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.
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
Mono Audio
Left + Right
Stereo
Pilot
Stereo Audio
Left - Right
RDS/
RBDS
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
0
15 19 23 38
Frequency (kHz)
53
57
RSSI, ultra-sonic noise (USN), and multi-path
interference are monitored simultaneously in forcing a
blend from stereo to mono. The metric, reflecting the
Figure 24. MPX Signal Spectrum
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
T< T release
Stereo
Blend level
Mono
Figure 25. Conceptual Illustration of Stereo-Mono Blend
40
Rev. 0.9
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