STA335BWSQS datasheet(49/70 Pages) STMICROELECTRONICS

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STA335BWSQS

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6.11.4

Limiter 2 attack/release threshold

The STA335BWSQS 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 F, bit 0 address 0x05. Each

channel can be mapped to either limiter or not mapped, meaning that channel will clip 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. 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 STA335BWSQS

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. 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 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 over-limiting 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

011

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