Changing Spectrum Analysis Settings |
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The commands on the Spectrum Analysis Options menu allow you to determine how spectral information is calculated and displayed.
From the Options menu, choose Auto Update to update the Spectrum Graph with any change in the cursor position or selection of the current file. A check mark is displayed next to the command when it is selected.
You can continue to make selections in the sound file with the Spectrum Analysis window open (just move the cursor or make selections as you normally would in Sound Forge).
From the Options menu, choose Settings. The Spectrum Settings dialog is displayed.
Use the Spectrum Settings dialog to adjust the FFT settings:
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) |
Frequecy 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:
Setting |
Description |
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. |
Hamming, Hanning, and Blackman |
Hamming, Hanning, and Blackman windows are commonly used in audio applications. |
Blackman-Harris |
Using the Blackman-Harris window results in the least sideband leakage at the expense of rounded graph peaks. |
Click the OK button.
The Spectrum Analysis window can display up to 64 spectrum graphs, each representing a point in time.
From the Options menu, choose Settings. The Spectrum Settings dialog is displayed.
Adjust the Slices displayed settings to specify the number of graphs displayed:
Slices displayed: specify the number of FFT slices displayed. Each slice represents FFT size samples in time.
Forward/Backward: 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.
Click the OK button to return to the spectrum graph.
In the spectrum graph, drag the Slice slider to show or hide graphs from the display.
From the Options menu, choose Settings. The Spectrum Settings dialog is displayed.
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.
Click the OK button.
From the Options menu, choose Settings. The Spectrum Settings dialog is displayed.
Select the Logarithmic graphing check box to display the x-axis in logarithmic mode rather than linear mode. In logarithmic mode, more area of the graph is devoted to lower frequencies.
Logarithmic
graphing affects the display only when Normal
display is selected on the Display menu.
Click the OK button.
From the Options menu, choose Settings. The Spectrum Settings dialog is displayed.
Adjust the Display Range controls:
Control |
Description |
Freq. Min |
Specify the lowest frequency displayed in graphs when Zoom to Range is selected on the Display menu. |
Max. |
Specify the highest frequency displayed in graphs when Zoom to Range is selected on the Display menu. |
Ceiling |
Specify the highest amplitude level displayed in graphs when Zoom to Range is selected on the Display menu. |
Floor |
Specify the lowest amplitude level displayed in graphs when Zoom to Range is selected on the Display menu. |
Click the OK button.
From the Options menu, choose Monitor Playback. A check mark is displayed next to the command when it is selected.
The Spectrum Graph will update in real time as the current file plays. The response of the graph will depend on the current FFT size and the speed of your computer.
From the Options menu, choose Monitor Input. A check mark is displayed next to the command when it is selected.
The Spectrum Graph will update in real time to any input to your selected sound card. This feature makes it possible to perform real-time analysis of room acoustics or other live sources.
From the Options menu, choose Settings. The Spectrum Settings dialog is displayed.
Adjust the Monitor Settings controls:
Select the Hold peaks during monitoring check box if you want to mark the highest value of each frequency bin.
Select the Maintain last monitored view 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.
Click the OK button.