Fix keyword spotting.

See also #1417

Author: csukuangfj

sherpa-onnx/csrc/keyword-spotter-impl.h

@@ -38,6 +38,8 @@ class KeywordSpotterImpl {
 
   virtual bool IsReady(OnlineStream *s) const = 0;
 
+  virtual void Reset(OnlineStream *s) const = 0;
+
   virtual void DecodeStreams(OnlineStream **ss, int32_t n) const = 0;
 
   virtual KeywordResult GetResult(OnlineStream *s) const = 0;

sherpa-onnx/csrc/keyword-spotter-transducer-impl.h

@@ -195,8 +195,24 @@ class KeywordSpotterTransducerImpl : public KeywordSpotterImpl {
     return s->GetNumProcessedFrames() + model_->ChunkSize() <
            s->NumFramesReady();
   }
+  void Reset(OnlineStream *s) const override { InitOnlineStream(s); }
 
   void DecodeStreams(OnlineStream **ss, int32_t n) const override {
+    for (int32_t i = 0; i < n; ++i) {
+      auto s = ss[i];
+      auto r = s->GetKeywordResult(true);
+      int32_t num_trailing_blanks = r.num_trailing_blanks;
+      // assume subsampling_factor is 4
+      // assume frameshift is 0.01 second
+      float trailing_slience = num_trailing_blanks * 4 * 0.01;
+
+      // it resets automatically after detecting 1.5 seconds of silence
+      float threshold = 1.5;
+      if (trailing_slience > threshold) {
+        Reset(s);
+      }
+    }
+
     int32_t chunk_size = model_->ChunkSize();
     int32_t chunk_shift = model_->ChunkShift();
 

sherpa-onnx/csrc/keyword-spotter.cc

@@ -157,6 +157,8 @@ bool KeywordSpotter::IsReady(OnlineStream *s) const {
   return impl_->IsReady(s);
 }
 
+void KeywordSpotter::Reset(OnlineStream *s) const { impl_->Reset(s); }
+
 void KeywordSpotter::DecodeStreams(OnlineStream **ss, int32_t n) const {
   impl_->DecodeStreams(ss, n);
 }

sherpa-onnx/csrc/keyword-spotter.h

@@ -129,6 +129,9 @@ class KeywordSpotter {
    */
   bool IsReady(OnlineStream *s) const;
 
+  // Remember to call it after detecting a keyword
+  void Reset(OnlineStream *s) const;
+
   /** Decode a single stream. */
   void DecodeStream(OnlineStream *s) const {
     OnlineStream *ss[1] = {s};

sherpa-onnx/csrc/sherpa-onnx-keyword-spotter-alsa.cc

@@ -106,13 +106,15 @@ as the device_name.
 
     while (spotter.IsReady(stream.get())) {
       spotter.DecodeStream(stream.get());
-    }
 
-    const auto r = spotter.GetResult(stream.get());
-    if (!r.keyword.empty()) {
-      display.Print(keyword_index, r.AsJsonString());
-      fflush(stderr);
-      keyword_index++;
+      const auto r = spotter.GetResult(stream.get());
+      if (!r.keyword.empty()) {
+        display.Print(keyword_index, r.AsJsonString());
+        fflush(stderr);
+        keyword_index++;
+
+        spotter.Reset(stream.get());
+      }
     }
   }
 

sherpa-onnx/csrc/sherpa-onnx-keyword-spotter-microphone.cc

@@ -150,13 +150,15 @@ for a list of pre-trained models to download.
   while (!stop) {
     while (spotter.IsReady(s.get())) {
       spotter.DecodeStream(s.get());
-    }
 
-    const auto r = spotter.GetResult(s.get());
-    if (!r.keyword.empty()) {
-      display.Print(keyword_index, r.AsJsonString());
-      fflush(stderr);
-      keyword_index++;
+      const auto r = spotter.GetResult(s.get());
+      if (!r.keyword.empty()) {
+        display.Print(keyword_index, r.AsJsonString());
+        fflush(stderr);
+        keyword_index++;
+
+        spotter.Reset(s.get());
+      }
     }
 
     Pa_Sleep(20);  // sleep for 20ms

sherpa-onnx/python/csrc/keyword-spotter.cc

@@ -67,6 +67,7 @@ void PybindKeywordSpotter(py::module *m) {
           py::arg("keywords"), py::call_guard<py::gil_scoped_release>())
       .def("is_ready", &PyClass::IsReady,
            py::call_guard<py::gil_scoped_release>())
+      .def("reset", &PyClass::Reset, py::call_guard<py::gil_scoped_release>())
       .def("decode_stream", &PyClass::DecodeStream,
            py::call_guard<py::gil_scoped_release>())
       .def(