o unify peak detection code in the simple recursive filter;

o rename bi-quad filter routines from fo_xxx to bqd_xxx;

o add high-pass bi-quad filter.
Use simple recursive filter for now instead of biquad filter.
o Switch to using signed 16-bit data from unsigned 8-bit;

o apply high-pass filter with the cutoff frequency of 1kHz to
reduce low-frequency noise and artefacts.