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+++ b/g3doc/sitemap.md
@@ -15,6 +15,7 @@
     *   Audio
       * AudioEngine
         * [ADM](/modules/audio_device/g3doc/audio_device_module.md)
+      * [Audio Coding](/modules/audio_coding/g3doc/index.md)
     *   Video
     *   DataChannel
     *   PeerConnection
diff --git a/modules/audio_coding/g3doc/index.md b/modules/audio_coding/g3doc/index.md
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@@ -0,0 +1,32 @@
+<?% config.freshness.owner = 'minyue' %?> <?% config.freshness.reviewed =
+'2021-04-13' %?>
+
+# The WebRTC Audio Coding Module
+
+WebRTC audio coding module can handle both audio sending and receiving. Folder
+[`acm2`][acm2] contains implementations of the APIs.
+
+*   Audio Sending Audio frames, each of which should always contain 10 ms worth
+    of data, are provided to the audio coding module through
+    [`Add10MsData()`][Add10MsData]. The audio coding module uses a provided
+    audio encoder to encoded audio frames and deliver the data to a
+    pre-registered audio packetization callback, which is supposed to wrap the
+    encoded audio into RTP packets and send them over a transport. Built-in
+    audio codecs are included the [`codecs`][codecs] folder. The
+    [audio network adaptor][ANA] provides an add-on functionality to an audio
+    encoder (currently limited to Opus) to make the audio encoder adaptive to
+    network conditions (bandwidth, packet loss rate, etc).
+
+*   Audio Receiving Audio packets are provided to the audio coding module
+    through [`IncomingPacket()`][IncomingPacket], and are processed by an audio
+    jitter buffer ([NetEq][NetEq]), which includes decoding of the packets.
+    Audio decoders are provided by an audio decoder factory. Decoded audio
+    samples should be queried by calling [`PlayoutData10Ms()`][PlayoutData10Ms].
+
+[acm2]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/acm2/;drc=854d59f7501aac9e9bccfa7b4d1f7f4db7842719
+[Add10MsData]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/include/audio_coding_module.h;l=136;drc=d82a02c837d33cdfd75121e40dcccd32515e42d6
+[codecs]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/codecs/;drc=883fea1548d58e0080f98d66fab2e0c744dfb556
+[ANA]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/audio_network_adaptor/;drc=1f99551775cd876c116d1d90cba94c8a4670d184
+[IncomingPacket]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/include/audio_coding_module.h;l=192;drc=d82a02c837d33cdfd75121e40dcccd32515e42d6
+[NetEq]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/neteq/;drc=213dc2cfc5f1b360b1c6fc51d393491f5de49d3d
+[PlayoutData10Ms]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/modules/audio_coding/include/audio_coding_module.h;l=216;drc=d82a02c837d33cdfd75121e40dcccd32515e42d6