fix(rtp): keep the voice activity bit out of the audio level's sign

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
   * Add RFC 2198 RED (redundant audio) support for all audio codecs #982
+  * Preserve the RFC 6464 audio-level voice activity bit when serializing #1032
 
 # 0.23.1
 

src/rtp/ext.rs

@@ -664,7 +664,11 @@ impl Extension {
             AudioLevel => {
                 let v1 = ev.audio_level?;
                 let v2 = ev.voice_activity?;
-                buf[0] = if v2 { 0x80 } else { 0 } | (-(0x7f & v1) as u8);
+                // `v1` is negative, so masking it before negating leaves the
+                // sign bit in place: for -37 the expression `-(0x7f & v1)`
+                // yields 0xA5, which already has bit 7 set. Taking the
+                // magnitude first keeps the sign out of the V bit.
+                buf[0] = if v2 { 0x80 } else { 0 } | (v1.unsigned_abs() & 0x7f);
                 Some(1)
             }
             TransmissionTimeOffset => {
@@ -1580,4 +1584,31 @@ mod test {
             vec![(12, &VideoOrientation), (14, &TransportSequenceNumber)]
         );
     }
+
+    #[test]
+    fn audio_level_round_trip() {
+        // The V bit (voice activity) must survive serialization for every
+        // level on the scale, not just when it is set.
+        let mut exts = ExtensionMap::empty();
+        exts.set(1, Extension::AudioLevel);
+
+        for level in -127..=0 {
+            for voice_activity in [false, true] {
+                let ev = ExtensionValues {
+                    audio_level: Some(level),
+                    voice_activity: Some(voice_activity),
+                    ..Default::default()
+                };
+
+                let mut buf = vec![0_u8; 8];
+                exts.write_to(&mut buf[..], &ev, ExtensionsForm::OneByte);
+
+                let mut ev2 = ExtensionValues::default();
+                exts.parse(&buf, ExtensionsForm::OneByte, &mut ev2);
+
+                assert_eq!(ev2.audio_level, Some(level));
+                assert_eq!(ev2.voice_activity, Some(voice_activity));
+            }
+        }
+    }
 }

src/rtp/header.rs

@@ -465,20 +465,24 @@ mod test {
         }
 
         let mut exts = ExtensionMap::empty();
+        // The expected bytes below carry the level with the V bit CLEAR, which
+        // is what `voice_activity: Some(false)` above asks for. They used to
+        // have bit 7 set (170 = 0xAA rather than 42 = 0x2A) because the
+        // serializer let the sign of the negative level spill into that bit.
         exts.set(3, Extension::AudioLevel);
 
         let buf1 = mk_header(47_000, 10_000, -42, false, &exts);
         let buf2 = mk_header(47_001, 12_000, -43, true, &exts);
         let buf3 = mk_header(47_002, 14_000, -44, false, &exts);
 
         let p1 = &[
-            144, 33, 183, 152, 0, 0, 39, 16, 0, 0, 0, 44, 0xBE, 0xDE, 0, 1, 48, 170, 0, 0,
+            144, 33, 183, 152, 0, 0, 39, 16, 0, 0, 0, 44, 0xBE, 0xDE, 0, 1, 48, 42, 0, 0,
         ];
         let p2 = &[
-            144, 161, 183, 153, 0, 0, 46, 224, 0, 0, 0, 44, 0xBE, 0xDE, 0, 1, 48, 171, 0, 0,
+            144, 161, 183, 153, 0, 0, 46, 224, 0, 0, 0, 44, 0xBE, 0xDE, 0, 1, 48, 43, 0, 0,
         ];
         let p3 = &[
-            144, 33, 183, 154, 0, 0, 54, 176, 0, 0, 0, 44, 0xBE, 0xDE, 0, 1, 48, 172, 0, 0,
+            144, 33, 183, 154, 0, 0, 54, 176, 0, 0, 0, 44, 0xBE, 0xDE, 0, 1, 48, 44, 0, 0,
         ];
 
         assert_eq!(&buf1, p1);
@@ -510,6 +514,8 @@ mod test {
         }
 
         let mut exts = ExtensionMap::empty();
+        // Same correction as above: the V bit is clear because the header
+        // asks for `voice_activity: Some(false)`.
         // An ID larger than 14 forces the 2-byte header extension form
         exts.set(15, Extension::AudioLevel);
 
@@ -518,13 +524,13 @@ mod test {
         let buf3 = mk_header(47_002, 14_000, -44, false, &exts);
 
         let p1 = &[
-            144, 33, 183, 152, 0, 0, 39, 16, 0, 0, 0, 44, 0x10, 0x00, 0, 1, 15, 1, 170, 0,
+            144, 33, 183, 152, 0, 0, 39, 16, 0, 0, 0, 44, 0x10, 0x00, 0, 1, 15, 1, 42, 0,
         ];
         let p2 = &[
-            144, 161, 183, 153, 0, 0, 46, 224, 0, 0, 0, 44, 0x10, 0x00, 0, 1, 15, 1, 171, 0,
+            144, 161, 183, 153, 0, 0, 46, 224, 0, 0, 0, 44, 0x10, 0x00, 0, 1, 15, 1, 43, 0,
         ];
         let p3 = &[
-            144, 33, 183, 154, 0, 0, 54, 176, 0, 0, 0, 44, 0x10, 0x00, 0, 1, 15, 1, 172, 0,
+            144, 33, 183, 154, 0, 0, 54, 176, 0, 0, 0, 44, 0x10, 0x00, 0, 1, 15, 1, 44, 0,
         ];
 
         assert_eq!(&buf1, p1);