2.10.2.3SlopeParameterComputation
Forn:1compression,theslopekcanbefoundby:
k=n–1meansk>0forn>1.Likewise,forcompression,
means–1<k<0forn>1.Thus,it
0.5 : 1 compression å k +
1
0.5
* 1 + 1
1 : 2 expansion å k + 2 * 1 + 1
Compression equation: k + *4 +
1
n
*1 å n + *
1
3
å *0.3333 : 1 compression
Expansion equation: k + *4 + n * 1 å n + *3 å 1 : *3 expansion
2.11OutputMixer
TAS5518C
8-ChannelDigitalAudioPWMProcessor
www.ti.com
SLES238A–SEPTEMBER2008–REVISEDJULY2009
Indevelopingtheequationsusedtodeterminethesubaddressoftheinputvaluerequiredtorealizea
givencompressionorexpansionwithinagivenregionoftheDRC,thefollowingconventionisadopted.
DRCtransfer=Inputincrease:Outputincrease
IftheDRCrealizesanoutputincreaseofndBforeverydBincreaseinthermsvalueoftheaudiointothe
DRC,a1:nexpansionisbeingperformed.IftheDRCrealizesa1-dBincreaseinoutputlevelforevery
n-dBincreaseinthermsvalueoftheaudiointotheDRC,ann:1compressionisbeingperformed.
k=n–1
Inbothexpansion(1:n)andcompression(n:1),nisimpliedtobegreaterthan1.Thus,forexpansion:
appearsthatkmustalwayslieintherangek>–1.
TheDRCimposesnosuchrestrictionandkcanbeprogrammedtovaluesasnegativeas–15.999.To
determinewhatresultswhensuchvaluesofkareentered,itisfirsthelpfultonotethatthecompression
andexpansionequationsforkareactuallythesameequation.Forexample,a1:2expansionisalsoa
0.5:1compression.
Ascanbeseen,thesamevalueforkisobtainedeitherway.Theabilitytochoosevaluesofklessthan–1
allowstheDRCtoimplementnegative-slopetransfercurveswithinagivenregion.Negative-slopetransfer
curvesareusuallynotassociatedwithcompressionandexpansionoperations,butthedefinitionofthese
operationscanbeexpandedtoincludenegative-slopetransferfunctions.Forexample,ifk=–4
Withk=–4,theoutputdecreases3dBforevery1dBincreaseinthermsvalueoftheaudiointothe
DRC.Astheinputincreasesinvolume,theoutputdecreasesinvolume.
TheTAS5518Cprovidesan8×2outputmixerforchannels1,2,3,4,5,and6.Forchannels7and8,the
TAS5518Cprovidesan8×3outputmixer.Thesemixersalloweachoutputtobeanymixofanytwo(or
three)signal-processedchannels.Thecontrolparametersfortheoutputcrossbarmixerareprogrammable
viatheI
2
Cinterface.AlloftheTAS5518Cfeaturesareavailablewhenthe8×2and8×3outputmixersare
configuredinthepass-throughoutputmixerconfiguration,wheretheaudiodatafromeachDAPchannel
mapsdirectlytothecorrespondingPWMchannel(i.e.,DAPchannel1toPWMchannel1,etc).
WhenmixingorremappingDAPchannelstodifferentPWMoutputchannelstherearelimitationsto
consider:
•Individualchannelmuteshouldnotbeused.
•Thesumoftheminimumchannelvolumeandmastervolumeshouldnotbebelow–109dB.
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