Si4720-A10-GM Silicon Laboratories Inc, Si4720-A10-GM Datasheet - Page 27

• Manufacturer Part Number: Si4720-A10-GM
• Description: RF Transceiver Broadcast FM Radio Transceiver
• Manufacturer: Silicon Laboratories Inc

Specifications of Si4720-A10-GM

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 19 below.
(OMODE = 1000)
(OMODE = 1100)
(OMODE = 0110)
(OMODE = 0000)
(OFALL = 0)
Left-Justified
0
(OFALL = 1)
(OFALL = 0)
(OFALL = 1)
(OFALL = 0)
I
2
Mono Audio
S
Left + Right
Figure 19. MPX Signal Spectrum
(MSB at 2
(MSB at 1
INVERTED
INVERTED
DOUT
DCLK
DCLK
DOUT
DFS
DCLK
DCLK
nd
st
DFS
rising edge)
rising edge)
15
Stereo
Pilot
DOUT
DOUT
DCLK
19
DFS
Frequency (kHz)
23
1 DCLK
MSB
1
1 DCLK
MSB
1
MSB
Figure 17. Left-Justified Digital Audio Format
Stereo Audio
1
2
Left - Right
2
MSB
38
Figure 18. DSP Digital Audio Format
2
1
Figure 16. I
3
3
LEFT CHANNEL
3
2
LEFT CHANNEL
LEFT CHANNEL
3
n-2
53
n-2
LEFT CHANNEL
2
RBDS
RDS/
S Digital Audio Format
n-2
57
n-1
Rev. 1.0
n-1
n-1
n-2
LSB
LSB
5.6.1. Stereo Decoder
The
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 Si4721 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
FM_BLEND_MONO_THRESHOLD property.
n
n
LSB
n-1
n
Si4720/21's
MSB
LSB
1
n
MSB
1
MSB
1 DCLK
2
1
MSB
can
1
2
3
2
2
RIGHT CHANNEL
3
Si4720/21-B20
integrated
be
3
3
RIGHT CHANNEL
RIGHT CHANNEL
RIGHT CHANNEL
n-2
forced
n-2
n-1
n-2
n-2
stereo
n-1
LSB
n-1
n-1
n
LSB
with
n
LSB
LSB
n
n
decoder
the
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