LM49352 datasheet(60/106 Pages) TI

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LM49352

SNAS467E – MARCH 2009 – REVISED MARCH 2013

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Table 44. DAC EFFECTS (0x71h) (continued)

Bits

Field

Description

DAC_EQ_ENB

This enables the DAC's 5-band Parametric EQ.

RSVD

Reserved

ADC_SCLP_ENB

This enables the DAC's Soft Clip Feature.

Table 45. HPF MODE (0x80h)

Bits

Field

Description

2:0

HPF_MODE

This configures the ADC's High Pass Filter (HPF). To calculate the –3dB cutoff frequency, multiply the

coefficient by the sample rate (Hz): fC = Xn.fS(Hz)

HPF_MODE

Coefficient

Filter Characteristics

fC = 220Hz for:

000

X0 = 0.0275

8kHz Voice

001

X1 = 0.01833

12kHz Voice

010

X2 = 0.01375

16kHz Voice

011

X3 = 0.009166

24kHz Voice

100

X4 = 0.006875

32kHz Voice

fC = 100Hz for:

101

X5 = 0.003125

32kHz Audio

110

X6 = 0.0020833

48kHz Audio

fC = 150Hz for:

111

X7 = 0.0015625

96kHz Audio

ALC Overview

The Automatic Level Control (ALC) system can be used to regulate the audio output level to a user defined

target level. The ALC feature is especially useful whenever the level of the audio input is unknown,

unpredictable, or has a large dynamic range. The main purpose of the ALC is to optimize the dynamic range of

the audio input to audio output path.

There are two separate and independent ALC circuits in the LM49352. One of the ALC circuits is located within

the DAC DSP effects block. The other ALC circuit is integrated into the ADC DSP effects block. The DAC ALC

controls the DAC digital gain. The ADC ALC controls the mono/auxiliary input amplifier gain or microphone

preamplifier gain. The dual ALCs can be used to regulate the level of the analog (AUX, MONO, MIC) and digital

(Port1 Data In, Port2 Data In) audio inputs. The ALC regulated output can be routed to any of the LM49352’s

amplifier outputs for playback. The ALC regulated output can also be routed to Audio Port1 or Audio Port2 for

digital data transmission via I2S or PCM.

Only audio inputs that are considered signals (rather than noise) are sent to the ALC’s peak detector block. The

peak detector compares the level of the audio input versus the ALC target level (TARGET_LEVEL). Signals

lower than the target level will be amplified and signals higher than the target level will be attenuated. Any audio

input that is lower than the level specified by the noise floor level (NOISE_FLOOR) will be considered as noise

and will be gated from the ALC’s peak detector in order to avoid noise pumping. So it is important to set

NOISE_FLOOR to correlate with the signal to noise ratio of the corresponding audio path. In some instances (ie.

Conference calls), it may be desirable to mute audio input signals that consist solely of background noise from

the audio output. This is accomplished by enabling the ALC’s noise gate (NG_ENB). When the noise gate is

enabled, signals lower than the noise floor level will be muted from the audio output.

If the audio input signal is below the target level, the ALC will increase the gain of the corresponding volume

control until the signal reaches the target level. The rate at which the ALC performs gain increases is known as

decay rate (DECAY RATE). But before each ALC gain increase the ALC must wait a predetermined amount of

time (HOLD TIME). If the audio input signal is above the target level, the ALC will decrease the gain of the

corresponding volume control until the signal reaches the target level. The rate at which the ALC performs

attenuation is known as attack rate (ATTACK RATE). The ALC’s peak detector tracks increases in audio input

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