SI4708-C Datasheet, PDF (15/40 Pages) Silicon Laboratories – BROADCAST FM RADIO TUNER FOR PORTABLE APPLICATIONS

Si4708/09-C
4.5. 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 7.
Mono Audio
Left + Right
Stereo
Pilot
Stereo Audio
Left - Right
RDS/
RBDS
High-fidelity stereo digital-to-analog converters (DACs)
drive analog audio signals onto the LOUT and ROUT
pins. The audio output may be muted with the DMUTE
bit. Volume can be adjusted digitally with the
VOLUME[3:0] bits. The volume dynamic range can be
set to either –28 dBFS (default) or –58 dBFS by setting
VOLEXT=1.
The soft mute feature is available to attenuate the audio
outputs and minimize audible noise in very weak signal
conditions. The soft mute attack and decay rate can be
adjusted with the SMUTER[1:0] bits where 00 is the
fastest setting. The soft mute attenuation level can be
adjusted with the SMUTEA[1:0] bits where 00 is the
most attenuated. The soft mute disable (DSMUTE) bit
may be set high to disable this feature.
0
15 19 23
38
53 57
Frequency (kHz)
Figure 7. MPX Signal Spectrum
The Si4708/09's integrated stereo decoder
automatically decodes the MPX signal. 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. Separate left and right channels are
obtained by adding and subtracting the (L+R) and (L-R)
signals, respectively. The Si4709 uses frequency
information from the 19 kHz stereo pilot to recover the
57 kHz RDS/RBDS signal.
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. The signal level range over which
the stereo to mono blending occurs can be adjusted by
setting the BLNDADJ[1:0] register. Stereo/mono status
can be monitored with the ST register bit and mono
operation can be forced with the MONO register bit.
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. All FM receivers
incorporate a de-emphasis filter which attenuates high
frequencies to restore a flat frequency response. Two
time constants, 50 or 75 µs, are used in various regions.
The de-emphasis time constant is programmable with
the DE bit.
4.6. Tuning
The Si4708/09 uses Silicon Laboratories’ patented and
proven frequency synthesizer technology including a
completely integrated VCO. The frequency synthesizer
generates the quadrature local oscillator signal used to
downconvert the RF input to a low intermediate
frequency. The VCO frequency is locked to the
reference clock and adjusted with an automatic
frequency control (AFC) servo loop during reception.
The tuning frequency is defined as:
Freq (MHz) = Spacing (kHz)  Channel + Bottom of Band (MHz)
Channel spacing of 50, 100 or 200 KHz is selected with
bits SPACE[1:0]. The channel is selected with bits
CHAN[9:0]. The bottom of the band is set to 76 MHz or
87.5 MHz with the bits BAND[1:0]. The tuning operation
begins by setting the TUNE bit. After tuning completes,
the seek/tune complete (STC) bit will be set and the
RSSI level is available by reading bits RSSI[7:0]. The
TUNE bit must be set low after the STC bit is set high in
order to complete the tune operation and clear the STC
bit.
Seek tuning searches up or down for a channel with an
RSSI greater than or equal to the seek threshold set
with the SEEKTH[7:0] bits. In addition, an optional SNR
and/or impulse noise detector may be used to qualify
valid stations. The SKSNR[3:0] bits set the SNR
threshold required. The SKCNT[3:0] bits set the impulse
noise threshold. Using the extra seek qualifiers can
reduce false stops and, in combination with lowering the
RSSI seek threshold, increase the number of found
stations. The SNR and impulse noise detectors are
disabled by default.
Two seek modes are available. When the seek mode
(SKMODE) bit is low and a seek is initiated, the device
seeks through the band, wraps from one band edge to
the other, and continues seeking. If the seek operation
Rev. 1.2
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