Using Spectrum in LabChart 7.1
These notes apply to LabChart v7.1 or later versions
LabChart 7.1 includes a completely redesigned Spectrum which has the following new features:
• Operates both online and offline.
• Multiple channels can be overlaid and analysed simultaneously in PSD plot
• Power Spectral Density (PSD) plot of magnitude vs. frequency.
• Spectrogram plot of colour coded magnitude vs. frequency vs. time.
• Data Pad calculations of PSD parameters over the current selection.
• Channel calculations of PSD parameters:
1. Percentage Total Power 4. Median Power Frequency
2. Maximum Power
5. Maximum Power Frequency
3. Minimum Power
6. Minimum Power Frequency
Spectrum in LabChart allows the analysis of the frequency distribution of component sine waves within a
signal. It can be used, for example, to identify and quantify various waveform components such as alpha, beta,
theta and delta waves in an ECG recording, or the harmonics and frequency distribution in a sound spectrum.
Figure 1. Spectrum in LabChart provides both a Power Spectral Density (PSD) pane and a Spectrogram plot. These
can be viewed concurrently or individually both offline and in real time. The PSD plot can be seen in the upper right
pane. The Spectrogram is shown below it. In both plots note the dominant frequency components at 10 Hz and 100
Hz which are derived from the point selection shown in the Chart View channel on the left hand pane.
Quantitative EMG - 2012A
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Using Spectrum in LabChart 7.1
Displaying LabChart Spectrum
• To open Spectrum, select the Spectrum option in the Window menu on the LabChart Toolbar.
• Alternatively, open Spectrum by clicking the Window→Spectrum View icon.
Figure 3.
Figure 2.
Figure 2. Using Spectrum, multiple channels can be analysed and displayed simultaneously. As shown in the figure,
in the PSD plot, two or more channels can be overlaid as desired by the user. In the Spectrogram pane, these separate
channels are plotted separately. Note that Spectrum will work over a single selection, over a selection within a block,
or even over multiple blocks of data.
Figure 3. Clicking the Settings menu in Spectrum brings up the Spectrum View Settings box where the Calculation
and Display settings are configured according to the user’s preferences and the required analysis.
Power Spectral Density plot (PSD)
Figure 4.
The Power Spectrum Density (PSD or Periodogram) plot
displays an estimation of the power of the input signal
against a selected frequency range. It is called a “density”
because the power is estimated over small frequency ranges
or “bins”, rather than at discrete frequencies. As in Chart
View, there are options for changing the scale in both the
Amplitude and Time axes.
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Using Spectrum in LabChart 7.1
Spectrogram
The Spectrogram (or STM) is a false-colour plot (i.e. 3 dimensional plot) of spectral power plotted against both
frequency and time. The spectrogram displays spectral power as a coded color against time and frequency.
• In Selection mode the spectrogram displays the color-coded spectral power of those windows of
data that are contained inside the LabChart selection.
• In Latest mode, each new section of input data obtained is windowed, transformed and then
displayed as a colored strip on the right hand edge of the spectrogram, and the spectrogram
is scrolled to the left – similar to the way that new data is displayed in the Chart View during
sampling.
Figure 5. The Spectrogram plot. The spectrogram above
shows power attenuation coded as a range of colors
from blue through to red. Red represents a higher power
density than blue.
Channel Calculations of Spectral Parameters
Another new feature in LabChart is Channel calculations of PSD parameters as shown in Figure 6 (below).
Figure 6. Channel 2 above
shows Mean Power Frequency
calculated using Source channel 1
and Spectrum analysis functions
provided in the Channel
calculations in Chart View.
Channel calculations, accessed from the channel drop-down menu in Chart View, provide the continuous
display of Spectral parameters.
In addition, Spectrum accessed via Channel calculations provides a number of configuration options
including, the Source Channel, the Upper and Lower Frequency boundaries, Spectral Edge Frequency
Threshold (if deriving the SEF Calculation type).
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Using Spectrum in LabChart 7.1
Data Pad Calculations of Spectral Parameters
As with other Data Pad functions, data available in Chart View channels can be sent to Data Pad where it
can be further processed by Spectrum functions as selected by the user (Fig. 7). To use Spectrum in Data
Pad simply select the desired parameters in the Data Pad Column setup and the Spectrum Data Pad Options
box. Then use any of the options available in Chart View to add channel data to Data Pad, including the
Setup→Timed Add to Data Pad.... option in the LabChart menu bar to add and calculate spectral information
in Data Pad in real-time.
Figure 7. Columns in Data Pad are
configured to use Spectrum similar to
other Data Pad functions. Selecting
Options.... in Data Pad Column
Setup brings up the mini window to
set the frequency limits and the SEF
Threshold.
Calculation and Display Options:
Calculation options
FFT size
• The Fast Fourier Transform works on a certain number of data points at a time over the selection
to be analyzed. The larger that number, the greater the frequency resolution (the number of
frequency “bins” into which the signal is resolved). Smaller FFT sizes give higher amplitude
accuracy, but lower frequency resolution.
• Sizes are available in 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192 and 16384 data points.
Data Window
• Select the data window to be applied to the data before an FFT is computed.
• Data windows reduce the influence of data at the edge of the data segments, which tends to
produce false peaks at high frequencies. Data window types available are: Cosine-Bell (the default),
Hamming, Blackman and Welch. Data windowing may also be turned off. Detailed information
on the Windowing functions is provided in the
Window Overlap
• Select the method of successive FFT generation.
• Available methods are 50% Overlap, which means that each successive FFT overlaps with the
previous one by 50%, or No Overlap. Overlapping gives more accurate results, but doubles the
computation time.
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Using Spectrum in LabChart 7.1
• For example, if No Overlap were chosen, a data selection of 1024 points with an FFT size of 512
points would be divided into two segments: points 1-512 and points 513-1024. If 50% Overlap
were chosen for the same data, the segments would change to points 1-512, points 256-768 and
points 513-1024.
Remove zero-frequency component
• When selected, the amplitude of the first frequency component (corresponding to the DC
component of the signal) will be set to zero.
Display options
Mode
• Amplitude gives a spectrum where the height at a particular frequency is the amplitude at that
frequency.
• Power gives a spectrum where the height at a particular frequency is the power contained in the
waveform at that frequency.
• Power Density gives a spectrum where the height at a particular frequency is the power divided by
the resolution in the frequency domain (bin width; measured in Hz).
• Power Attenuation [dB] gives a spectrum with a wide range of powers. The power is expressed in
logarithmic units decibels (dB), relative to the reference power (attenuation = 10 * log (power /
reference power))
• Reference Power: The reference power is an arbitrary number that is used to provide a relative
value to the Power. The default reference is 0.001.
Spectrogram Colors
• The Spectrogram displays spectral power as a coded color against time and frequency. A user can
choose from a variety of color scales to represent the spectral power.
PSD Averaging (Latest)
• When in Latest Mode, the PSD Plot content changes over time as new sections of input data are
obtained. The user can specify how many of these sections are averaged in latest mode, which may
provide a smoother plot.
Detailed information about the Spectrum features is available in the Help Center in LabChart.
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