DA7212 datasheet(32/129 Pages) DIALOG

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DA7212

Ultra-low power stereo codec

Company confidential

Datasheet

Revision 3c

24-Nov-2015

32 of 129

13.8 Automatic level control (ALC)

For improved sound recordings of signals with a large volume range, the DA7212 offers a fully-

configurable automatic recording level control (ALC) for microphone inputs. This is enabled via the

ALC_L_EN and ALC_R_EN controls, and can be enabled independently on either left or right

channel. It is recommended that the ALC is only enabled in stereo as this applies the same gain to

both channels and so protects the pan of stereo signals.

The ALC monitors the digital signal after the ADC and adjusts the microphones’ analogue and digital

gain to maintain a constant recording level, whatever the analogue input signal level.

Operation of ALC is illustrated in Figure 9. When the input signal volume is high, the ALC system will

reduce the overall gain until the output volume is below the specified maximum value. When the

input signal volume is low, the ALC will increase the gain until the output volume increases above the

specified minimum value. If the output signal is within the desired signal level (between the specified

minimum and maximum levels), the ALC does nothing.

The maximum and the minimum thresholds that trigger a gain change of the ALC are programmed

by the ALC_THRESHOLD_MAX and ALC_THRESHOLD_MIN controls.

ALC Input

ALC Gain

ALC Output

Release time

Attack time

Figure 9: Principle of operation of the ALC

The total gain is made up of an analogue gain, which is applied to the microphone PGAs, and a

digital gain, which is implemented in the filtering stage. The ALC block monitors and controls the gain

of the microphone PGAs and the ADC. Note that although the ALC is controlling the gain, it does not

modify any of the registers MIC_1_AMP_GAIN, MIC_2_AMP_GAIN, ADC_L_DIGITAL_GAIN and

ADC_R_DIGITAL_GAIN. These registers are ignored while the ALC is in operation.

The minimum and maximum levels of digital gain that can be applied by the ALC are controlled using

ALC_ATTEN_MAX and ALC_GAIN_MAX.

Similarly the minimum and maximum levels of analogue gain are controlled by ALC_ANA_GAIN_MIN

and ALC_ANA_GAIN_MAX. The rates at which the gain is changed are defined by the attack and

decay rates in register ALC_CTRL2. When attacking, the gain decreases with ALC_ATTACK rate.

When decaying, the gain increases with ALC_RELEASE rate.

The hold-time is defined by ALC_HOLD in the ALC_CTRL3 register. This controls the length of time

that the system maintains the current gain level before starting to decay. This prevents unwanted

changes in the recording level when there is a short-

lived ‘spike’ in input volume, for example when

recording speech.

Typically the attack rate should be much faster than the decay rate, as it is necessary to reduce

rapidly increasing waveforms as quickly as possible, whereas fast release times will result in the

DA7212 Datasheet(PDF) 32 Page - Dialog Semiconductor

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