SSM2804CBZ-R7 datasheet(15/36 Pages) AD

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SSM2804

Rev. 0 | Page 15 of 36

ALC Compression and Limiter Modes

The ALC implemented on the SSM2804 has two operation

modes: compression mode and limiter mode. When the ALC

is triggered for medium-level input signals, the ALC is in com-

pression mode. In this mode, an increase of the output signal is

one-third the increase of the input signal. For example, if the

input signal increases by 3 dB, the ALC reduces the amplifier

gain by 2 dB and, thus, the output signal increases by only 1 dB.

As the input signal becomes very large, the ALC transitions to

limiter mode. In this mode, the output stays at a given threshold

level, VTH, even if the input signal grows larger. As an example of

limiter mode operation, when a large input signal increases by

3 dB, the ALC reduces the amplifier gain by 3 dB and, thus, the

output increases by 0 dB. When the amplifier gain is reduced to

1 dB, the ALC cannot reduce the gain further, and the output

increases again. This is because the total range of the ALC opera-

tion has bottomed out due to extreme input voltage at high gain. To

avoid potential speaker damage, the maximum input amplitude

should not be large enough to exceed the maximum attenuation

(to a level of 1 dB) of the limiter mode.

Attack Time, Hold Time, and Release Time

When the amplifier input signal exceeds a preset threshold,

the ALC reduces amplifier gain rapidly until the output voltage

settles to a target level. This target level is maintained for a certain

period. If the input voltage does not exceed the threshold again,

the ALC increases the gain gradually.

The attack time is the time taken to reduce the gain from maxi-

mum to minimum. The hold time is the time that the reduced

gain is maintained. The release time is the time taken to increase

the gain from minimum to maximum. These times are shown

in Table 8. The attack time and the release time can be set using

the ALC 1 control register (Address 0x0A).

Table 8. ALC Attack, Hold, and Release Times

Time1

Duration

Attack Time

32 μs to 4 ms (per 0.5 dB step)

Hold Time

90 ms to 120 ms

Release Time

4 ms to 512 ms (per 0.5 dB step)

1 The attack time and release time can be adjusted using the I2C interface.

The hold time cannot be adjusted.

Soft-Knee Compression

Often performed using sophisticated DSP algorithms, soft-knee

compression provides maximum sound quality with effective

speaker protection. Instead of using a fixed compression setting

prior to limiting, the SSM2804 allows for a much more subtle

transition into limiting mode, preserving the original sound

quality of the source audio. Figure 31 to Figure 33 show the

various soft-knee compression settings that can be selected

using the COMP bit settings (Bits[D6:D5] of Register 0x0B).

0.5

1.0

1.5

2.0

2.5

0.05

0.10 0.15 0.20

0.30 0.35

0.40

0.45

0.25

0.50

INPUT VOLTAGE (V)

00 (COMPRESSION MODE 1)

01 (COMPRESSION MODE 2)

10 (COMPRESSION MODE 3)

11 (LIMITER MODE)

2.7V × 0.78 = 2.106V

Figure 31. Adjustable Compression Settings, PVDD = 2.7 V,

ALC Threshold Level = 78%

0.5

1.5

2.0

2.5

1.0

3.0

3.5

0.1

0.2

0.3

0.4

0.6

0.7

0.8

0.9

0.5

1.0

INPUT VOLTAGE (V)

00 (COMPRESSION MODE 1)

01 (COMPRESSION MODE 2)

10 (COMPRESSION MODE 3)

11 (LIMITER MODE)

3.6V × 0.78 = 2.808V

Figure 32. Adjustable Compression Settings, PVDD = 3.6 V,

ALC Threshold Level = 78%

0.5

1.5

2.0

2.5

1.0

3.0

4.0

3.5

4.5

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

1.0

2.0

INPUT VOLTAGE (V)

00 (COMPRESSION MODE 1)

01 (COMPRESSION MODE 2)

10 (COMPRESSION MODE 3)

11 (LIMITER MODE)

5.0V × 0.78 = 3.9V

Figure 33. Adjustable Compression Settings, PVDD = 5.0 V,

ALC Threshold Level = 78%

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