diff --git a/speechx/speechx/decoder/ctc_beam_search_decoder.h b/speechx/speechx/decoder/ctc_beam_search_decoder.h
index 479754c3689c9fc3f6b684b6321a02d31c93a02c..6347bba87d8d2ec4b782e0626f1f82e1e4f6e2ba 100644
--- a/speechx/speechx/decoder/ctc_beam_search_decoder.h
+++ b/speechx/speechx/decoder/ctc_beam_search_decoder.h
@@ -23,7 +23,7 @@
 
 namespace ppspeech {
 
-class CTCBeamSearch : public DecoderInterface {
+class CTCBeamSearch : public DecoderBase {
   public:
     explicit CTCBeamSearch(const CTCBeamSearchOptions& opts);
     ~CTCBeamSearch() {}
diff --git a/speechx/speechx/decoder/ctc_prefix_beam_search_decoder.h b/speechx/speechx/decoder/ctc_prefix_beam_search_decoder.h
index eef8823da4aa2522cdf36521b3deb134a1e9c7f5..ef96ecd935cc7919cce9f3f907ac8b3b9c9071e8 100644
--- a/speechx/speechx/decoder/ctc_prefix_beam_search_decoder.h
+++ b/speechx/speechx/decoder/ctc_prefix_beam_search_decoder.h
@@ -1,3 +1,4 @@
+// Copyright (c) 2020 Mobvoi Inc (Binbin Zhang)
 // Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -12,6 +13,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// modified from https://github.com/wenet-e2e/wenet/blob/main/runtime/core/decoder/ctc_prefix_beam_search.cc
+
 #pragma once
 
 #include "decoder/ctc_beam_search_opt.h"
@@ -21,7 +24,7 @@
 
 namespace ppspeech {
 class ContextGraph;
-class CTCPrefixBeamSearch : public DecoderInterface {
+class CTCPrefixBeamSearch : public DecoderBase {
   public:
     explicit CTCPrefixBeamSearch(const std::string vocab_path,
                                  const CTCBeamSearchOptions& opts);
diff --git a/speechx/speechx/decoder/ctc_prefix_beam_search_score.h b/speechx/speechx/decoder/ctc_prefix_beam_search_score.h
index da2fb80a9d3e0498a2f7f71cb4b74f9dc5e42bff..908be1d69d3ffb1c4ee3063bd93151a17e4d3d38 100644
--- a/speechx/speechx/decoder/ctc_prefix_beam_search_score.h
+++ b/speechx/speechx/decoder/ctc_prefix_beam_search_score.h
@@ -13,6 +13,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// modified from https://github.com/wenet-e2e/wenet/blob/main/runtime/core/decoder/ctc_prefix_beam_search.h
+
 #pragma once
 
 #include "base/common.h"
diff --git a/speechx/speechx/decoder/ctc_tlg_decoder.h b/speechx/speechx/decoder/ctc_tlg_decoder.h
index cf8a9b7303a48b276f5f6eef5ea1a0d2a970eecf..f250ac25e26fcfb2abbf4b0331401cbd42e51021 100644
--- a/speechx/speechx/decoder/ctc_tlg_decoder.h
+++ b/speechx/speechx/decoder/ctc_tlg_decoder.h
@@ -54,7 +54,7 @@ struct TLGDecoderOptions {
     }
 };
 
-class TLGDecoder : public DecoderInterface {
+class TLGDecoder : public DecoderBase {
   public:
     explicit TLGDecoder(TLGDecoderOptions opts);
     ~TLGDecoder() = default;
diff --git a/speechx/speechx/decoder/decoder_itf.h b/speechx/speechx/decoder/decoder_itf.h
index eec9bc3d4e0c08c275054af2d9dda54ff47c1de7..2289b3173dca346440ef293ace2fd3ead0a44587 100644
--- a/speechx/speechx/decoder/decoder_itf.h
+++ b/speechx/speechx/decoder/decoder_itf.h
@@ -51,7 +51,10 @@ class DecoderInterface {
     virtual std::vector<std::pair<double, std::string>> GetNBestPath() = 0;
 
     virtual std::vector<std::pair<double, std::string>> GetNBestPath(int n) = 0;
+};
 
+class DecoderBase : public DecoderInterface {
+  protected:
     // start from one
     int NumFrameDecoded() { return num_frame_decoded_ + 1; }
 
diff --git a/speechx/speechx/nnet/u2_nnet.cc b/speechx/speechx/nnet/u2_nnet.cc
index c92c96aaae86af65cd000aae73bea297584a02fd..ff6a4dc37d14bae255e1ea428849c605a8f6a529 100644
--- a/speechx/speechx/nnet/u2_nnet.cc
+++ b/speechx/speechx/nnet/u2_nnet.cc
@@ -1,3 +1,4 @@
+// Copyright 2022 Horizon Robotics. All Rights Reserved.
 // Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -12,6 +13,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// modified from https://github.com/wenet-e2e/wenet/blob/main/runtime/core/decoder/asr_model.cc
+
 #include "nnet/u2_nnet.h"
 
 #ifdef USE_PROFILING
diff --git a/speechx/speechx/nnet/u2_nnet.h b/speechx/speechx/nnet/u2_nnet.h
index a37a88f2fe6fefb391ce262f7acfd88a8b441120..48dd8193b5f0858827d71a8747f753dda79d9936 100644
--- a/speechx/speechx/nnet/u2_nnet.h
+++ b/speechx/speechx/nnet/u2_nnet.h
@@ -1,3 +1,4 @@
+// Copyright 2022 Horizon Robotics. All Rights Reserved.
 // Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -12,6 +13,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// modified from https://github.com/wenet-e2e/wenet/blob/main/runtime/core/decoder/asr_model.h
 #pragma once
 
 #include "base/common.h"