Add 3-band parametric EQ to speaker output

[gburlingame]

Summary

• Adds a 3-band parametric EQ (peaking filters) applied to the speaker output in speaker_pipeline_output(), immediately before DAC output via I2S
• Header-only biquad implementation using Q28 fixed-point arithmetic with int64_t accumulators (Direct Form I)
• Three bands: 1825 Hz/-7.7 dB, 3542 Hz/-10.0 dB, 166 Hz/-6.1 dB — targeting mid and upper-mid resonances plus a low-frequency cut

Details

• Coefficients pre-computed from Audio EQ Cookbook peaking EQ formulas at Fs = 48 kHz
• EQ is applied to both L/R channels independently using static filter state
• AEC reference signal is derived after EQ, so it accurately models speaker output
• Estimated CPU cost: ~1.5 ms per 15 ms frame (~10% of thread budget at 100 MHz effective clock)

Files Changed

File	Change
satellite-xmos-firmware/src/eq/biquad_eq.h	New — header-only biquad EQ implementation
satellite-xmos-firmware/src/eq/bode_plot_eq.png	New — frequency response plot
satellite-xmos-firmware/src/eq/EQ_CHANGES.md	New — detailed design documentation
satellite-xmos-firmware/src/main.c	Modified — integrate EQ into speaker output path

Test plan

• Build firmware and verify no compilation errors
• Flash device and confirm audio output with EQ applied
• Verify frequency response matches expected Bode plot
• Confirm AEC still functions correctly with EQ in the signal path

🤖 Generated with Claude Code