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Si4770/77-A20
4.12. 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 Si4770/77-
A20 incorporate 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 μ s.
4.13. Analog Audio and FM MPX
High-fidelity digital-to-analog converters (DACs) drive
analog audio signals or the FM MPX signal onto the
LOUT/MPXOUT and ROUT pins. At powerup time the
user can configure the analog outputs for either audio or
MPX output. In applications where MPX and audio
outputs are required simultaneously, the analog MPX
signal can be driven onto the MPXOUT pin and the
audio signals can be sourced from the digital audio
interface.
The audio output may be muted. Volume is adjusted
digitally. It is necessary that the volume be maintained
at maximum levels to ensure the highest dynamic range
audio outputs to the external audio processing stage in
a car radio.
4.14. Soft Mute
The soft mute feature is available to attenuate the audio
outputs and minimize audible noise in compromised
signal conditions. The Si4770/77-A20 triggers soft mute
by monitoring signal metrics such as on-channel RSSI
or SNR. The thresholds for activating soft mute are
programmable, as are soft mute attenuation levels and
attack and decay rates. The Si4770/77-A20 provides
the soft mute feature in FM and AM bands.
4.15. AM/FM Dynamic Bandwidth Control
The AM/FM IF channel bandwidth is dynamically
optimized according to on-channel RSSI, and with the
aid of the adjacent and alternate channel RSSI metric.
4.16. Seek and Valid Station Qualification
The seek function will search up or down the selected
frequency band for a valid channel. A valid channel is
qualified according to a series of programmable signal
indicators and thresholds. The seek function can be
made to stop at the band edge and provide an interrupt,
or wrap the band and continue seeking until arriving at
the original departure frequency. The device sets
interrupts with found valid stations or, if the seek results
in zero found valid stations, the device indicates failure
and again sets an interrupt.
The Si4770/77-A20 seek functionality is performed
completely on-chip or can be brought out to a
companion processor. The Si4770/77-A20 can provide
base values for signal quality variables to a companion
processor for qualification or can further process the
base values to qualify valid or invalid stations.
The Si4770/77-A20 uses RSSI, SNR, and frequency
offset to qualify stations. These variables have
programmable thresholds to tailor the seek function to
the subjective tastes of customers.
RSSI is employed first to screen all possible candidate
stations. SNR and frequency offset are subsequently
used in screening the RSSI qualified stations. The more
thresholds the system engages, the higher the
confidence that any found stations will indeed be valid
broadcast stations; however, the more challenging
levels the thresholds are set to, the longer the overall
seek time as more stations and more qualifiers will be
assessed.
It is recommended that RSSI be set to a midlevel
threshold in conjunction with an SNR threshold set to a
level delivering acceptable audio performance. This
trade-off will eliminate very low RSSI stations whilst
keeping the seek time to acceptable levels. Generally,
the time to auto-scan and store valid channels for an
entire AM or FM band with all thresholds engaged is
very short depending on the band content.
Seek is initiated using the AM and FM seek commands.
The RSSI and SNR threshold settings are adjustable
using properties.
Rev. 0.9
41
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