STA350B datasheet(62/94 Pages) STMICROELECTRONICS

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STA350B, STA350BW

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DocID018572 Rev 7

8.11.4

Limiter 2 attack/release threshold

The STA350Bx 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 E (addr 0x04) on page 46.

Each channel can be mapped to either limiter or not mapped, meaning that the channel

clips when 0 dBfs is exceeded. Each limiter looks at the present value of each channel that

is mapped to it, selects the maximum absolute value of all these channels, performs the

limiting algorithm on that value, and then if needed adjusts the gain of the mapped channels

in unison.

The limiter attack thresholds are determined by the LxAT registers if EATHx[7] bits are set

to 0 else the thresholds are determined by EATHx[6:0]. It is recommended in anti-clipping

mode to set this to 0 dBfs, which corresponds to the maximum unclipped output power of an

FFX amplifier. Since gain can be added digitally within the STA350Bx, it is possible to

exceed 0 dBfs or any other LxAT setting. When this occurs, the limiter, when active,

automatically starts 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. Gain

reduction occurs on a peak-detect algorithm. Setting the EATHx[7] bits to 1 selects the

anti-clipping mode.

The limiter release thresholds are determined by the LxRT registers if ERTHx[7] bits are set

to 0, else the thresholds are determined by ERTHx[6:0]. Setting the ERTHx[7] bits to 1

automatically selects the anti-clipping mode. The release of the limiter, when the gain is

again increased, is dependent on an RMS-detect algorithm. The output of the volume/limiter

block is passed through an RMS filter. The output of this filter is compared to the release

threshold, determined by the Release Threshold register. When the RMS filter output falls

below the release threshold, the gain is again increased at a rate dependent upon the

Release rate register. The gain can never be increased past its set value and, therefore, the

release only occurs 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

overlimiting can reduce the dynamic range to virtually zero and cause program material to

sound “lifeless”.

In AC mode, the attack and release thresholds are set relative to full-scale. In DRC mode,

the attack threshold is set relative to the maximum volume setting of the channels mapped

to that limiter and the release threshold is set relative to the maximum volume setting plus

the attack threshold.

L2AT3

L2AT2

L2AT1

L2AT0

L2RT3

L2RT2

L2RT1

L2RT0

0110

1001

STA350B Datasheet(PDF) 62 Page - STMicroelectronics

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