build/iirfilt: extending description for create(), create_sos() methods
@@ -2720,20 +2720,28 @@ LIQUID_FFTFILT_DEFINE_API(LIQUID_FFTFILT_MANGLE_CCCF,
/* Infinite impulse response (IIR) filter */ \
typedef struct IIRFILT(_s) * IIRFILT(); \
\
-/* Create infinite impulse reponse filter from external coefficients */ \
-/* _b : feed-forward coefficients, [size: _nb x 1] */ \
-/* _nb : number of feed-forward coefficients */ \
-/* _a : feed-back coefficients, [size: _na x 1] */ \
-/* _na : number of feed-back coefficients */ \
+/* Create infinite impulse reponse filter from external coefficients. */ \
+/* Note that the number of feed-forward and feed-back coefficients do */ \
+/* not need to be equal, but they do need to be non-zero. */ \
+/* Furthermore, the first feed-back coefficient \(a_0\) cannot be */ \
+/* equal to zero, otherwise the filter will be invalid as this value is */ \
+/* factored out from all coefficients. */ \
+/* For stability reasons the number of coefficients should reasonably */ \
+/* not exceed about 8 for single-precision floating-point. */ \
+/* _b : feed-forward coefficients (numerator), [size: _nb x 1] */ \
+/* _nb : number of feed-forward coefficients, _nb > 0 */ \
+/* _a : feed-back coefficients (denominator), [size: _na x 1] */ \
+/* _na : number of feed-back coefficients, _na > 0 */ \
IIRFILT() IIRFILT(_create)(TC * _b, \
unsigned int _nb, \
TC * _a, \
unsigned int _na); \
\
/* Create IIR filter using 2nd-order secitons from external */ \
-/* coefficients */ \
+/* coefficients. */ \
/* _B : feed-forward coefficients [size: _nsos x 3] */ \
/* _A : feed-back coefficients [size: _nsos x 3] */ \
+/* _nsos : number of second-order sections (sos), _nsos > 0 */ \
IIRFILT() IIRFILT(_create_sos)(TC * _B, \
TC * _A, \
unsigned int _nsos); \
