DRC Input Level

DRC − Compensated Output

1:1 Transfer Function

Implemented Transfer Function

Region

M0014-01

TAS5518C

8-ChannelDigitalAudioPWMProcessor

SLES238A–SEPTEMBER2008–REVISEDJULY2009

www.ti.com

Figure2-17illustratesatypicalDRCtransferfunction.

Figure2-17.DynamicRangeCompression(DRC)TransferFunctionStructure

ThethreeregionsshowninFigure2-17aredefinedbythreesetsofprogrammablecoefficients:

•ThresholdsT1andT2defineregionboundaries.

•OffsetsO1andO2definetheDRCgaincoefficientsettingsatthresholdsT1andT2,respectively.

•Slopesk0,k1,andk2definewhethercompressionorexpansionistobeperformedwithinagiven

region.Themagnitudesoftheslopesdefinethedegreeofcompressionorexpansiontobeperformed.

Thethreesetsofparametersarealldefinedinlogarithmicspaceandadheretothefollowingrules:

•ThemaximuminputsampleintotheDRCisreferencedat0dB.Allvaluesbelowthismaximumvalue

thenhavenegativevaluesinlogarithmic(dB)space.

•ThesamplesinputintotheDRCare32-bitwordsandconsistoftheupper32bitsofthe48-bitword

formatusedbythedigitalaudioprocessor(DAP).The48-bitDAPwordisderivedfromthe32-bitserial

datareceivedattheserial-audioreceiveportbyadding8bitsofheadroomabovethe32-bitwordand

8bitsofcomputationalprecisionbelowthe32-bitword.Iftheaudio-processingstepsbetweentheSAP

inputandtheDRCinputresultinnoaccumulativeboostorcut,theDRCoperatesonthe8bitsof

headroomandthe24MSBsoftheaudiosample.Undertheseconditions,a0-dB(maximumvalue)

audiosample(0x7FFFFFFF)isseenattheDRCinputasa–48-dBsample(8bits×–6dB/bit=–48

dB).

•ThresholdsT1andT2define,indB,theboundariesofthethreeregionsoftheDRC,asreferencedto

thermsvalueofthedataintotheDRC.Zero-valuedthresholdsettingsreferencethemaximum-valued

rmsinputintotheDRCandnegative-valuedthresholdsreferenceallotherrmsinputlevels.

Positive-valuedthresholdshavenophysicalmeaningandarenotallowed.Inaddition,zero-valued

thresholdsettingsarenotallowed.

AlthoughtheDRCinputislimitedto32-bitwords,theDRCitselfoperatesusingthe48-bitwordformatof

theDAP.The32-bitsamplesinputintotheDRCareplacedintheupper32bitsofthis48-bitwordspace.

Thismeansthatthethresholdsettingsmustbeprogrammedas48-bit(25.23format)numbers.

CAUTION

Zero-valuedandpositive-valuedthresholdsettingsarenotallowedandcause

unpredictablebehaviorifused.

•OffsetsO1andO2define,indB,theattenuation(cut)orgain(boost)appliedbytheDRC-derivedgain

coefficientatthethresholdpointsT1andT2,respectively.Positiveoffsetsaredefinedascuts,and

thusboostorgainselectionsarenegativenumbers.Offsetsmustbeprogrammedas48-bit(25.23

format)numbers.

Description 34SubmitDocumentationFeedback

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2.10.1DRCImplementation

window

* 1

ȏn(1 * ae)

ȏn(1 * aa)

ȏn(1 * ad)

2.10.2Compression/ExpansionCoefficientComputationEngineParameters

TAS5518C

8-ChannelDigitalAudioPWMProcessor

www.ti.com

SLES238A–SEPTEMBER2008–REVISEDJULY2009

•Slopesk0,k1,andk2definewhethercompressionorexpansionistobeperformedwithinagiven

region,andthedegreeofcompressionorexpansiontobeapplied.Slopesareprogrammedas28-bit

(5.23format)numbers.

ThethreeelementscomprisingtheDRCinclude:(1)anrmsestimator,(2)acompression/expansion

coefficientcomputationengine,and(3)anattack/decaycontroller.

•RMSestimator—ThisDRCelementderivesanestimateofthermsvalueoftheaudiodatastreaminto

theDRC.FortheDRCblocksharedbyCh1andCh2,twoestimatesarecomputed—anestimateofthe

Ch1audiodatastreamintotheDRC,andanestimateoftheCh2audiodatastreamintotheDRC.The

outputsofthetwoestimatorsarethencompared,sample-by-sample,andthelarger-valuedsampleis

forwardedtothecompression/expansioncoefficientcomputationengine.

Twoprogrammableparameters,aeand(1–ae),settheeffectivetimewindowoverwhichtherms

estimateismade.FortheDRCblocksharedbyCh1andCh2,theprogrammableparametersapplyto

bothrmsestimators.Thetimewindowoverwhichthermsestimationiscomputedcanbedetermined

by:

•Compression/expansioncoefficientcomputation—ThisDRCelementconvertstheoutputoftherms

estimatortoalogarithmicnumber,determinestheregionwheretheinputresides,andthencomputes

andoutputstheappropriatecoefficienttotheattack/decayelement.Sevenprogrammableparameters,

T1,T2,O1,O2,k0,k1,andk2,definethethreecompression/expansionregionsimplementedbythis

element.

