Frequency Frame Scope (DSP Blockset)

Display frequency-domain frame-based data.

Library

DSP Sinks

Description

The Frequency Frame Scope block is similar to the Time Frame Scope, but plots frequency-domain data instead of time-domain data. For a complete discussion of this block's axis properties, line properties, scope window, and frame-based operation, see the Time Frame Scope block reference.

The block assumes that the each length-M input frame is a vector of magnitude data corresponding to M ascending frequencies. That is, each data point in the input frame, u, is assumed to correspond to a unique frequency value, u=u(f), where f_i₊₁>f_i.

In order to correctly scale the frequency axis (i.e., to determine the frequencies that the data in the input frame should be plotted against), the block needs to know the sample period of the original time-domain sequence represented by the frequency-domain data. The Sample time of original time-series parameter allows you to specify this in two different ways:

Auto-Detect from Input Sample Period. A value of -1 for this parameter instructs the block to reconstruct the frequency data from the frame-period of the frequency-domain input. This parameter setting is appropriate when each frame of frequency-domain data shares the same length as the frame of time-domain data that it was generated from.

This is the case when the FFT is computed on the same number of points as are contained in the time-domain input (as shown above). Blocks that use the FFT internally, for example those in the Power Spectrum Estimation library, usually provide an FFT size parameter to specify the number of frequency points in the output. When this parameter is set to the size of the time-domain input, the frequency-domain output and time-domain input have the same frame size.

When the frequency-domain input to the Frequency Frame Scope block is related to the original time-domain signal in this way, the block can compute the original time-domain sample period from the frequency-domain input frame size and frame period.

Note that the auto-detect mode makes the following two assumptions:

The sample period of the time-domain signal in the simulation is equal to the period with which the physical signal was originally sampled. For example, the mtlb signal imported from the workspace in the model above has an actual sample period of 1/Fs = 1/7418. Although the Sample time parameter in the Signal From Workspace block can legitimately be set to any value, for the auto-detect mode to compute the correct frequency scaling, the Sample time parameter must be set to the original sample period of 1/7418.
Consecutive frames containing the time-domain signal do not overlap each other; that is, a particular signal sample does not appear in more than one sequential frame.

Enter the Appropriate Time-Domain Sample Period. Enter the sample period of the original time-series. This is necessary when the frame size of frequency-domain data is not identical to the frame size of the time-domain data it was generated from.

This is the case when the FFT is computed on a different number of samples (more or fewer) than are contained in the time-domain input. When the FFT size parameter of FFT-based blocks is set to a value other than the size of the input frame, the block either zero-pads or truncates the input before performing the FFT, and the frequency-domain output and time-domain input have different frame sizes.

When the time-domain signal is zero-padded or truncated before transformation to the frequency domain, the Frequency Frame Scope block cannot automatically compute the original time-domain sample period. You should specify the original sample period explicitly in the Sample time of original time-series parameter.

You also need to explicitly specify the time-series' actual sample period when either of the assumptions listed above for the auto-detect mode are not valid.

The Frequency units parameter specifies whether the frequency axis values should be in units of Hertz or rads/sec, and the Frequency range parameter specifies the range of frequencies over which the magnitudes in the input should be plotted. The available options are [0..Fs/2], [-Fs/2..Fs/2], and [0..Fs], where F_s is the original time-domain signal's sample frequency (F_s/2 is the Nyquist frequency). All of the FFT-based blocks in the DSP Blockset, including those in the Power Spectrum Estimation library, compute the FFT at frequencies in the range [0,F_s).

If the Frequency units parameter specifies Hertz, the spacing between frequency points is 1/(M*T_s). For Frequency units of rads/sec, the spacing between frequency points is 2

/(M*T_s). The Amplitude scaling parameter allows you to select magnitude or dB scaling along the y-axis.

The scope updates the display for each new input frame.

Dialog Box

Frequency units: The frequency units for the x-axis, Hertz or rads/sec.
Frequency range: The frequency range over which to plot the data, [0..Fs/2], [-Fs/2..Fs/2], or [0..Fs], where F_s is the sample frequency of the original time-domain signal, 1/T_s.
Sample time of original time series: The sample period of the original time-domain signal, T_s. Set to -1 to auto-detect the time-domain sample period from the frame period and frame size of the frequency-domain input (use only if the time-domain data was not zero-padded prior to the FFT).
Y-Axis title: The text to be displayed to the left of the y-axis.
Amplitude scaling: The scaling for the y-axis, dB or Magnitude.
Number of input channels: The number of channels (columns) in the input matrix.
Axis properties: Select to expose the Axis Properties panel. See Time Frame Scope for more information.
Line properties: Select to expose the Line Properties panel. See Time Frame Scope for more information.