Datasheet

LM49350, LM49350RLEVAL
www.ti.com SNAS359D –SEPTEMBER 2008–REVISED JUNE 2012
Table 47. DAC EFFECTS (0x71h)
Bits Field Description
0 DAC_ALC_ENB This enables the DAC's Auto Level Control.
1 DAC_PK_ENB This enables the DAC's Peak Detector.
2 DAC_EQ_ENB This enables the DAC's 5-band Parametric EQ.
3 DAC_3D_ENB This enables the DAC's Stereo Widening Circuit.
4 ADC_SCLP_ENB This enables the DAC's Soft Clip Feature.
Table 48. HPF MODE (0x80h)
Bits Field Description
2:0 HPF_MODE This configures the ADC's High Pass Filter. To calculate the –3dB cutoff frequency, multiply the
coefficient by the sample rate (Hz): f
C
= X
N
.f
S(Hz)
HPF_MODE Coefficient Filter Characteristics
f
C
= 220Hz for:
000 X
0
= 0.0275 8kHz Voice
001 X
1
= 0.01833 12kHz Voice
010 X
2
= 0.01375 16kHz Voice
011 X
3
= 0.009166 24kHz Voice
100 X
4
= 0.006875 32kHz Voice
f
C
= 100Hz for:
101 X
5
= 0.003125 32kHz Audio
110 X
6
= 0.0020833 48kHz Audio
f
C
=150Hz for:
111 X
7
= 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 LM49350. 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 auxiliary input amplifier gain or microphone preamplifier
gain. The dual ALCs can be used to regulate the level of the analog (Stereo Auxiliary, mono differential, Stereo
MIC/LINE) and digital (Port1 Data In, Port2 Data In) audio inputs. The ALC regulated output can be routed to any
of the LM49350’s amplifier outputs for playback. The ALC regulated output can also be routed to Audio Port1 or
Audio Port2 for digital data transmission via I
2
S 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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