•Attack/decaycontrol—ThisDRCelementcontrolsthetransitiontimeofchangesinthecoefficient

computedinthecompression/expansioncoefficientcomputationelement.Fourprogrammable

parametersdefinetheoperationofthiselement.Parametersadand(1–ad)setthedecayorrelease

timeconstanttobeusedforvolumeboost(expansion).Parametersaaand(1–aa)settheattacktime

constanttobeusedforvolumecuts.Thetransitiontimeconstantscanbedeterminedby:

SevenprogrammableparametersareassignedtoeachDRCblock:twothresholdparameters—T1and

T2,twooffsetparameters—O1andO2,andthreeslopeparameters—k0,k1,andk2.Thethreshold

parametersestablishthethreeregionsoftheDRCtransfercurve,theoffsetsanchorthetransfercurveby

establishingknowngainsettingsatthethresholdlevels,andtheslopeparametersdefinewhetheragiven

regionisacompressionoranexpansionregion.

TheaudioinputstreamintotheDRCmustpassthroughDRC-dedicatedprogrammableinputmixers.

Thesemixersareprovidedtoscalethe32-bitinputintotheDRCtoaccountforthepositioningofthe

audiodatainthe48-bitDAPwordandthenetgainorattenuationinsignallevelbetweentheSAPinput

andtheDRC.Theselectionofthresholdvaluesmusttakethegain(attenuation)ofthesemixersinto

account.TheDRCimplementationexamplesthatfollowillustratetheeffectthesemixershaveon

establishingthethresholdsettings.

T2establishestheboundarybetweenthehigh-volumeregionandthemid-volumeregion.T1establishes

theboundarybetweenthemid-volumeregionandthelow-volumeregion.Boththresholdsaresetin

logarithmicspace,andwhichregionisactiveforanygivenrmsestimatoroutputsampleisdeterminedby

thelogarithmicvalueofthesample.

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Not Recommended For New Designs

No Discontinuity

T1 * T2

k1 ) O2 For (

)

2.10.2.1ThresholdParameterComputation

SUB_ADDRESS_ENTRY

*64

*6.0206

+ 10.63

2.10.2.2OffsetParameterComputation

INPUT

DESIRED

) 24.0824 dB

6.0206

INPUT

–21 dB ) 24.0824 dB

6.0206

+ 0.51197555

+ 0.1000_0011_0001_1101_0100

+ 0x00000041886A in 25.23 format

TAS5518C

8-ChannelDigitalAudioPWMProcessor

SLES238A–SEPTEMBER2008–REVISEDJULY2009

www.ti.com

ThresholdT2servesasthefulcrumorpivotpointintheDRCtransferfunction.O2definestheboost

(>0dB)orcut(<0dB)implementedbytheDRC-derivedgaincoefficientforanrmsinputlevelofT2.If

O2=0dB,thevalueofthederivedgaincoefficientis1(0x00800000in5.23format).k2istheslopeof

theDRCtransferfunctionforrmsinputlevelsaboveT2,andk1istheslopeoftheDRCtransferfunction

forrmsinputlevelsbelowT2(andaboveT1).ThelabelingofT2asthefulcrumstemsfromthefactthat

therecannotbeadiscontinuityinthetransferfunctionatT2.Theusercan,however,settheDRC

parameterstorealizeadiscontinuityinthetransferfunctionattheboundarydefinedbyT1.Ifno

discontinuityisdesiredatT1,thevaluefortheoffsettermO1mustobeythefollowingequation.

T1andT2arethethresholdsettingsindB,k1istheslopeforregion1,andO2istheoffsetindBatT2.If

theuserchoosestoselectavalueofO1thatdoesnotobeytheaboveequation,adiscontinuityatT1is

realized.

DecreasinginvolumefromT2,theslopek1remainsineffectuntiltheinputlevelT1isreached.If,atthis

inputlevel,theoffsetofthetransferfunctioncurvefromthe1:1transfercurvedoesnotequalO1,there

isadiscontinuityatthisinputlevelasthetransferfunctionissnappedtotheoffsetcalledforbyO1.Ifno

discontinuityiswanted,O1and/ork1mustbeadjustedsothatthevalueofthetransfercurveatinputlevel

T1isoffsetfromthe1:1transfercurvebythevalueO1.Theexamplesthatfollowillustrateboth

continuousanddiscontinuoustransfercurvesatT1.

DecreasinginvolumefromT1,startingatoffsetlevelO1,slopek0definesthecompression/expansion

activityinthelowerregionoftheDRCtransfercurve.

Forthresholds,

=–6.0206T

INPUT

=–6.0206T

SUB_ADDRESS_ENTRY

If,forexample,itisdesiredtosetT1=–64dB,thenthesubaddressentryrequiredtosetT1to–64dBis:

T1isenteredasa48-bitnumberin25.23format.Therefore:

T1=10.63=01010.10100001010001111010111

=0x00000550A3D7in25.23format

TheoffsetssettheboostorcutappliedbytheDRC-derivedgaincoefficientatthethresholdpoint.An

equivalentstatementisthatoffsetsrepresentthedepartureoftheactualtransferfunctionfroma1:1

transferatthethresholdpoint.Offsetsare25.23-formatted,48-bitlogarithmicnumbers.Theyare

computedbythefollowingequation.

Gainsorboostsarerepresentedasnegativenumbers;cutsorattenuationsarerepresentedaspositive

numbers.Forexample,toachieveaboostof21dBatthresholdT1,theI

Ccoefficientvalueenteredfor

O1mustbe:

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Part #	TAS5518CPAG
Description	8 CHANNEL DIGITAL AUDIO PWM PROCESSOR
Category	IC
Availability	In Stock
Qty	2

TAS5518CPAG

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