Explore Signals

Step 3 of 5 in Using Signal Analyzer App

You can use theSignal Analyzerapp to visualize signals in the time, frequency, and time-frequency domains. The panner and zoom actions enable you to quickly navigate through a signal, and options to change axes units and scale let you customize display settings. You can link displays in the time domain, and extract signal regions of interest.

Plot Signals

Select a signal by clicking its name in the Workspace browser or the Signal table. Then plot your selection by dragging it to a display. This action also selects the check box to the left of the signalNameon the Signal table. You can also plot a signal by selecting this check box. The app displays a set of axes with the time-domain waveform and aTimetab with options to control the view. The app does not support plotting in the frequency and time-frequency domains for non-finite signals.

If you drag a matrix from the Workspace browser to a display, the app automatically plots each column as a separate signal, up to a maximum of 10 columns. The app creates signals in the Signal table for the remaining columns, but you must drag the additional signals to the display.

Note

Signals with no time information are plotted in units of samples on thex-axis. Signals with time information are plotted in units of time on thex-axis. To plot several signals on the same display, ensure that they all have time information or are all in samples. Otherwise, you get a warning.

View Signals on Multiple Plots

ClickDisplay Grid创建或删除显示。

Move Signals Between Displays

To move a signal from one display to another, click the plotted line or select its name on itsLegend, for example,. Click the resulting thicker line and drag it to the target display.

Note

If you move the real part or the imaginary part of a complex signal from one display to another, the app moves both parts of the signal.

Visualize Signal Spectra

Use theSignal Analyzerapp to analyze signals in the frequency domain. To activate the frequency-domain view of a signal, clickSpectrumon theDisplaytab and selectSpectrum. The app displays a set of axes with the signal spectrum, and aSpectrumtab with options to control the view.

If the panner is activated and is zoomed in on a particular region of interest, or if you zoom in on a region of the signal in the time plot using one of the zoom actions on theDisplaytab, the spectrum in the display corresponds to the region of interest, not the whole signal.
You cannot zoom out in frequency beyond the Nyquist range.
You cannot plot the spectrum of a non-finite signal.
To change the frequency scale, selectLinearorLogin theFrequency Scalelist. The app does not support a logarithmic scale when a complex-valued signal is plotted.
To see a time plot and a spectrum plot of the same signal side-by-side, use different displays. Drag the signal to two displays. ClickTimeorSpectrumon theDisplaytab to control what is plotted on each display.

Signal Analyzerscales the spectrum so that, if the frequency content of a signal falls exactly within a bin, its amplitude in that bin is the true average power of the signal. For example, the average power of a sinusoid is one-half the square of the sinusoid amplitude. For more details, seeMeasure Power of Deterministic Periodic Signals. For more information on howSignal Analyzercomputes spectra, seeSpectrum Computation in Signal Analyzer.

Note

By default, the app displays the spectrum in decibels. To display the spectrum in linear scale, deselect theSpectrum in dBcheck box.
When displaying a spectrum,Signal Analyzerconverts the power to dB using10 log₁₀(Power).

If any complex signals are plotted,Signal Analyzerdisplays centered two-sided spectra.

If a signal is nonuniformly sampled, thenSignal Analyzerinterpolates the signal to a uniform grid to compute spectral estimates. The app uses linear interpolation and assumes a sample time equal to the median of the differences between adjacent time points. For a nonuniformly sampled signal to be supported, the median time interval and the mean time interval must obey

$\frac{1}{100} < \frac{Median time interval}{Mean time interval} < 100.$

Visualize Persistence Spectra

Use theSignal Analyzerapp to visualize the persistence spectrum of a signal: The persistence spectrum contains time-dependent probabilities of occurrence of signals at given frequency locations and power levels. This type of spectrum is useful for detecting brief events.

