User`s guide

Spectrum Scope
5-409
5Spectrum Scope
Purpose Compute and display the short-time FFT of each input signal.
Library DSP Sinks
Description The Spectrum Scope block computes and displays the magnitude-squared FFT
of the input. The input be a 1-D vector or a 2-D matrix of any frame status.
When the input is a 1-by-N sample-based vector or M-by-N sample-based
matrix, you must select the
Buffer input check box. Each of the N vector
elements (or M
∗N matrix elements) is then treated as an independent channel,
and the block buffers and displays the data in each channel independently.
When the input is frame-based, you can leave the input as is, or rebuffer data
by checking the
Buffer input check box and specifying the new buffer size. In
the latter case, you can also specify an optional
Buffer overlap.
Buffering 1-D vector inputs is recommended. In this case, the inputs are
buffered into frames (the length of which are specified in the
Buffer size
parameter), where each 1-D input vector becomes a row in the buffered
outcome. If a 1-D vector input is left unbuffered, you will get a warning because
the block is computing the FFT of a scalar; though the scope window appears,
it is unlikely you will be able to see the plot, and a warning is also displayed on
the scope itself. It is not recommended that you leave 1-D inputs unbuffered.
The number of input samples that the block buffers before computing and
displaying the magnitude FFT is specified by the
Buffer size parameter, M
o
.
The
Buffer overlap parameter, L, specifies the number of samples from the
previous buffer to include in the current buffer. The number of new input
samples the block acquires before computing and displaying the magnitude
FFT is the difference between the
Buffer size and Buffer overlap, M
o
-L.
The display update period is (M
o
-L)∗T
s
, where T
s
is the input sample period,
and is equal to the input sample period when the
Buffer overlap is M
o
-1. For
negative
Buffer overlap values, the block simply discards the appropriate
number of input samples after the buffer fills, and updates the scope display at
a slower rate than the zero-overlap case.
When the
FFT length check box is deselected and the input is buffered, the
block uses the buffer size as the FFT size. If the check box is deselected and the
input is not buffered, the block uses the input size as the FFT size. When the
check box is selected, the
FFT length parameter, N
fft
, is enabled, and specifies