ReleaseTime

E

Limiterthreshold

GAIN

A

C

B

D

AttackTime

Limiterthreshold

Limiterthreshold

HoldTime

Limiterthreshold

TPA2029D1

www.ti.com

SLOS661A – DECEMBER 2011 – REVISED APRIL 2012

APPLICATION INFORMATION

AUTOMATIC GAIN CONTROL

The Automatic Gain Control (AGC) feature provides continuous automatic gain adjustment to the amplifier

through an internal PGA. This feature enhances the perceived audio loudness and at the same time prevents

speaker damage from occurring (Limiter function).

The AGC works by detecting the audio input envelope. The gain changes depending on the amplitude, the limiter

level, the compression ratio, and the attack and release time. The gain changes constantly as the audio signal

increases and/or decreases to create the compression effect. The gain step size for the AGC is 0.5 dB. If the

audio signal has near-constant amplitude, the gain does not change. Figure 15 shows how the AGC works.

A.

Gain decreases with no delay; attack time is reset. Release time and hold time are reset.

B.

Signal amplitude above limiter level, but gain cannot change because attack time is not over.

C.

Attack time ends; gain is allowed to decrease from this point forward by one step. Gain decreases because the

amplitude remains above limiter threshold. All times are reset

D.

Gain increases after release time finishes and signal amplitude remains below desired level. All times are reset after

the gain increase.

E.

Gain increases after release time is finished again because signal amplitude remains below desired level. All times

are reset after the gain increase.

Figure 15. Input and Output Audio Signal vs Time

Copyright © 2011–2012, Texas Instruments Incorporated

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