To activate the persistence spectrum, clickSpectrumon theDisplaytab and selectPersistence Spectrum. The app displays a set of axes with the persistence spectrum, and aPersistence Spectrumtab with options to control the view. By default, the app displays the persistence spectrum in decibels. On thePersistence Spectrumtab, toggle between decibels and linear scale using theSpectrum in dBcheck box. You cannot zoom out in frequency beyond the Nyquist range.

Note

You can plot the persistence spectrum of only one signal per display.

For more information on howSignal Analyzercomputes persistence spectra, seePersistence Spectrum in Signal Analyzer.

For complex input signals,Signal Analyzerdisplays centered two-sided persistence spectra.

Visualize Signal Spectrograms

Use theSignal Analyzerapp to analyze a signal in the time-frequency domain. To activate the spectrogram view of a signal, clickTime-Frequencyon theDisplaytab and selectSpectrogram. The app displays a set of axes with the signal spectrogram, and aSpectrogramtab with options to control the view.

Note

You can plot the spectrogram of only one signal per display.

If the panner is activated and is zoomed in on a particular region of interest, or if you zoom in on a region of the signal in the time plot using one of the zoom actions on theDisplaytab, the spectrogram in the display corresponds to the region of interest, not the whole signal.
You cannot zoom out in frequency beyond the Nyquist range.
You cannot plot the spectrogram of a non-finite signal.
To change the frequency scale, selectLinearorLogin theFrequency Scalelist. The app does not support a logarithmic scale when a complex-valued signal is plotted.
To see a time plot and a spectrogram plot of the same signal side-by-side, use different displays. Drag the signal to two displays. ClickTimeorTime-Frequencyon theDisplaytab to control what is plotted on each display.

For more information on howSignal Analyzercomputes spectrograms, seeSpectrogram Computation in Signal Analyzer.

Note

By default, the app displays the spectrogram in decibels. To display the spectrogram in linear scale, deselect theSpectrum in dBcheck box.

The reassignment technique sharpens the time and frequency localization of spectrograms by reassigning each power spectrum estimate to the location of its center of energy. If your signal contains well-localized temporal or spectral components, then this option generates a spectrogram that is easier to read and interpret. To apply reassignment to a spectrogram, checkReassignin theSpectrogramtab.

$\frac{1}{100} < \frac{Median time interval}{Mean time interval} < 100.$

For complex input signals,Signal Analyzerdisplays centered two-sided spectrograms.

Visualize Signal Scalograms

Use theSignal Analyzerapp to visualize the scalogram of a signal. The scalogram is useful for identifying signals with low-frequency components and for analyzing signals whose frequency content changes rapidly with time. You need a Wavelet Toolbox™ license to use the scalogram view.

To activate the scalogram view of a signal, clickTime-Frequencyon theDisplaytab and selectScalogram. The app displays a set of axes with the signal scalogram and aScalogramtab with options to control the view.

Note

You can plot the scalogram of only one signal per display.

If the panner is activated and is zoomed in on a particular region of interest, the scalogram in the display corresponds to the whole signal, not just the region of interest.Signal Analyzer执行一个光学zooming, using interpolation to display a smooth curve.
如果你放大信号的一个区域me plot using one of the zoom actions on theDisplaytab, the scalogram in the display corresponds to the whole signal, not just the region of interest.Signal Analyzer执行一个光学zooming, using interpolation to display a smooth curve.
To see a time plot and a scalogram plot of the same signal side by side, use different displays. On theDisplaytab, clickDisplay Grid, create a side-by-side pair of displays, and drag-and-drop the signal on both displays. ClickTimeorTime-Frequencyon theDisplaytab to control what is plotted on each display.

Note

Scalogram view does not support complex signals.
Scalogram view does not support nonuniformly sampled signals.

For more information on howSignal Analyzercomputes scalograms, seeScalogram Computation in Signal Analyzer.

Zoom and Pan Through Signals

TheSignal Analyzerapp features a panner that enables you to zoom in on and navigate through signals to see how they change in frequency and time. To activate the panner, on theDisplaytab, clickPanner.

