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The BLK_CON function computes a “fast convolution” of a digital signal and an impulse-response sequence. It returns the filtered signal.

This routine is written in the IDL language. Its source code can be found in the file blk_con.pro in the lib subdirectory of the IDL distribution.


The following lines create the plot shown at the top of this topic:

; Create a filter of length P = 32
filter = REPLICATE(1.0 ,32) ;Set all points to 1.0
filter(2*INDGEN(16)) = 0.5 ;Set even points to 0.5
; Create a sampled signal with random noise
signal = SIN((FINDGEN(1000)/35.0)^2.5)
noise = (RANDOMU(SEED,1000)-.5)/2.
signal = signal + noise
; Convolve the filter and signal using block convolution
result = BLK_CON(filter, signal)
plot1 = PLOT(signal, TITLE='Signal + Noise', $
  DIMENSIONS=[960,480], LAYOUT=[2,1,1])
plot2 = PLOT(result, TITLE='Convolved Filter and Signal', $
  LAYOUT=[2,1,2], /CURRENT)


Result = BLK_CON( Filter, Signal [, B_LENGTH=scalar] [, /DOUBLE] )

Return Value

This function returns a vector with the same length as Signal. If either of the input arguments are double-precision or the DOUBLE keyword is set, the result is double-precision, otherwise the result is single-precision.



A P-element floating-point vector containing the impulse-response sequence of the digital filter.


An n-element floating-point vector containing the discrete signal samples.



A scalar specifying the block length of the subdivided signal segments. If this parameter is not specified, a near-optimal value is chosen by the algorithm based upon the length P of the impulse-response sequence. If P is a value less than 11 or greater than 377, then B_LENGTH must be specified.

B_LENGTH must be greater than the filter length, P, and less than the number of signal samples.


Set this keyword to force the computation to be done using double-precision arithmetic.

Version History

Pre 4.0


See Also


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