Datasheet
LM49370
SNAS356D –FEBRUARY 2007–REVISED MARCH 2012
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Table 13. ADC_2 (0x07h) (continued)
Bit Field Description
s
4:2 AGC_FRAME_TIME This sets the frame time to be used by the AGC algorithm. In a given frame, the AGC's peak detector
determines the peak value of the incoming microphone audio signal and compares this value to the target
value of the AGC defined by AGC_TARGET (bits [3:1] of register (0x08h)) in order to adjust the microphone
preamplifier's gain accordingly. AGC_FRAME_TIME basically sets the sample rate of the AGC to adjust for a
wide variety of speech patterns.
(1)
AGC_FRAME_TIME Time (ms)
000
2
96
001
2
128
010
2
192
011
2
256
100
2
384
101
2
512
110
2
768
111
2
1000
6:5 ADC_CLK This selects the clock to be used by the audio ADC system.
ADC_CLK Source
00
2
MCLK
01
2
PLL_OUTPUT
10
2
I2S_CLK_IN
11
2
PCM_CLK_IN
7 NGZXDD If set, the noise gate will not wait for a zero crossing before mute/unmuting. This bit should be set if the ADC's
HPF is disabled and if there is a large DC or low frequency component at the ADC input.
NGZXDD Result
0 Noise Gate operates on ZXD events
1 Noise Gate operates on frame boundaries
(1) Refer to the AGC Overview for further detail.
AGC_1 CONFIGURATION REGISTER
This register is used to control the LM49370's Automatic Gain Control.
(2)
Table 14. AGC_1 (0x08h)
Bit Field Description
s
If set, the AGC controls the analog microphone preamplifier gain into the system. This feature is useful for
0 AGC_ENABLE
microphone signals that are routed to the ADC.
3:1 AGC_TARGET This programs the target level of the AGC. This will depend on the expected transients and desired headroom.
Refer to AGC_TIGHT (bit 7 of 0x09h) for more detail.
AGC_TARGET Target Level
000
2
−6 dB
001
2
−8 dB
010
2
−10 dB
011
2
−12 dB
100
2
−14 dB
101
2
−16 dB
110
2
−18 dB
111
2
−20 dB
If set, signals below the noise gate threshold are muted. The noise gate is only activated after a set period of
4 NOISE_GATE_ON
signal absence.
(2) See the AGC Overview.
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