The panner renders signals in their entire duration. To select a region of interest, click the panner and drag to create a zoom window. Use the mouse to resize or slide the zoom window along the length of the signal.

If the spectrum of the signal is plotted, it corresponds to the region of interest, not the whole signal. For more details, seeSpectrum Computation in Signal Analyzer.
If the persistence spectrum of the signal is plotted, it corresponds to the region of interest, not the whole signal. For more details, seePersistence Spectrum in Signal Analyzer.
If the spectrogram of the signal is plotted, it corresponds to the region of interest, not the whole signal. For more details, seeSpectrogram Computation in Signal Analyzer.
If the scalogram of the signal is plotted, it corresponds to the whole signal, not the region of interest.Signal Analyzer执行一个光学zooming, using interpolation to display a smooth curve. For more details, seeScalogram Computation in Signal Analyzer.
You cannot zoom out in frequency beyond the Nyquist range.

Edit Time Information and Link Displays in Time

Use theSignal Analyzerapp to add time information to signals. In the Signal table, select the signals whose time information you want to add or modify. Add time information to the signals by clickingTime Valuesin theAnalyzertab.

Note

You cannot edit the time information of a timetable or time series with inherent time information.
You cannot edit the time information of a labeled signal set.
You cannot edit the time information for individual channels of a multichannel signal. You must edit the time information for the whole signal.

你可以表达的信息sample rate or sample time, and a start time. You can also add explicit time values using a numeric vector, adurationarray, or a MATLAB^®expression. Time values must be unique and cannot beNaN, but they need not be uniformly spaced. The app derives a sample rate from the time values and displays it in theTimecolumn of the Signal table. For more details, seeEdit Sample Rate and Other Time Information.

Note

Filtering and scalogram view do not support nonuniformly sampled signals.

If a signal is nonuniformly sampled, thenSignal Analyzerinterpolates the signal to a uniform grid to compute spectral estimates. The app uses linear interpolation and assumes a sample time equal to the median of the differences between adjacent time points. The derived sample rate in the Signal table has an asterisk to indicate that the signal is nonuniformly sampled. For a nonuniformly sampled signal to be supported, the median time interval and the mean time interval must obey

$\frac{1}{100} < \frac{Median time interval}{Mean time interval} < 100.$

Note

The interpolation is used only to compute spectral estimates. Time plots are not resampled.
You can link display time spans so that plot responses are synchronized when you pan and zoom horizontally. The signals in the displays you want to link must contain time information. To link the time span of a display to the time spans of the displays linked already, select the display and, on theDisplaytab, selectLink Time. To unlink a display, select it and clearLink Time.

Note

SelectingLink Timelinks the selected display to the complete collection of displays that have already been linked.

Displays with linked time spans have the following operations synchronized:
- Panning by selecting and dragging the plot or by using the display panner.
- Zooming in, zooming out, or zooming on the time axis. Zooming in or out on one display affects only the time axis in the remaining linked displays.
- Fitting data to view. The app stretches the common time axis so that it shows the span from the earliest to the latest time among all signals in the linked displays.
- If the axes of two displays are linked in time, then the time cursors in the displays are linked.
The time axis of a linked display might update as you add or remove signals.

Note

Frequency axes are never linked between displays.

Extract Signal Regions of Interest

TheSignal Analyzerapp enables you to extract regions of interest from the signals you are studying and export them for further analysis. To extract regions of interest, select the display that has them. On theDisplaytab, clickExtract Signals, or right-click the display and selectExtract Signals.

SelectBetween Time Limitsto extract a region of interest defined by the time limits of the selected display. To change the time limits, you can use the panner, select one of the zoom actions on theDisplaytab, or change the limit values on theDisplay,Time,Spectrogram, orScalogramtabs.
SelectBetween Time Cursorsto extract a region of interest defined by the locations of the time-domain cursors in the selected display.
If a signal has time information, you can preserve the start time of the region of interest by checkingPreserve Start Time.