SI4730-C40-GM Datasheet, PDF (22/42 Pages) Silicon Laboratories – BROADCAST AM/FM RADIO RECEIVER

Si4730/31-C40
(OFALL = 1)
INVERTED
DCLK
(OFALL = 0)
DCLK
I²S
(OMODE = 0000)
DFS
DOUT
(OFALL = 1)
INVERTED
DCLK
(OFALL = 0)
DCLK
Left-Justified
(OMODE = 0110)
DFS
DOUT
1 DCLK
1
MSB
LEFT CHANNEL
1 DCLK
2
3
n-2
n-1
n
1
2
LSB
MSB
Figure 10. I²S Digital Audio Format
RIGHT CHANNEL
3
n-2
n-1
n
LSB
LEFT CHANNEL
RIGHT CHANNEL
1
2
3
MSB
n-2
n-1
n
LSB
1
2
3
MSB
Figure 11. Left-Justified Digital Audio Format
n-2
n-1
n
LSB
(OFALL = 0)
DCLK
DFS
(OMODE = 1100)
DOUT
(MSB at 1^st rising edge)
DOUT
(OMODE = 1000) (MSB at 2^nd rising edge)
1
MSB
1 DCLK
LEFT CHANNEL
RIGHT CHANNEL
2
3
n-2
n-1
n
1
2
3
n-2
n-1
LEFT CHANNEL
LSB
MSB
RIGHT CHANNEL
1
2
3
n-2
n-1
n
1
2
3
n-2
MSB
LSB
MSB
Figure 12. DSP Digital Audio Format
n
LSB
n-1
n
LSB
5.6. Stereo Audio Processing
The output of the FM demodulator is a stereo
multiplexed (MPX) signal. The MPX standard was
developed in 1961, and is used worldwide. Today's
MPX signal format consists of left + right (L+R) audio,
left – right (L–R) audio, a 19 kHz pilot tone, and
RDS/RBDS data as shown in Figure 13 below.
Mono Audio
Left + Right
Stereo
Pilot
Stereo Audio
Left - Right
RDS/
RBDS
0
15 19 23
38
53 57
Frequency (kHz)
Figure 13. MPX Signal Spectrum
5.6.1. Stereo Decoder
The Si4730/31's integrated stereo decoder
automatically decodes the MPX signal using DSP
techniques. The 0 to 15 kHz (L+R) signal is the mono
output of the FM tuner. Stereo is generated from the
(L+R), (L–R), and a 19 kHz pilot tone. The pilot tone is
used as a reference to recover the (L–R) signal. Output
left and right channels are obtained by adding and
subtracting the (L+R) and (L–R) signals respectively.
The Si4731 uses frequency information from the 19 kHz
stereo pilot to recover the 57 kHz RDS/RBDS signal.
5.6.2. Stereo-Mono Blending
Adaptive noise suppression is employed to gradually
combine the stereo left and right audio channels to a
mono (L+R) audio signal as the signal quality degrades
to maintain optimum sound fidelity under varying
reception conditions. Stereo/mono status can be
monitored with the FM_RSQ_STATUS command. Mono
operation can be forced with the
FM_BLEND_MONO_THRESHOLD property.
22
Rev. 1.0