DAC Noise Floor Filter & Stereo Fix

Overview

DAC Noise Floor Filter & Stereo Fix (authored by FK) addresses one of the most frustrating audio anomalies on Qualcomm Snapdragon smartphones: persistent background hiss, ground loop electromagnetic interference, and elevated noise floors when listening through sensitive wired earphones.

Audiophiles using multi-driver balanced armature (BA) in-ear monitors or low-impedance dynamic IEMs frequently report hearing an annoying “ocean roar” or persistent electrical static whenever an app requests audio focus, even when music is paused. This occurs because OEM mixer path configurations often set the analog headphone amplifier (HPHL/HPHR) to excessive base gain, relying on downstream software digital attenuation to control volume.

This module replaces stock mixer path definitions for Qualcomm’s WCD9335 (“Tasha”) and SKUK audio codecs, calibrating the analog gain structure to push the noise floor below the threshold of human hearing while preserving genuine stereo imaging.

Technical Architecture & How It Works

The module targets the core ALSA mixer path configurations in /vendor/etc/:

• mixer_paths_tasha.xml
• mixer_paths_skuk.xml
• mixer_paths_qvr.xml

1. Analog Amplifier Floor Suppression

Standard OEM configurations often leave the headphone amplifier registers (HPHL Volume and HPHR Volume) elevated at values like 15 or 20, amplifying thermal Johnson-Nyquist noise generated by internal DAC traces.

The module adjusts these analog registers:

<!-- Optimized Headphone Power Amplifier (HPH) Gain Staging -->
<ctl name="HPHL Volume" value="9" />
<ctl name="HPHR Volume" value="9" />

<!-- Digital Headroom Optimization -->
<ctl name="RX1 Digital Volume" value="84" />
<ctl name="RX2 Digital Volume" value="84" />

By reducing the analog gain multiplier and driving the signal with clean, high-resolution digital volume, the electrical signal-to-noise ratio (SNR) is dramatically improved.

2. Stereo Channel Isolation

Certain OEM ROM updates accidentally introduce mono downmixing or unbalanced stereo crosstalk compensation into mixer_paths.xml. This module explicitly restores discrete left/right routing across SLIM RX1 and SLIM RX2 channels, restoring wide soundstage geometry and pin-point instrument separation.

Compatibility & Usage

• Supported Hardware: Devices featuring Qualcomm Snapdragon 820, 821, 835, 845, and related platforms equipped with WCD9335 or compatible Qualcomm audio codecs.
• Verification: Connect sensitive earphones, set volume to 10%, and pause playback. The background hiss present prior to module installation should be completely suppressed.