LC07424LP

No.A1619-29/32

Description of ALC operation

‹ Note › [xxxx] is a register name.

The ALC (Automatic Level Control) function performs automatic adjustment so that the audio level becomes the set value.

The amplifier gain of PGA (Programmable Gain Amplifier) is automatically controlled so that the ADC output audio level

becomes equal to the ALC value [ALC_VAL].

The PGA gain variable range is +34dB to -14dB. This range helps setting the maximum value [ALC_VMAX].

=› By setting [ALC_VMAX] to the maximum value (+34dB), the ALC function can be used to the maximum limit.

=› By setting [ALC_VMAX] to the gain zero (+0dB), there is no gain in the "+" direction. Namely, the operation becomes

equivalent to the limiter function.

ALC settings

• Power down function

When [ALC_PDX]=0, the ALC circuit enters the power DOWN mode.

• System operation

This is done by digitally processing and feeding back the ADC data.

For this purpose, the ADC function becomes necessary.

Therefore, to activate the ALC function;

[ALC_PDX]=1 (ALC power DOWN mode OFF )

[ALC_OFF]=0 (ALC function ON)

[ADC_A_PDX]=1, [ADC_D_PDX]=1 (ADC activated)

Manual mode (ALC Function OFF)

With [ALC_OFF]=1, the manual mode becomes effective.

The PGA gain becomes the [ALC_DVL] and [ALC_DVR] values.

ALC operating

There are "attack" and "recovery" operations. See the operation chart on the next page.

(1) Attack operations

When the PGA output exceeds the ALC value [ALC_VAL], the PGA gain is lowered in a ratio of the attack

factor [ALC_FA].

With zero cross detection [ALC_ZCD]=1, the gain reduction between zero crosses is limited by the limit

value [ALC_ATLIM].

(2) Recovery operations

When the PGA output is 2dB less than the ALC value [ALC_VAL] and this state continues for the recovery

standby period [ALC_RWT], the PGA gain is increased in a ratio of the recovery factor [ALC_FR].

The PGA gain increase continues while the state in which the gain is 2dB less than the ALC value

[ALC_VAL].

The PGA gain increment between zero crosses is maximum 1 dB.

Functions common to (1) and (2)

• Zero cross detection

With [ALC_ZCD]=1, PGA gain change occurs only when the PGA output is in the zero cross timing.

With [ALC_ZCD]=0, PGA gain change is made regardless of whether or not PGA output is zero cross.

• Zero cross timeout

If the PGA output does not zero cross, the zero cross signal is generated internally if there is no zero cross

signal for the period of zero cross timeout value [ALC_ZCDTM].

• Average output amplitude control

In the attack operation by spike noise, etc., the average output amplitude becomes small.

To prevent this, the recover speed is raised automatically above the [ALC_FR] set value in the case of

excessively large input for a short period (tSN).