SI4770-77-A20 Datasheet, PDF (40/54 Pages) Silicon Laboratories – HIGH-PERFORMANCE CONSUMER ELECTRONICS

Si4770/77-A20
4.11. 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 24.
Mono Audio
Left + Right
Stereo
Pilot
Stereo Audio
Left - Right
RDS/
RBDS
0
15 19 23
38
53 57
Frequency (kHz)
Figure 24. MPX Signal Spectrum
4.11.1. Stereo Decoder
The Si4770/77-A20'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.
4.11.2. 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. Signal metrics such as on-channel
RSSI, ultra-sonic noise (USN), and multi-path
interference are monitored simultaneously in forcing a
blend from stereo to mono. The metric, reflecting the
poorest signal quality, takes priority and the stereo
signal is blended appropriately. The thresholds for
activating stereo-mono blend are programmable, as are
the levels for a fully blended state. The attack and decay
rates for each metric are programmable. The pilot
detection metric is additionally available for read-out.
Stereo thld
SNR (dB)
Mono thld
Stereo thld
RSSI (dBuV)
Mono thld
Mono thld
Multi-path %
Stereo thld
Stereo
Blend level
Mono
40
T< T_release
Figure 25. Conceptual Illustration of Stereo-Mono Blend
Rev. 0.9