Fix µ-law decoder bias (33 → 0x84) — root cause of distorted call audio

b52ca5ef4bdec301774b6596c1318dc6eb94b9e7 · 2026-05-19 14:00:17 -0700 · SteveStudio2

lib/listen-bridge.js decoded Twilio's G.711 µ-law media frames with
MULAW_BIAS=33. The G.711 decode bias is 0x84 (132), added inside the
<< exponent shift. At 33 the bias was under-counted 4×: quiet samples
(exponent 0) stayed correct but every loud sample was nonlinearly
crushed (0x00 decoded to -19551 instead of -32124). That amplitude-
dependent distortion made both the recorded .mp3 and the live-listen
feed sound horrible. Verified: the decoder now matches canonical
G.711 on all 256 µ-law byte values.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Files touched

M lib/listen-bridge.js

Diff

commit b52ca5ef4bdec301774b6596c1318dc6eb94b9e7
Author: SteveStudio2 <steve@designerwallcoverings.com>
Date:   Tue May 19 14:00:17 2026 -0700

    Fix µ-law decoder bias (33 → 0x84) — root cause of distorted call audio
    
    lib/listen-bridge.js decoded Twilio's G.711 µ-law media frames with
    MULAW_BIAS=33. The G.711 decode bias is 0x84 (132), added inside the
    << exponent shift. At 33 the bias was under-counted 4×: quiet samples
    (exponent 0) stayed correct but every loud sample was nonlinearly
    crushed (0x00 decoded to -19551 instead of -32124). That amplitude-
    dependent distortion made both the recorded .mp3 and the live-listen
    feed sound horrible. Verified: the decoder now matches canonical
    G.711 on all 256 µ-law byte values.
    
    Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 lib/listen-bridge.js | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/lib/listen-bridge.js b/lib/listen-bridge.js
index d01920b..644e99a 100644
--- a/lib/listen-bridge.js
+++ b/lib/listen-bridge.js
@@ -106,7 +106,12 @@ function finalizeArchive(callId) {
 
 // µ-law → linear PCM16 LE decoder (G.711 standard).
 // Bit-fiddly but small. Adapted from RFC 5391.
-const MULAW_BIAS = 33;
+// G.711 decode bias is 0x84 (132) and is added INSIDE the << exponent shift.
+// This was 33 — under-counted by 4× — which left quiet samples (exponent 0)
+// correct but nonlinearly crushed every loud sample (e.g. 0x00 decoded to
+// -19551 instead of -32124). That amplitude-dependent distortion is what made
+// the recordings and the live-listen feed sound horrible.
+const MULAW_BIAS = 0x84;
 const MULAW_CLIP = 32635;
 function mulawDecodeOne(mulawByte) {
   mulawByte = ~mulawByte & 0xff;

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