STA308 View Datasheet(PDF) - STMicroelectronics
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STA308 Datasheet PDF : 33 Pages
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Basic Limiter and Volume Flow Diagram.
Gain/Volume
Limiter
Input
Gain
Attenuation
RMS
STA308
Saturation
Output
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 STA308 includes 2 independent limiter blocks. The purpose of the limiters is to automatically reduce the
dynamic range of the input signal 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; 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 STA308 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
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