ref: 0a5f275112d871f18435dbe76294c11d5f99ecf9
dir: /h/butlp.h/
typedef struct { SPFLOAT sr, freq; SPFLOAT lfreq; SPFLOAT a[7]; SPFLOAT pidsr; } sp_butlp; int sp_butlp_create(sp_butlp **p); int sp_butlp_destroy(sp_butlp **p); int sp_butlp_init(sp_data *sp, sp_butlp *p); int sp_butlp_compute(sp_data *sp, sp_butlp *p, SPFLOAT *in, SPFLOAT *out);