TAS5705PAPR datasheet(36/71 Pages) TI

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RMSVoltage

Estimator

RMSVoltage

Estimator

RMSVoltage

Estimator

RMSVoltage

Estimator

AudioInput

CH1

AudioInput

CH2

AudioInput

CH3

AudioInput

CH4

Cut

Attack/DecayControl

DRC-Derived

GainCoefficient

Volume

{

Compression/Offset

AudioIn

AudioOut

B0309-01

TAS5705

SLOS549 – JUNE 2008 ...................................................................................................................................................................................................... www.ti.com

The threshold settings must be programmed as 32-bit (9.23 format) numbers.

Zero-valued and positive-valued threshold settings are not allowed, and cause unpredictable behavior if used.

• 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.

• Slopes k1 and k2 define the compression is to be performed within a given region, and the degree of

compression to be applied. Slopes are programmed as 26-bit (3.23 format) numbers.

DRC Implementation

Figure 4.13.1-1 shows the three elements comprising the DRC: (1) an RMS estimator, (2) a compression

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 though Ch4 , the individual channel estimates are computed. The

outputs of the estimators are then compared, sample-by-sample, and the largest-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

though Ch4, the programmable parameters apply to both RMS estimators. The time window over which the

RMS estimation is computed can be determined by

• Compression 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. The programmable parameters, T1, T2, O1, O2, K1, and

K2, define the compression regions for both DRCs 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

signal amplitude boost (expansion). Parameters aa and (1 – aa) set the attack time constant to be used for

signal amplitude cuts. The transition-time constants can be determined by

Figure 43. DRC Block Diagram

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