From 7240fbeda25af65e613137db12d9935f4c1c110d Mon Sep 17 00:00:00 2001
From: Jianjun Zhu <jianjun.zhu@intel.com>
Date: Tue, 2 Jul 2024 13:59:52 +0800
Subject: [PATCH] Add a note for voice activity explanation.

---
 index.bs | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/index.bs b/index.bs
index bedd997..d75727c 100644
--- a/index.bs
+++ b/index.bs
@@ -414,8 +414,8 @@ platform UI or media keys, thereby improving the user experience.
         </li>
         <li>
           <dfn enum-value for=MediaSessionAction>voiceactivity</dfn>:
-          the action's intent is to notify the action handler that a voice
-          activity is started.
+          the action's intent is to notify the web page that a voice activity
+          has been detected by the microphone.
         </li>
       </ul>
     </p>
@@ -547,10 +547,10 @@ platform UI or media keys, thereby improving the user experience.
       steps defined to [$set a track's muted state$].
     </p>
     <p>
-      A user agent MUST invoke {{MediaSessionActionHandler}} for
-      {{MediaSessionAction/voiceactivity}} only when the voice activity is
-      detected from a microphone with one or more live {{MediaStreamTrack}}s. A
-      user agent MAY ignore a {{MediaSessionAction/voiceactivity}} action if all
+      A user agent MUST invoke the {{MediaSessionActionHandler}} for
+      {{MediaSessionAction/voiceactivity}} only when voice activity is detected
+      from a microphone with one or more live {{MediaStreamTrack}}s. A user
+      agent MAY ignore a {{MediaSessionAction/voiceactivity}} action if all
       {{MediaStreamTrack}}s associated with the source are not
       {{MediaStreamTrack/muted}}. It is RECOMMENDED for user agents to set a
       minimal interval for invoking {{MediaSessionActionHandler}} for
@@ -558,6 +558,19 @@ platform UI or media keys, thereby improving the user experience.
       policies.
     </p>
 
+    <p class=note>
+      {{MediaSessionAction/voiceactivity}} only indicates the start of a voice
+      activity. Application may display a notification if the user is speaking
+      while the {{MediaStreamTrack}} is muted, or start an {{AudioWorklet}} for
+      audio processing. No action is defined for the end of a voice activity.
+      Unlike other actions which are explicitely triggered by the user,
+      {{MediaSessionAction/voiceactivity}} also depends on the voice activity
+      detection algorithm of the user agent or the system. For privacy and power
+      efficiency concern, web page may not be notified if the second voice
+      activity started soon after last {{MediaSessionAction/voiceactivity}}
+      action.
+    </p>
+
     <p class=note>
       A page should only register a {{MediaSessionActionHandler}} for a <a>media
       session action</a> when it can handle the action given that the user agent