fbank: update to better match kaldifeat

Author: abb128

src/april_model.c

@@ -93,6 +93,9 @@ AprilASRModel aam_create_model(const char *model_path) {
     //aam->fbank_opts.snip_edges         = aam->params.snip_edges;
     aam->fbank_opts.snip_edges = true;
 
+    aam->fbank_opts.remove_dc_offset = true;
+    aam->fbank_opts.preemph_coeff = 0.97f;
+
     ASSERT_OR_FREE_AAM_AND_RETURN_NULL(aam, aam->x_dim[0] == aam->params.batch_size);
     ASSERT_OR_FREE_AAM_AND_RETURN_NULL(aam, aam->x_dim[1] == aam->fbank_opts.pull_segment_count);
     ASSERT_OR_FREE_AAM_AND_RETURN_NULL(aam, aam->x_dim[2] == aam->fbank_opts.num_bins);

src/fbank.c

@@ -185,6 +185,7 @@ void fbank_accept_waveform(OnlineFBank fbank, float *wave, size_t wave_count) {
         sonicReadFloatFromStream(fbank->sonic_stream, wave, wave_count);
     }
 
+    float preemph_coeff = fbank->opts.preemph_coeff;
     for(ssize_t i=0;; i++) {
         if((fbank->temp_segment_avail + 1) > fbank->temp_segments_y){
             LOG_WARNING("fbank ran out of space. Please call fbank_pull_segments. Can't eat wave");
@@ -228,12 +229,33 @@ void fbank_accept_waveform(OnlineFBank fbank, float *wave, size_t wave_count) {
             ssize_t wave_idx = start_idx + j;
             if(wave_idx < 0){
                 ssize_t ll_idx = fbank->prev_leftover_count + wave_idx;
-                fbank->data[j] = fbank->prev_leftover[ll_idx] * fbank->window[j];
+                fbank->data[j] = fbank->prev_leftover[ll_idx];
             } else {
-                fbank->data[j] = wave[start_idx + j] * fbank->window[j];
+                fbank->data[j] = wave[start_idx + j];
             }
         }
 
+        // Not included: dither
+
+        // Apply remove dc offset
+        if(fbank->opts.remove_dc_offset) {
+            float sum = 0;
+            for(int j=0; j<fbank->padded_window_size; j++) sum += fbank->data[j];
+            float mean = sum / fbank->padded_window_size;
+            for(int j=0; j<fbank->padded_window_size; j++) fbank->data[j] -= mean;
+        }
+
+        // Apply preemphasize
+        if(preemph_coeff > 0.0f) {
+            for(int j=fbank->padded_window_size-1; j>0; --j)
+                fbank->data[j] -= preemph_coeff * fbank->data[j - 1];
+            fbank->data[0] -= preemph_coeff * fbank->data[0];
+        }
+
+        // Apply window function
+        for(int j=0; j<fbank->padded_window_size; j++)
+            fbank->data[j] *= fbank->window[j];
+
         double *dptr = fbank->data;
         double *rptr = fbank->ret;
         memcpy((char *)(rptr+1), dptr, fbank->padded_window_size * sizeof(double));

src/fbank.h

@@ -60,6 +60,9 @@ typedef struct FBankOptions {
 
     // If false, speed feature will be unavailable
     bool use_sonic;
+
+    bool remove_dc_offset; // true
+    float preemph_coeff; // 0.97
 } FBankOptions;
 
 OnlineFBank make_fbank(FBankOptions opts);