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2.10.1 DRC Implementation

window

n(1 * ae)

ta +

n(1 * aa)

n(1 * ad)

2.10.2 Compression/Expansion Coefficient Computation Engine Parameters

TAS5508

8-Channel Digital Audio PWM Processor

SLES091D – FEBRUARY 2004 – REVISED JULY 2009

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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.

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

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

• 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:

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

estimator to a logarithmic number, determines the region where 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.

• 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:

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

The audio input stream into the DRC must pass through DRC-dedicated programmable input mixers.

These mixers are provided to scale the 32-bit input into the DRC to account for the positioning of the

audio data in the 48-bit DAP word and the net gain or attenuation in signal level between the SAP input

and the DRC. The selection of threshold values must take the gain (attenuation) of these mixers into

account. The DRC implementation examples that follow illustrate the effect these mixers have on

establishing the threshold settings.

T2 establishes the boundary between the high-volume region and the mid-volume region. T1 establishes

the boundary between the mid-volume region and the low-volume region. Both thresholds are set in

logarithmic space, and which region is active for any given rms estimator output sample is determined by

the logarithmic value of the sample.

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