From c438da9603439c4a496242cb0c803393e34874f0 Mon Sep 17 00:00:00 2001
From: Richie McIlroy <33632126+richiemcilroy@users.noreply.github.com>
Date: Sat, 17 Oct 2026 03:27:28 +0900
Subject: [PATCH] fix(recording): keep each mixed Instant source at its own
 level

amix's default normalisation divided every input by the number of inputs,
so with system audio on the microphone was recorded at half amplitude
(-6 dB) while a microphone-only recording, which skips the mixer, was not.
The mixer now sums its inputs at their own levels.
---
 crates/recording/src/sources/audio_mixer.rs | 25 ++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/crates/recording/src/sources/audio_mixer.rs b/crates/recording/src/sources/audio_mixer.rs
index 498c73aeb7f..974a774c52c 100644
--- a/crates/recording/src/sources/audio_mixer.rs
+++ b/crates/recording/src/sources/audio_mixer.rs
@@ -1047,7 +1047,11 @@ fn construct_filter_graph(infos: &[AudioInfo]) -> Result<FilterGraphParts, ffmpe
     let mut amix = filter_graph.add(
         &ffmpeg::filter::find("amix").expect("Failed to find amix filter"),
         "amix",
-        &format!("inputs={}:duration=longest", abuffers.len()),
+        // normalize=0: each source keeps its own level. amix's default
+        // divides every input by the number of inputs (the microphone at
+        // -6 dB beside system audio) and ramps that scale whenever an input
+        // drops out.
+        &format!("inputs={}:duration=longest:normalize=0", abuffers.len()),
     )?;
 
     let aformat_args = format!(
@@ -1547,6 +1551,25 @@ mod test {
         assert_eq!(probe.mixer.sources[0].late_samples_dropped, 0);
     }
 
+    #[test]
+    fn pcm_mixed_sources_keep_their_own_level() {
+        let mut probe = PcmProbe::new(2);
+        for chunk in 0..50u64 {
+            probe.send(0, chunk * 20, 960, (chunk % 10 == 0).then_some(0));
+            probe.send(1, chunk * 20, 960, (chunk % 10 == 5).then_some(0));
+            probe.tick(chunk * 20 + 20);
+        }
+        let peak = |channel: usize| {
+            probe
+                .pcm
+                .iter()
+                .map(|sample| sample[channel].abs())
+                .fold(0.0f32, f32::max)
+        };
+        assert_eq!(peak(0), 0.5);
+        assert_eq!(peak(1), 0.5);
+    }
+
     #[test]
     fn pcm_timestamp_jitter_within_tolerance_keeps_samples_contiguous() {
         let mut probe = PcmProbe::new(1);