Add silenceThreshold parameter to MFCC (#543)

Add silenceThreshold parameter default to 1e-9.

Get rid of pointer to compressor function because it is not flexible
as soon as one needs to run functions with different number of
parameters (amp2db vs linear) or different threshold log values (amb2db
db threshold vs log)

src/algorithms/spectral/mfcc.cpp

@@ -28,7 +28,7 @@ const char* MFCC::name = "MFCC";
 const char* MFCC::category = "Spectral";
 const char* MFCC::description = DOC("This algorithm computes the mel-frequency cepstrum coefficients of a spectrum. As there is no standard implementation, the MFCC-FB40 is used by default:\n"
 "  - filterbank of 40 bands from 0 to 11000Hz\n"
-"  - take the log value of the spectrum energy in each mel band\n"
+"  - take the log value of the spectrum energy in each mel band. Bands energy values below silence threshold will be clipped to its value before computing log-energies\n"
 "  - DCT of the 40 bands down to 13 mel coefficients\n"
 "There is a paper describing various MFCC implementations [1].\n"
 "\n"
@@ -93,10 +93,13 @@ void MFCC::configure() {
                   INHERIT("liftering"));
   _logbands.resize(parameter("numberBands").toInt());
 
-  setCompressor(parameter("logType").toString());
-
+  _logType = parameter("logType").toLower();
+  _silenceThreshold = parameter("silenceThreshold").toReal();
+  _dbSilenceThreshold = 10 * log10(_silenceThreshold);
+  _logSilenceThreshold = log(_silenceThreshold);
 }
 
+
 void MFCC::compute() {
 
   // get the inputs and outputs
@@ -111,30 +114,25 @@ void MFCC::compute() {
 
   // take the dB amplitude of the spectrum
   for (int i=0; i<int(bands.size()); ++i) {
-    _logbands[i] = (*_compressor)(bands[i]);
+    if (_logType == "dbpow") {
+      _logbands[i] = pow2db(bands[i], _silenceThreshold, _dbSilenceThreshold);
+    }
+    else if (_logType == "dbamp") {
+      _logbands[i] = amp2db(bands[i], _silenceThreshold, _dbSilenceThreshold);
+    }
+    else if (_logType == "log") {
+      _logbands[i] = lin2log(bands[i], _silenceThreshold, _logSilenceThreshold);
+    }
+    else if (_logType == "natural") {
+      _logbands[i] = bands[i]; 
+    }
+    else {
+      throw EssentiaException("MFCC: Bad 'logType' parameter");
+    }
   }
 
   // compute the DCT of these bands
   _dct->input("array").set(_logbands);
   _dct->output("dct").set(mfcc);
   _dct->compute();
 }
-
-void MFCC::setCompressor(std::string logType){
-  if (logType == "natural"){
-    _compressor = linear;
-  }
-  else if (logType == "dbpow"){
-    _compressor = pow2db;
-  }
-  else if (logType == "dbamp"){
-    _compressor = amp2db;
-  }
-  else if (logType == "log"){
-    _compressor = log;
-  }
-  else{
-    throw EssentiaException("MFCC: Bad 'logType' parameter");
-  }
-
-}

src/algorithms/spectral/mfcc.h

@@ -36,11 +36,10 @@ class MFCC : public Algorithm {
   Algorithm* _dct;
 
   std::vector<Real> _logbands;
-
-  typedef  Real (*funcPointer)(Real);
-  funcPointer _compressor;
-
-  void setCompressor(std::string logType);
+  std::string _logType;
+  Real _silenceThreshold;
+  Real _dbSilenceThreshold;
+  Real _logSilenceThreshold;
 
  public:
   MFCC() {
@@ -68,6 +67,7 @@ class MFCC : public Algorithm {
     declareParameter("weighting", "type of weighting function for determining triangle area","{warping,linear}","warping");
     declareParameter("normalize", "'unit_max' makes the vertex of all the triangles equal to 1, 'unit_sum' makes the area of all the triangles equal to 1","{unit_sum,unit_max}", "unit_sum");
     declareParameter("type", "use magnitude or power spectrum","{magnitude,power}", "power");
+    declareParameter("silenceThreshold", "silence threshold for computing log-energy bands", "(0,inf)", 1e-9);
     declareParameter("dctType", "the DCT type", "[2,3]", 2);
     declareParameter("liftering", "the liftering coefficient. Use '0' to bypass it", "[0,inf)", 0);
     declareParameter("logType","logarithmic compression type. Use 'dbpow' if working with power and 'dbamp' if working with magnitudes","{natural,dbpow,dbamp,log}","dbamp");