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Rev. 0.9
8.1.3. Interrupts
The interrupt status can be monitored by sending command 0x15 GET_INT_STATUS. Note that the command
response varies based on FM or AM mode. Interrupt pins can be configured by sending command 0x1C
INTB_PIN_CFG.
Property 0x0000 INT_CTL_ENABLE enables top-level interrupt sources and interrupt repetition characteristics.
Property group 0x1200 configures RSQ (RSSI, SNR) interrupts. Property group 0x1300 configures ACF (Softmute,
Channel Filter Bandwidth, HIBLEND, HICUT) interrupts. Note that HIBLEND only applies to FM mode operation.
Property group 0x4000 configures RDS interrupts.
8.1.4. Noise Blankers
8.1.4.1. FM
Property group 0x2100 configures the noise blankers for FM mode.
The FM noise blanking system consists of a detector and audio sample blanker after the FM demodulator.
The detector compares average USN with USN scaled by the FM_NB_DETECT_THRESHOLD property setting. If
the scaled USN is greater than the filtered USN audio samples will be blanked. Lower values of
FM_NB_DETECT_THRESHOLD result in more aggressive noise blanking.
For weak signal levels, the USN of only the L+R portion of the MPX spectrum is used as the detector input. For
higher signal levels, the USN of the entire MPX spectrum is used as the detector input. The signal levels at which
the switch between weak/strong blanking occurs is set by the FM_NB_STRONG_WEAK_SWITCH property (by
default, SNR < 9 dB for weak signal blanking and SNR >11 dB for strong signal blanking). For strong signal levels
(RSSI > 50 dB
μ
V) the noise blanker is disabled as configured by
FM_NB_STRONG_SIGNAL_SHUTOFF.
The number of samples to blank is set by the FM_NB_INTERVAL property setting. Higher values result in more
aggressive noise blanking, as more samples are replaced with repaired ones.
8.1.4.2. AM
Property group 0x2100 configures the noise blankers for AM modes.
The AM noise blanking system consists of a wide band detector and I/Q sample blanker before the first channel
filter and a narrow band detector and I/Q sample blanker after the first channel filter.
The detectors compare average USN with USN scaled by the AM_NB_DETECT_THRESHOLD (wide band) and
AM_SECONDARY_NB_DETECT_THRESHOLD (narrow band) property setting. If the scaled USN is greater than
the filtered USN audio samples will be blanked. Lower values of AM_NB_DETECT_THRESHOLD or
AM_SECONDARY_NB_DETECT_THRESHOLD result in more aggressive noise blanking.
The narrow band noise blanker with pre-channel filter is enabled if ±1 channel adjacent blocker is detected. In this
mode, the wide band detector AM_NB_DETECT_THRESHOLD is used.
The narrow band noise blanker without pre-channel filter is enabled if ±2 or ±3 channel adjacent blocker is
detected. In this mode, the narrow band detector AM_SECONDARY_NB_DETECT_THRESHOLD is used.
The wide band noise blanker is enabled if no adjacent blocker is detected. In this mode, the wide band detector
AM_NB_DETECT_THRESHOLD is used.
8.1.5. Pop Filter
Property group 0x3200 configures the pop filter for FM mode operation.
Two independent pop filter engines can be configured for minimum and maximum limiting of the MPX using RSSI
and USN as the metric. Limiting ranging from minimum to maximum occurs at the specified attack rate as the RSSI
or USN metric increases above the minimum threshold and reaches the maximum threshold. Limiting ranging from
maximum to minimum occurs at the specified release rate as the RSSI or USN metric decreases below the
maximum threshold and reaches the minimum threshold. The resulting limiting level is the maximum of the result of
the two pop filter engines. Refer to Figure 10.
