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fix(ios,audio): use AVAudioSession mode .default + correct mono downmix

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Two changes that together address the user-reported 'speaker selector
not working' AND 'audio quality bad' symptoms on iPhone:

1. AppDelegate.swift: AVAudioSession mode .voiceChat -> .default

   .voiceChat binds the underlying AudioUnit's output element to a
   SINGLE physical transducer (the receiver/earpiece) at session-
   configure time. overrideOutputAudioPort updates AVAudioSession's
   route metadata so currentRoute.outputs reports Speaker, but the
   AudioUnit's output binding is stale and audio keeps routing to
   the original transducer. Net: tapping Speaker in the picker
   flipped the route in our log but produced no audible change.

   .voiceChat also enables iOS's telephony processing chain (forced
   mono output, aggressive AGC, heavy noise gating) which explains
   the 'garbled / watery / metallic' quality complaints.

   .default mode uses iOS's standard audio graph: stereo output, no
   AGC, no telephony post-processing, AudioUnit re-binds live when
   the route changes. Same mode Music.app and most non-telephony
   apps use. Trade-off: we lose iOS hardware AEC. If users report
   speakerphone echo we'll add software AEC (DEC-030).

   Category options unchanged \u2014 .allowBluetoothHFP +
   .allowBluetoothA2DP still permit BT headsets in both directions.

2. engine.rs::build_output_stream: mono device downmix fix

   The mixing path at dev_channels==1 previously wrote only the L
   channel of AudioHandler's stereo output into the single mono
   device channel and discarded R entirely. Anything panned right
   in the stereo voice mix was silently lost \u2014 on .voiceChat
   speakerphone (forced mono device) this manifested as quiet
   remote speakers being inaudible. Fix: when dev_channels==1,
   output = (L + R) * 0.5 instead of just L. The dev_channels>=2
   branch is unchanged.

   With change #1 iOS will typically expose stereo so this branch
   is rarely hit, but the fix is correct for any genuinely-mono
   sink (some BT car-audio profiles, USB mono headsets).
This commit is contained in:
EdisonJwa
2026-05-17 00:18:48 +08:00
parent 2f6cdd3ad4
commit 6a4dbad60e
2 changed files with 45 additions and 40 deletions
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crates/chanora_audio/src/engine.rs
+18 -7
View File
@@ -1042,14 +1042,25 @@ where
let l = (a_l as f64 + frac * (b_l - a_l) as f64) as f32 * gain;
let r = (a_r as f64 + frac * (b_r - a_r) as f64) as f32 * gain;
let base = frame_idx * dev_channels;
if dev_channels >= 1 {
if dev_channels == 1 {
// Mono output device (typical on iOS
// .voiceChat / phone-call audio path):
// downmix L+R to a single channel
// rather than dropping the R side.
// Without the downmix, anything panned
// right in the AudioHandler stereo mix
// is silently lost \u2014 on speakerphone
// this manifested as quiet remote
// speakers being inaudible.
out[base] = T::from_f32_sample((l + r) * 0.5);
} else {
out[base] = T::from_f32_sample(l);
}
if dev_channels >= 2 {
out[base + 1] = T::from_f32_sample(r);
}
for c in 2..dev_channels {
out[base + c] = T::from_f32_sample(0.0);
if dev_channels >= 2 {
out[base + 1] = T::from_f32_sample(r);
}
for c in 2..dev_channels {
out[base + c] = T::from_f32_sample(0.0);
}
}
pos += resample_ratio;
}