zero-valued outside the interval: all that is left is the part where they overlap; the "view through the window". Applications of window functions include spectral analysis, filter design, and beamforming. In typical applications, the window functions used are non-negative smooth "bell-shaped" curves, though rectangle, triangle, and other functions can be used.

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