Changing Spectrum Analysis Settings

Click the Settings button in the Spectrum Analysis window to display the Spectrum Settings dialog. This dialog contains controls to determine how spectral information is calculated and displayed.

Item

Description

FFT size

Choose a setting from the drop-down list to set the size in samples of the analysis window and number of discrete frequencies analyzed. Higher numbers produce increased frequency resolution at the expense of lower time resolution and longer computational time. The following example illustrates the results of 128-point and 2048-point FFT sizes on a 44,100 Hz sound file:

FFT Size

Time Analyzed (seconds)

Frequency Bands

Frequency Bandwidth (Hz)

128

0.003 (128/44100)

64 (128/2)

345 (44100/128)

2048

0.046 (2048/44100)

1024 (2048/2)

22 (44100/2048)

FFT overlap

Specify the percentage of overlap between FFT analysis windows. Overlapping allows for more accurate analysis. Lower settings decrease the number of distinct analysis functions performed, which decreases processing time. High settings allow for more analysis, but can result in slow processing.

Smoothing window

Choose a setting from the drop-down list to determine the window function applied to the input data before analysis. The window function affects the sharpness of peaks in an FFT graph and the leakage into neighboring frequencies. The effects of these windows can be easily seen by analyzing a single sine wave:

Blackman-Harris

Using the Blackman-Harris window results in the least sideband leakage at the expense of rounded graph peaks.

Hamming Hanning

Hamming and Hanning windows are commonly used in audio applications.

Rectangle

No window is applied. Using a rectangular window results in a very sharp peak but high leakage.

Triangle

A triangle window (also called a Bartlett or Parzen window) results in less leakage than the rectangle window.

Slices displayed

The Spectrum Analysis window can display up to 64 spectrum graphs, each representing a point in time.

Type a value in the Slices displayed box to specify the number of FFT slices displayed. Each slice represents FFT size samples in time.

When displaying multiple slices, click the Forward or Backward radio button to determine whether slices are layered in chronological order. When Forward is selected, the first slice is displayed in the foreground; when Backward is selected, the last slice is displayed in the foreground.

In the spectrum graph, drag the Slice slider to show or hide graphs from the display.

Set sonogram resolution

Select the Set sonogram resolution check box and specify the number of FFT samplings used in the sonogram. When this check box is not selected, the number of samplings is determined by the length of the selection and the FFT overlap setting.

Increasing the samplings increases the horizontal graph resolution but requires more processing time.

Display Range

The Display Range controls allow you to adjust the range of each graph.

Channel

Choose a setting from the drop-down list to choose which graph you want to edit, or choose All Channels to edit all graphs.

Sync graphs

If you are analyzing a multichannel file, select the Sync graphs check box to synchronize the displays so you can view the same region of the FFT in all channels.

Logarithmic graphing

Select the check box to display the x-axis in logarithmic mode rather than linear mode. In logarithmic mode, more of the graph is devoted to lower frequencies.

Logarithmic graphing affects the display only when Normal Display is selected.

Freq. Min

Specify the lowest frequency displayed in the graphs.

The Freq Min and Max settings affect the display only when Normal Display is selected.

Max

Specify the highest frequency displayed in the graphs.

Ceiling

Specify the highest amplitude displayed in the graphs.

Floor

In the box, specify the lowest amplitude displayed in the graphs.

Hold peaks during monitoring

Select this check box if you want to mark the highest value of each frequency bin.

Maintain last monitored view

Select this check box if you want to maintain the state of the spectrum graph when you stop playback. If this check box is not selected, the graph will revert to the cursor position.