Add CONTRIBUTING.md with contribution guidelines (includes DCO)

CONTRIBUTING.md

@@ -0,0 +1,147 @@
+# Contributing to AudioNoise
+
+Thank you for your interest in contributing to AudioNoise! This project is a learning-focused digital audio effects toolkit.
+
+## Getting Started
+
+### Prerequisites
+
+- GCC compiler
+- ffmpeg (for audio format conversion)
+- aplay (Linux audio playback) or equivalent
+- Python 3 with numpy and matplotlib (for visualizer)
+
+### Building
+
+```bash
+make convert
+```
+
+### Testing an Effect
+
+```bash
+make flanger   # or echo, phaser, fm, discont
+```
+
+This will:
+1. Convert `BassForLinus.mp3` to raw format
+2. Process it through the selected effect
+3. Play the result
+4. Save an MP3 of the output
+
+## How to Contribute
+
+### Reporting Bugs
+
+Open an issue with:
+- Description of the bug
+- Steps to reproduce
+- Expected vs actual behavior
+- Your environment (OS, compiler version)
+
+### Suggesting Features
+
+Open an issue describing:
+- What problem the feature solves
+- Proposed implementation approach
+- Any DSP references or algorithms involved
+
+### Submitting Code
+
+1. **Fork the repository**
+2. **Create a feature branch** from `main`:
+   ```bash
+   git checkout -b feature/my-new-effect
+   ```
+3. **Make your changes** following the code style below
+4. **Test thoroughly** with the provided Makefile targets
+5. **Commit with DCO sign-off** (see below):
+   ```bash
+   git commit -s -m "Add reverb effect using Schroeder algorithm"
+   ```
+6. **Push and open a Pull Request**
+
+### Developer Certificate of Origin (DCO)
+
+All contributions to this project **must** include a DCO sign-off. This certifies that you have the right to submit the work under the project's license (GPL-2.0).
+
+Add a sign-off line to your commit message:
+```
+Signed-off-by: Your Name <[email protected]>
+```
+
+You can do this automatically with the `-s` flag:
+```bash
+git commit -s -m "Your commit message"
+```
+
+By signing off, you certify that:
+- (a) The contribution was created by you and you have the right to submit it under GPL-2.0; or
+- (b) The contribution is based on previous work covered under an appropriate open source license; or
+- (c) The contribution was provided to you by someone who certified (a) or (b).
+- (d) You understand this project and contribution are public.
+
+See the full DCO text at: https://developercertificate.org/
+
+## Code Style
+
+This project uses a minimal, kernel-like C style:
+
+- **Indentation**: Tabs
+- **Line length**: Keep reasonable (no hard limit, but ~80 chars preferred)
+- **Braces**: K&R style (opening brace on same line)
+- **Comments**: Use `//` for inline, `/* */` for block
+- **Naming**: lowercase_with_underscores for functions/variables
+
+### Adding a New Effect
+
+1. Create `myeffect.h` with:
+   ```c
+   static inline void myeffect_init(float pot1, float pot2, float pot3, float pot4)
+   {
+       // Initialize effect state
+       fprintf(stderr, "myeffect: param1=%g\n", pot1);
+   }
+
+   static inline float myeffect_step(float in)
+   {
+       // Process one sample
+       return in;  // Modified output
+   }
+   ```
+
+2. Add to `convert.c`:
+   ```c
+   #include "myeffect.h"
+   // ...
+   EFF(myeffect),  // in effects[] array
+   ```
+
+3. Add to `Makefile`:
+   ```makefile
+   effects = ... myeffect
+   myeffect_defaults = 0.5 0.5 0.5 0.5
+   HEADERS = ... myeffect.h ...
+   ```
+
+### Design Principles
+
+- **Zero latency**: Single sample in, single sample out
+- **No FFT**: IIR filters and delay lines only
+- **Hardware-friendly**: Code should be portable to embedded (RP2354)
+- **Simple math**: Prefer fast approximations over library calls
+- **Four pots**: All effects controlled by four float parameters (0.0-1.0)
+
+## License
+
+This project is licensed under GPL-2.0. By contributing, you agree that your contributions will be licensed under the same terms.
+
+### GPL-2.0 Requirements
+
+- Modified files must carry prominent notices stating changes and dates
+- Derivative works must be licensed under GPL-2.0
+- Keep copyright headers intact
+
+## Questions?
+
+Open an issue for any questions about contributing.

flanger.h

@@ -12,7 +12,7 @@ static inline void flanger_init(float pot1, float pot2, float pot3, float pot4)
 	fprintf(stderr, " freq=%g Hz", pot1*pot1*10);
 	fprintf(stderr, " delay=%g ms", pot2*10);
 	fprintf(stderr, " depth=%g", pot3);
-	fprintf(stderr, " feedback=%g\n", pot1*pot1*10);
+	fprintf(stderr, " feedback=%g\n", pot4);
 }
 
 static inline float flanger_step(float in)