Helix at Home (HaH) Processor 🎸

Overview

Helix at Home (HaH) is a synthesizable multi-effects guitar processor implemented in SystemVerilog on the DE1-SoC FPGA. It uses the on-board audio codec ADC and DAC to process live audio input from an electric guitar through a configurable chain of digital audio effects.

This project was created to explore audio DSP in hardware, with a focus on real-time streaming, pipelined processing, and FPGA-based system design.

The design was initially forked from the audio-loopback-only example in De1-SoC-Verilog-Audio-HW-FX by Navraj Kambo, which provided basic audio codec configuration and loopback functionality.

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Audio Processing Chain

Audio is processed sequentially through the following effect chain:

Gain → Gate → EQ → Compressor → Distortion → EQ → Chorus → Delay → Reverb → Gain

This structure mimics a traditional digital pedalboard signal flow.

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Hardware Interface (DE1-SoC)

User Controls

• Switches & Buttons

  • Select effects and parameters
  • Increment / decrement parameter values

• HEX Displays

  • Show selected effect and parameter
  • Display current parameter value

• LEDs

  • Visual feedback for parameter values and selection state

Audio I/O

• Input: DE1-SoC LINE IN (electric guitar)
• Output: DE1-SoC LINE OUT

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Audio & System Specifications

Feature	Value
Input	Mono (left channel duplicated for guitar input)
Output	Stereo
Audio Resolution	16-bit
Sampling Rate	48 kHz
Internal Clock	50 MHz
Max FX Slots	16
Parameters per FX	Up to 8
Latency	34 Samples (under 1ms)

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Usage Instructions

The following sections outline how to use the board

Hardware + Software Needed

Hardware

• x1 DE1 SoC Board
  • x1 USB Blaster Cable
  • x1 Power Cable
• x2 AUX Cable
  • x1 whatever you connect to output, can be speacker, headphone, audio interface, etc
• x1 1/4 inch to AUX adapter
• x1 Guitar or your choice

Software

• Quartus Prime
  • Quartus Prime Version 18.1.0 Build 09/12/2018 SJ Lite Edition was used to develop this project

How to install and run on the board

1. Clone this repo into the machine with Quartus Prime
2. Open quartus prime and use the Programmer tool the program the board with the ./AudioFX.sof that is in the root directory of this repo, should not need any changes to project file or repo to do this
3. Click KEY[0] to reset the board
4. Connect one of the AUX cables to the green "LINE IN" port at the top left of the board and then connect the 1/4 inch to AUX adapter to the other end then plug that into you guitar
5. Connect the other AUX cable to the blue "LINE OUT" port at the top left of the board and then connect it to the output device
6. PLAY

How to control FX

HEX5-HEX2 display the currently selected FX number and parameter, with the exact mappings shown in the table below; values not listed correspond to no parameter. The current parameter value (ranging from 0 to 255) is visualized on LEDR[9:0] as a bar graph, where only LEDR[9] lit represents a value of 0 and LEDR[8:0] illuminate as the value increases. KEY[2] and KEY[3] are used to decrease and increase the selected parameter value, respectively. The active FX module is selected using SW[9:6], while SW[3:1] choose the parameter within that FX, and SW[0] mutes the output signal with HEX[0] being a line if it is muted.

FX Mapping Table

FX	Parameter	FX Number	Parameter Number
Gain1	fx_gain	F0	P0
Gate	fx_threshold	F1	P0
Gate	fx_attack	F1	P1
Gate	fx_release	F1	P2
EQ1	fx_sub_gain	F2	P0
EQ1	fx_low_gain	F2	P1
EQ1	fx_mid_gain	F2	P2
EQ1	fx_high_gain	F2	P3
Compressor	fx_threshold	F3	P0
Compressor	fx_ratio	F3	P1
Compressor	fx_attack	F3	P2
Compressor	fx_release	F3	P4
Distortion	fx_drive	F4	P0
Distortion	fx_makeup_gain	F4	P1
Distortion	fx_mix	F4	P2
EQ2	fx_sub_gain	F5	P0
EQ2	fx_low_gain	F5	P1
EQ2	fx_mid_gain	F5	P2
EQ2	fx_high_gain	F5	P3
Chorus	fx_rate	F6	P0
Chorus	fx_depth	F6	P1
Chorus	fx_mix	F6	P2
Delay	fx_time	F7	P0
Delay	fx_feedback	F7	P1
Delay	fx_mix	F7	P2
Reverb	fx_size	F8	P0
Reverb	fx_damping	F8	P1
Reverb	fx_mix	F8	P2
Gain2	fx_gain	F9	P0

NOTES:

• You should see the FX Number values on HEX5-HEX4, with HEX5 always being F
• You should see the Parameter Number values on HEX3-HEX2, with HEX3 always being P
• The order of FX number represented how the order the FX are connect in series, with the output of F0 being connected to the input of F1 so on and so forth
• Descriptions for what the parameters do are in the next section

Implemented Effects & Parameters

Gain

Applies a gain multiplier to the signal.

Parameters

• fx_gain: Gain multiplier (0–255), where 128 = unity

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Gate

Noise gate using peak envelope smoothing. Signals above the threshold pass through, while signals below are muted.

Parameters

• fx_threshold: Threshold (scaled 0–24480 relative to audio_in)
• fx_attack: Time for gate to open after exceeding threshold
• fx_release: Time for gate to close after falling below threshold

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EQ

3-band EQ that splits the signal into low, mid, and high frequency bands.

Parameters

• fx_sub_gain: Sub-band gain (128 = unity)
• fx_low_gain: Low-band gain (128 = unity)
• fx_mid_gain: Mid-band gain (128 = unity)
• fx_high_gain: High-band gain (128 = unity)

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Compressor

Peak-envelope-based compressor that reduces signal levels above a threshold while leaving lower levels unchanged.

Parameters

• fx_threshold: Threshold (0–24480 scaled to input)
• fx_ratio: Compression ratio from 1:1 (0) to 20:1 (255)
• fx_attack: Time before gain reduction begins
• fx_release: Time before gain reduction is released

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Distortion

Non-linear distortion using a 3rd-order polynomial approximation of tanh(x):

        {  2/3           , x ≥ 1
f(x) =  {  x − x³/3      , −1 < x < 1
        {  2/3           , x ≤ −1

Parameters

• fx_drive: Input gain into non-linearity

  • 0 = unity, 255 ≈ 32.875×

• fx_makeup_gain: Output gain (128 = unity)

• fx_mix: Dry/wet mix

  • 0 = all dry, 255 = all wet

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Chorus

Creates a thicker sound by duplicating and delaying the input signal, then modulating it with a triangle-wave LFO before mixing it back with the original.

Parameters

• fx_rate: LFO frequency
• fx_depth: LFO modulation depth
• fx_mix: Dry/wet mix

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Delay

Implements an echo effect using delay lines with feedback.

Parameters

• fx_time: Delay time (larger value = longer delay)
• fx_feedback: Feedback amount (capped at 0.875 to prevent runaway)
• fx_mix: Dry/wet mix

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Reverb

Implements a Schroeder Reverberator using parallel feedback comb filters followed by series all-pass filters to simulate room reverberation.

Parameters

• fx_size: Controls room size (delay lengths)
• fx_damping: Controls reverberation damping
• fx_mix: Dry/wet mix

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Tools, Technologies & Platform

• FPGA Board: DE1-SoC
• HDL: SystemVerilog
• Audio Codec: DE1-SoC on-board codec
• Synthesis & Programming: Intel Quartus
• Simulation: ModelSim