TAS5518 datasheet(35/103 Pages) TI

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Introduction

SLES115 — August 2004

TAS5518

•

Thresholds T1 and T2 define, in dB, the boundaries of the three regions of the DRC, as referenced to the

rms value of the data into the DRC. Zero valued threshold settings reference the maximum valued rms

input into the DRC and negative valued thresholds reference all other rms input levels. Positive valued

thresholds have no physical meaning and are not allowed. In addition, zero valued threshold settings are

not allowed.

Although the DRC input is limited to 32-bit words, the DRC itself operates using the 48-bit word format of the

DAP. The 32-bit samples input into the DRC are placed in the upper 32 bits of this 48-bit word space. This

means that the threshold settings must be programmed as 48-bit (25.23 format) numbers.

CAUTION: Zero valued and positive valued threshold settings are not allowed and

cause unpredictable behavior if used.

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Offsets O1 and O2 define, in dB, the attenuation (cut) or gain (boost) applied by the DRC-derived gain

coefficient at the threshold points T1 and T2 respectively. Positive offsets are defined as cuts, and thus

boost or gain selections are negative numbers. Offsets must be programmed as 48-bit (25.23 format)

numbers.

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Slopes k0, k1, and k2 define whether compression or expansion is to be performed within a given region,

and the degree of compression or expansion to be applied. Slopes are programmed as 28-bit (5.23 format)

numbers.

1.11.1 DRC Implementation

The three elements comprising the DRC: (1) an rms estimator, (2) a compression/expansion coefficient

computation engine, and (3) an attack/decay controller.

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RMS estimator—This DRC element derives an estimate of the rms value of the audio data stream into

the DRC. For the DRC block shared by CH1 and CH2, two estimates are computed—an estimate of the

CH1 audio data stream into the DRC, and an estimate of the CH2 audio data stream into the DRC. The

outputs of the two estimators are then compared, sample-by-sample, and the larger valued sample is

forwarded to the compression/expansion coefficient computation engine.

Two programmable parameters, ae and (1 – ae), set the effective time window over which the rms estimate

is made. For the DRC block shared by CH1 and CH2, the programmable parameters apply to both rms

estimators. The time window over which the rms estimation is computed can be determined by:

twindow +

* 1

FS n(1 * ae)

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Compression/expansion coefficient computation—This DRC element converts the output of the rms

estimator to a logarithmic number, determines the region that the input resides, and then computes and

outputs the appropriate coefficient to the attack/decay element. Seven programmable parameters—T1,

T2, O1, O2, k0, k1, and k2—define the three compression/expansion regions implemented by this

element.

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Attack/decay control—This DRC element controls the transition time of changes in the coefficient

computed in the compression/expansion coefficient computation element. Four programmable

parameters define the operation of this element. Parameters ad and 1 − ad set the decay or release time

constant to be used for volume boost (expansion). Parameters aa and 1 − aa set the attack time constant

to be used for volume cuts. The transition time constants can be determined by:

ta +

* 1

FS n(1 * aa)

td +

* 1

FS n(1 * ad)

1.11.2

Compression/Expansion Coefficient Computation Engine Parameters

There are seven programmable parameters assigned to each DRC block: two threshold parameters - T1 and

T2, two offset parameters - O1 and O2, and three slope parameters - k0, k1, and k2. The threshold parameters

establish the three regions of the DRC transfer curve, the offsets anchor the transfer curve by establishing

known gain settings at the threshold levels, and the slope parameters define whether a given region is a

compression or an expansion region.

TAS5518 Datasheet(PDF) 35 Page - Texas Instruments

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