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STA306

Basic Limiter and Volume Flow Diagram.

A limiter is basically a variable gain device, where the amount of gain applied depends on the input signal level.

As the name implies, compression limits the dynamic range of the signal.

The STA306 includes 2 independent limiter blocks.

The purpose of the limiters is to automatically reduce the dynamic range of the input signal to prevent the out-

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

ment 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; address 0x02, bit 7.

Each channel can be mapped to either limiter or not mapped. Non-mapped channels will clip when 0dBFS 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 LxAT registers. It is recommended in anti-clipping mode to

set this to 0dBFS, which corresponds to the maximum unclipped output power of a DDX amplifier. Since gain

can be added digitally within the STA306 it is possible to exceed 0dBFS or any other LxAT 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 (uncompression), when the gain is again increased, is dependent on a RMS-detect algo-

rithm. 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 register. When the RMS filter output falls below

the release threshold, the gain is again increased (uncompressed) at a rate dependent upon the Release Rate

register. The gain can never be increased past its 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". 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.

Table 7. Channel Limiter Mapping as a function of CxLS bits.

CxLS(1,0)

Channel Limiter Mapping

00

Channel has limiting disabled

01

Channel is mapped to limiter #1

10

Channel is mapped to limiter #2

Gain

Attenuation

Saturation

RMS

Limiter

Gain/Volume

Input

Output