# HubAudio Engineering Principles

**Document:** Engineering Principles

**Version:** 0.1 (Draft)

**Status:** Draft

**Applies to:** Entire HubAudio Project

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# 1. Purpose

This document defines the engineering principles that govern the design,
development, documentation and maintenance of the HubAudio platform.

These principles apply equally to:

- Hardware
- Firmware
- Documentation
- Architecture
- Development tools

Every engineering decision shall be consistent with these principles.

Whenever a principle cannot be respected, the reason shall be documented
through an Architecture Decision Record (ADR).

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# 2. Vision

HubAudio is not simply an electronic board.

HubAudio is an embedded digital audio platform designed to evolve over
many years while remaining understandable, maintainable and reliable.

The project values engineering quality over feature quantity.

Every design decision should reduce complexity rather than increase it.

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# 3. Core Values

The project follows these values, listed in order of importance.

1. Understandability
2. Maintainability
3. Reliability
4. Modularity
5. Extensibility
6. Performance

Performance is important.

Understanding the system is more important.

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# 4. Fundamental Principle

## EP-001 — Code That Fits in Your Head

This is the fundamental engineering principle of HubAudio.

The concept applies to every engineering artifact.

An engineering artifact includes:

- Source code
- Electrical schematics
- PCB layout
- Documentation
- Architecture diagrams
- Firmware modules
- Test procedures
- Design notes

Every engineering artifact should remain understandable by a single
engineer during a normal working session.

When understanding an artifact requires reading many unrelated parts of
the project, the design should be reconsidered.

Complexity shall never be removed by hiding it elsewhere.

Complexity shall instead be isolated inside the component responsible
for it.

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### EP-001.a — One Sheet, One Story

Each schematic sheet shall describe one subsystem.

Examples:

- ESP32 System Controller
- Power Management
- DSP Engine
- Radio Engine
- Bluetooth Engine
- USB Interface

A schematic shall never mix unrelated functions simply to reduce the
number of pages.

Readability is always preferred over compactness.

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### EP-001.b — One Document, One Topic

Each document shall describe one subject.

If a document becomes too large, it should be divided into multiple
documents.

Documentation shall remain easy to navigate.

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### EP-001.c — One Module, One Responsibility

Each firmware module shall implement one responsibility only.

Modules communicate through interfaces.

Implementation details remain internal.

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### EP-001.d — One PCB Area, One Function

PCB placement should reflect the logical architecture.

Subsystems should remain visually identifiable.

Power, DSP, Radio, Bluetooth and Controller sections should remain
clearly separated whenever practical.

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# 5. Engineering Principles

## EP-002 — Single Responsibility

Every hardware and software component shall have one clearly defined
responsibility.

Responsibilities shall never overlap.

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## EP-003 — Architecture Before Implementation

The design process always follows this order:

Architecture

↓

Documentation

↓

Implementation

↓

Verification

Implementation shall never drive architecture.

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## EP-004 — Hardware Independence

Firmware shall communicate with logical devices.

Hardware details shall remain confined inside the Hardware Abstraction
Layer whenever practical.

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## EP-005 — Modular Hardware

Each subsystem should, whenever technically possible, provide:

- Independent power control
- Independent reset
- Diagnostic capability
- Firmware update capability

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## EP-006 — Digital First

Audio shall remain digital until analog conversion is explicitly
required.

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## EP-007 — Observable System

Every subsystem shall expose sufficient diagnostic information.

A system that cannot be observed cannot be maintained efficiently.

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## EP-008 — Serviceability

Maintenance shall be considered from the beginning of the project.

Firmware update, diagnostics and hardware verification are part of the
system architecture.

They are not optional features.

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## EP-009 — Documentation is Part of the Product

Documentation is part of the engineering deliverable.

Every important design decision shall be documented.

Outdated documentation shall be considered a defect.

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## EP-010 — Continuous Improvement

Engineering decisions are based on evidence.

Whenever a better solution becomes available, it shall be evaluated
using objective engineering criteria.

Previous decisions are never protected by pride.

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# 6. Engineering Rules

The following practical rules derive directly from these principles.

- Small classes
- Small functions
- Explicit interfaces
- No hidden side effects
- Clear ownership
- Minimal coupling
- Meaningful names
- Predictable behaviour

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# 7. Definition of Done

A development activity is complete only when:

✓ Implementation completed

✓ Documentation updated

✓ Review completed

✓ Tests completed

✓ Future maintenance considered

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# 8. Scope

These principles apply to every future revision of HubAudio unless
explicitly superseded by a newer approved version.

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# 9. References

- ADR-000 — Engineering Principles

- Mark Seemann
  *Code That Fits in Your Head*

- Robert C. Martin
  *Clean Architecture*

- John Ousterhout
  *A Philosophy of Software Design*