U4255BM-M Datasheet, PDF (11/32 Pages) ATMEL Corporation – AM / FM RECEIVER IC

Soft Mute
U4255BM-M
The soft mute functionality is implemented to reduce the output level of the FM demodu-
lator at low input signal levels to limit the noise at MPX output in this case. If the input
level falls below an adjustable threshold, continuously the output of the FM demodulator
is continuously muted with decreasing input level until a maximum mute value is
reached. The threshold for the start of soft mute and the maximum mute can be
adjusted. The signal level for 3 dB mute can be set by means of Bit 68 to 70 and the
maximum value for soft mute can be selected by Bit 67. The steepness and the time
constant of the soft mute can be adjusted by the resistor and capacitor between Pin 34
and Pin 29.
The field strength signal available at Pin 9 is used for soft mute. Therefore, the soft mute
threshold referred to the input of the FM mixer depends on the gain from FM mixer input
to the field strength sensor.
Table 18. Soft Mute Threshold
Relative Soft Mute Threshold
Soft mute OFF
-18 dB
-15 dB
-12 dB
-9 dB
-6 dB
-3 dB
0 dB
Bit 70
0
0
0
0
1
1
1
1
Bit 69
0
0
1
1
0
0
1
1
Bit 68
0
1
0
1
0
1
0
1
Table 19. Maximum Soft Mute
Maximum Value of Soft Mute
28 dB
24 dB
Bit 67
0
1
Figure 6. Soft Mute
+
Pin 34
FS (Pin9)
Bit 68..70
Pin 29
Bit 67
Gain FM-Demodulator
V3
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4519C–AUDR–07/02