STA308A datasheet(45/63 Pages) STMICROELECTRONICS

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STA308A

Registers

45/63

7.2.49

Limiter 1 attack/release threshold (0x30)

7.2.50

Limiter 2 attack/release rate (0x31)

7.2.51

Limiter 2 attack/release threshold (0x32)

7.2.52

Bit description

The STA308A includes two independent limiter blocks. The purpose of the limiters is to

automatically reduce the dynamic range of a recording to prevent the outputs from clipping

in anti-clipping mode or to actively reduce the dynamic range for a better listening

environment such as a night-time listening mode which is often needed for DVDs. The two

modes are selected via the DRC bit in Configuration Register B, bit 7 address 0x02. Each

channel can be mapped to either limiter or not mapped, meaning that channel will clip when

0 dBFS is exceeded. Each limiter will look at the present value of each channel that is

mapped to it, select the maximum absolute value of all these channels, perform the limiting

algorithm on that value, and then if needed adjust the gain of the mapped channels in

unison.

The limiter attack thresholds are determined by the LnAT registers. It is recommended in

anti-clipping mode to set this to 0 dBFS, which corresponds to the maximum unclipped

output power of a DDX amplifier. Since gain can be added digitally within the STA308A it is

possible to exceed 0 dBFS or any other LnAT setting, when this occurs, the limiter, when

active, will automatically start reducing the gain. The rate at which the gain is reduced when

the attack threshold is exceeded is dependent upon the attack rate register setting for that

limiter. The gain reduction occurs on a peak-detect algorithm.

The release of limiter, when the gain is again increased, is dependent on a RMS-detect

algorithm. The output of the volume/limiter block is passed through a RMS filter. The output

of this filter is compared to the release threshold, determined by the Release Threshold

increased at a rate dependent upon the Release Rate register. The gain can never be

increased past it's set value and therefore the release will only occur if the limiter has

already reduced the gain. The release threshold value can be used to set what is effectively

a minimum dynamic range, this is helpful as over-limiting can reduce the dynamic range to

virtually zero and cause program material to sound lifeless.

L1AT3

L1AT2

L1AT1

L1AT0

L1RT3

L1RT2

L1RT1

L1RT0

L2A3

L2A2

L2A1

L2A0

L2R3

L2R2

L2R1

L2R0

L2AT3

L2AT2

L2AT1

L2AT0

L2RT3

L2RT2

L2RT1

L2RT0

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