Meaning of backgroundNdxFilename in IvTest?

[lbjcom]

I am testing ivector based ASV system by following the ALIZE tutorial. But I don't know the exact meaning of the backgroundNdxFilename parameter in the IvTest configuration. Would you please explain the meaning of the parameter?

• ivTest_Plda.cfg

%numThread				2
verboseLevel				1


outputFilename 				res/scores_PLDA_lengthnorm.txt
outputScoreFormat			ascii
gender					M
ndxFilename				ndx/ivTest_plda_target-seg.ndx
targetIdList				ndx/trainModel.ndx
scoring					plda
ivNorm					false
ivNormIterationNb			1

pldaLoadModel				true
pldaEigenVoiceMatrix			plda_F
pldaEigenChannelMatrix			plda_G
pldaSigmaMatrix				plda_Sigma
pldaMeanVec				plda_originalMean
pldaMinDivMean				plda_minDivMean

loadVectorFilesExtension		.y
loadVectorFilesPath			./iv/lengthNorm/
testVectorFilesPath			./iv/lengthNorm/
matrixFilesPath				./mat/
saveMatrixFilesExtension		.matx
loadMatrixFilesExtension		.matx

ivNormLoadParam				true
ivNormEfrMatrixBaseName			EfrMat
ivNormEfrMeanBaseName			EfrMean

%This parameter!!
backgroundNdxFilename			ndx/Plda.ndx 



loadMatrixFormat			DB
saveMatrixFormat			DB
ivNormEfrMode				EFR
iVectSize				10
pldaEigenVoiceNumber			5
pldaEigenChannelNumber			2
pldaNbIt				10
pldaLoadInitMatrices			false