Si4708/09-B
Confidential Rev. 1.4 15
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.
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.
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:
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
was unable to find a channel, the seek failure/band limit
(SF/BL) bit will be set high and the device will return to
the channel selected before the seek operation began.
When the SKMODE bit is high and a seek is initiated,
the device seeks through the band until the band limit is
reached and the SF/BL bit will be set high. A seek
operation is initiated by setting the SEEK and SEEKUP
bits. After the seek operation completes, the STC bit will
be set, and the RSSI level and tuned channel are
available by reading bits RSSI[7:0] and bits
READCHAN[9:0]. During a seek operation
READCHAN[9:0] is also updated and may be read to
determine seek progress. The STC bit will be set after
the seek operation completes. The channel is valid if the
seek operation completes and the SF/BL bit is set low.
At other times, such as before a seek operation or after
a seek completes and the SF/BL bit is set high, the
channel is valid if the AFC Rail (AFCRL) bit is set low
and the value of RSSI[7:0] is greater than or equal to
SEEKTH[7:0]. Note that if the AFCRL bit is set, the
audio output is muted as in the softmute case discussed
in Section “4.5. Stereo Audio Processing”. The SEEK bit
must be set low after the STC bit is set high in order to
complete the seek operation and clear the STC and
SF/BL bits. The seek operation may be aborted by
setting the SEEK bit low at any time.
The device can be configured to generate an interrupt
on GPO when a tune or seek operation completes.
Setting the seek/tune complete (STCIEN) bit and
GPO[1:0] = 01 will configure GPO for a 5 ms low
interrupt when the STC bit is set by the device.
For additional recommendations on optimizing the seek
function, consult "AN349: Si4708/09 Programming
Guide."
Freq (MHz) Spacing (kHz) Channel Bottom of Band (MHz)+=
