REAL signal process
The AC signal from the APD is amplified, passed to analog-to-digital convertor for digitalization and processed. As Figure 5 shown, the first step of processing is to detrend by subtracting the low-frequency (below 30 Hz) envelope from the raw signal. The second step is to use a bandpass filter (0.1~7 kHz) to extract the signal in the audio frequency range. The final step uses spectral subtraction to reduce noise.[32] After processing, the signal is normalized into the interval [-1, 1] and converted into audio.