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4.8. 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.
Figure 13. MPX Signal Spectrum
4.8.1. Stereo Decoder
The Si4734/35'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 Si4735 uses frequency information from the 19 kHz
stereo pilot to recover the 57 kHz RDS/RBDS signal.
4.8.2. 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.
4.9. De-emphasis
Pre-emphasis and de-emphasis is a technique used by
FM broadcasters to improve the signal-to-noise ratio of
FM receivers by reducing the effects of high-frequency
interference and noise. When the FM signal is
transmitted, a pre-emphasis filter is applied to
accentuate the high audio frequencies. The Si4734/35
incorporates a de-emphasis filter which attenuates high
frequencies to restore a flat frequency response. Two
time constants are used in various regions. The de-
emphasis time constant is programmable to 50 or 75 µs
and is set by the FM_DEEMPHASIS property.
4.10. Stereo DAC
High-fidelity stereo digital-to-analog converters (DACs)
drive analog audio signals onto the LOUT and ROUT
pins. The audio output may be muted. Volume is
adjusted digitally with the RX_VOLUME property.
4.11. Soft Mute
The soft mute feature is available to attenuate the audio
outputs and minimize audible noise in very weak signal
conditions. The softmute attenuation level is adjustable
using the FM_SOFT_MUTE_MAX_ATTENUATION and
AM_SOFT_MUTE_MAX_ATTENUATION properties.
4.12. RDS/RBDS Processor (Si4735 Only)
The Si4735 implements an RDS/RBDS* processor for
symbol decoding, block synchronization, error
detection, and error correction.
The Si4735 device is user configurable and provides an
optional interrupt when RDS is synchronized, loses
synchronization, and/or the user configurable RDS
FIFO threshold has been met.
The Si4735 reports RDS decoder synchronization
status, and detailed bit errors in the information word for
each RDS block with the FM_RDS_STATUS command.
The range of reportable block errors is 0, 1–2, 3–5, or
6+. More than six errors indicates that the
corresponding block information word contains six or
more non-correctable errors, or that the block
checkword contains errors.
*Note:
RDS/RBDS is referred to only as RDS throughout the
remainder of this document.
0
57
53
38
23
19
15
Frequency (kHz)
Modula
tion Level
Stereo Audio
Left - Right
RDS/
RBDS
Mono Audio
Left + Right
Stereo
Pilot
Содержание SI4734-B20
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