Decompression
A decompressor is used at the receiving end to increase the dynamic range of the audio signal by
reducing the large signal and amplifying the small signal.
The steady-state output of the decompressor is the square of the input signal, that is, when the input
signal is increased or decreased by 1dB, the output signal is correspondingly increased or decreased
by 2dB. Generally, in a voice communication system, the dynamic range can be converted from 30
dB of the input signal to 60 dB of the output signal by audio compression technology. The user can
open the compression module by configuring register 0x34[6]=1’b1.
OMISSIS
Figure 6.10 Decompressing the time domain response
At the same time, the user can set the compressor's 0dB compression point by configuring Register
0x2D[7:4].
Bandpass filter
The HR_C6000 has an optional bandpass filter built into the FM receiver. The signal bandwidth is
300Hz to 3400Hz. The bandpass filter can be turned on by configuring the register 0x34[7]=1’b1.
6.2.1 CTCSS reception
The CTCSS air signal generates phase information through the phase detector, and the DC offset of
the signal is cancelled by the frequency offset calibration module. After the limiting processing, the
high frequency audio portion is filtered by the 4th order IIR 300Hz low pass filter.
The frequency response amplitude detection result is compared with the preset threshold value.
When the threshold is greater than the threshold, the voice enable is enabled, and the output
interrupt signal is sent to the peripheral to open the speaker and the voice path.
OMISSIS
Figure 6.11 CTCSS Receive Block Diagram
Detailed usage is detailed in Appendix A2.2.1.
6.2.2 CDCSS reception
The CDCSS modulation consists of an FM demodulation module that uses a non-coherent
demodulation scheme. CDCSS signal reception includes key steps such as differential phase
discrimination, frequency offset estimation, decision, and golay decoding. The flow of back-end
baseband processing is shown in Figure 6.12.
OMISSIS
Figure 6.12 Receive Baseband Processing Flowchart
The CDCSS baseband signal is filtered by a low-pass filter to remove some of the out-of-band noise
and reduced to an amplitude value by FM demodulation. Then the next two LPFs further filter out
the noise and audio signals and then perform frequency offset compensation. The signal obtained at
