Si4734/35-C40
24 Rev. 1.0
5.9. 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.
5.10. 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.
5.11. 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.
5.12. 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.
5.13. Tuning
The tuning frequency is directly programmed using the
FM_TUNE_FREQ and AM_TUNE_FREQ commands.
The Si4734/35 supports channel spacing steps of
10 kHz in FM mode and 1 kHz in AM/SW/LW mode.
5.14. Seek
Seek tuning will search up or down for a valid channel.
Valid channels are found when the receive signal
strength indicator (RSSI) and the signal-to-noise ratio
(SNR) values exceed the set threshold. Using the SNR
qualifier rather than solely relying on the more
traditional RSSI qualifier can reduce false stops and
increase the number of valid stations detected. Seek is
initiated using the FM_SEEK_START and
AM_SEEK_START commands. The RSSI and SNR
threshold settings are adjustable using properties (see
Table 15).
5.15. Reference Clock
The Si4734/35 reference clock is programmable,
supporting RCLK frequencies in Table 12. Refer to
Table 3, “DC Characteristics,” on page 5 for switching
voltage levels and Table 9, “FM Receiver
Characteristics,” on page 12 for frequency tolerance
information.
An onboard crystal oscillator is available to generate the
32.768 kHz reference when an external crystal and load
capacitors are provided. Refer to "2. Typical Application
Schematic (QFN)" on page 17. This mode is enabled
using the POWER_UP command. Refer to Table 14,
“Selected Si473x Commands,” on page 27.
The Si4734/35 performance may be affected by data
activity on the SDIO bus when using the integrated
internal oscillator. SDIO activity results from polling the
tuner for status or communicating with other devices
that share the SDIO bus. If there is SDIO bus activity
while the Si4734/35 is performing the seek/tune
function, the crystal oscillator may experience jitter,
which may result in mistunes, false stops, and/or lower
SNR.
For best seek/tune results, Silicon Laboratories
recommends that all SDIO data traffic be suspended
during Si4734/35 seek and tune operations. This is
achieved by keeping the bus quiet for all other devices
on the bus, and delaying tuner polling until the tune or
seek operation is complete. The seek/tune complete
(STC) interrupt should be used instead of polling to
determine when a seek/tune operation is complete.
5.16. Control Interface
A serial port slave interface is provided, which allows an
external controller to send commands to the Si4734/35
and receive responses from the device. The serial port
can operate in three bus modes: 2-wire mode, 3-wire
mode, or SPI mode. The Si4734/35 selects the bus
mode by sampling the state of the GPO1 and GPO2
pins on the rising edge of RST
. The GPO1 pin includes
an internal pull-up resistor, which is connected while
