 Negative values are converted to positive by FUNCABS.
In order to get its envelope, we need to remove high frequencies. Please
open a monitor for FFTMON (U_17) to see the spectrum distribution.
300Hz has come back to the original position. However, there are some frequency
unneeded over 5kHz. Let's remove them by a LPF (Low Pass Filter).
RMZLPF (U_21) is a LPF (FIR type by Remez Exchange Method). To see the
characteristics, open a monitor for RMZLPF.
This LPF leaves low frequencies under 800Hz and eliminates high frequencies
over 2500Hz.
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