Document FIR filter requirements, fixes #114

PaulStoffregen · Feb 7, 2016 · 29b4ded · 29b4ded
1 parent a25efdf
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Showing 3 changed files with 18 additions and 3 deletions.
diff --git a/examples/Effects/Filter_FIR/Filter_FIR.ino b/examples/Effects/Filter_FIR/Filter_FIR.ino
@@ -62,7 +62,7 @@ AudioControlSGTL5000 audioShield;
 
 struct fir_filter {
   short *coeffs;
-  short num_coeffs;
+  short num_coeffs;    // num_coeffs must be an even number, 4 or higher
 };
 
 // index of current filter. Start with the low pass.

diff --git a/filter_fir.h b/filter_fir.h
@@ -41,8 +41,11 @@ class AudioFilterFIR : public AudioStream
 		coeff_p = cp;
 		// Initialize FIR instance (ARM DSP Math Library)
 		if (coeff_p && (coeff_p != FIR_PASSTHRU) && n_coeffs <= FIR_MAX_COEFFS) {
-			arm_fir_init_q15(&fir_inst, n_coeffs, (q15_t *)coeff_p,
-				&StateQ15[0], AUDIO_BLOCK_SAMPLES);
+			if (arm_fir_init_q15(&fir_inst, n_coeffs, (q15_t *)coeff_p,
+			  &StateQ15[0], AUDIO_BLOCK_SAMPLES) != ARM_MATH_SUCCESS) {
+				// n_coeffs must be an even number, 4 or larger
+				coeff_p = NULL;
+			}
 		}
 	}
 	void end(void) {

diff --git a/gui/index.html b/gui/index.html
@@ -2218,6 +2218,18 @@ <h3>Functions</h3>
 	<h3>Examples</h3>
 	<p class=exam>File &gt; Examples &gt; Audio &gt; Effects &gt; Filter_FIR
 	</p>
+	<h3>Known Issues</h3>
+	<p>Your filter's impulse response array must have an even length.  If you have
+		add odd number of taps, you must add an extra zero to increase the length
+		to an even number.
+	</p>
+	<p>The minimum number of taps is 4.  If you use less, add extra zeros to increase
+		the length to 4.
+	</p>
+	<p>The impulse response must be given in reverse order.  Many filters have
+		symetrical impluse response, making this a non-issue.  If your filter has
+		a non-symetrical response, make sure the data is in reverse time order.
+	</p>
 	<h3>Notes</h3>
 	<p>FIR filters requires more CPU time than Biquad (IIR), but they can
 		implement filters with better phase response.