Si4702/03-D30
Rev. 0.6 17
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.
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 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 Si4702/03-D30 uses Silicon Laboratories’ patented
and proven frequency synthesizer technology including
a completely integrated VCO. 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 spacings of 50, 100 or 200 kHz are selected
using bits SPACE[1:0]. The channel is selected with bits
CHAN[9:0]. Band selection for Japan, Japan wideband,
or Europe/U.S./Asia is set with 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, optional SNR
and/or impulse noise detector criteria 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 for backwards compatibility.
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
is unable to find a valid channel, the seek failure/band
limit (SF/BL) bit is set high and the device returns 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 is set high. A seek operation
is initiated by setting the SEEK and SEEKUP bits. After
the seek operation completes, the STC bit is 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 and report seek progress.
The STC bit is 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 a
valid channel is found but 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. Setting the STC bit low
clears STC status 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 GPIO2 when a tune or seek operation completes.
Setting the seek/tune complete (STCIEN) bit and
GPIO2[1:0] = 01 will configure GPIO2 for a 5 ms low
interrupt when the STC bit is set by the device.
For additional recommendations on optimizing the seek
function, consult "AN230: Si4700/01/02/03
Programming Guide."
Freq (MHz) Spacing (kHz) Channel Bottom of Band (MHz)+=
