ListenAid

ListenAid - intuitive & clean realtime audio spectrum analysis.

ListenAid extends far beyond a basic spectrum analyzer. It functions as a highly sophisticated acoustic measurement tool optimized for live audio engineering and system tuning (leveraging transfer functions and multi-resolution processing), delivering uncompromising performance through Metal and Accelerate hardware acceleration.

Audio Engine & Core DSP • Lock-Free Processing (RingBuffer): Thread-safe ring buffer via atomic operations (UnsafeAtomic) ensures non-blocking, zero-dropout audio. • Low-Level Math (vDSP): Uses Apple's Accelerate framework for vectorized windowing, division, and interpolation to drastically minimize CPU load. • Real-Time Transfer Function: Live reference vs. live channel division (vDSP_vdiv) instantly measures PA or room frequency responses.

5 different Analysis models: • STFT: Standard Short-Time Fourier Transform. • CWT: Continuous Wavelet Transform for high-frequency time resolution; optimized via integral images. • CQT: Constant-Q Transform with variable smoothing. • QTAM: Amplitude demodulation in the frequency domain. • Multi-Resolution: Concurrently processes/crossfades Low, Mid, and High FFT windows (with latency compensation) for pristine low-end frequency and high-end time resolution.

Metal GPU Rendering • Spectrogram is GPU-offloaded; maintains 60 FPS even at massive 65,536 FFT bin sizes. • Phase Reassignment: Analyzes consecutive frame phase differences, shifting energy to exact sub-bin frequencies to significantly sharpen visual traces. • Simultaneous FFT Traces: Concurrently plots Live, Reference, and Secondary traces with customizable colors and opacities. • Dynamic Viewport (ViewportMath): Continuous X/Y pan and zoom. The X-axis toggles dynamically between logarithmic Hertz and linear musical scales.

Analytical Tools & Precision • Sub-Bin Peak Detection: Calculates precise inter-bin frequencies using complex FFT phase correction or logarithmic-parabolic interpolation (LSPD). • Pitch Conversion (FrequencyConverter): Translates peaks to exact MIDI notes, showing cent deviations and allowing adjustable concert pitch. • Weighting & Tilt: A- and C-Weighting filters; adjustable display tilt (e.g., +4.5 dB/octave). • Smoothing & Averaging: Exponential, RMS, or Peak Hold averaging; psychoacoustic fractional octave smoothing (1/96 to 1 full octave).

System Tests & Signal Generation • Sine Generator: Artifact-free oscillator with amplitude fade envelopes; outputs up to the Nyquist frequency. • Channel Control (SystemTestManager): Routes the generator or loop player via Normal, Solo L/R, or Phase Invert (L-channel inverted for stereo alignment). • Loop Playback: Imports and loops custom audio files to test spaces using known reference tracks.

Workflow & Connectivity • Multipeer Control (MultipeerManager): Uses Apple Multipeer Connectivity (mesh network) to fully remote-control an iPad at the mixing desk (volume, mute, generator, playback) from an iPhone. • Smart Routing: AudioEngine safely intercepts hardware changes and manages BT profiles (HFP for input, A2DP for high-quality output). • Preset System: Instantly save and recall all DSP and UI configurations via slots A, B, and Y.