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(why_12m_looks_noisy)=
# Why 12m Looks "Noisy" on the Raspberry Pi

> NOTE: This document was split out of the main FAQ for readability.

(1-observations)=
## 1. Observations

- **Test tone at 24.926 MHz (12m)** looks _clean_ on the scope.
- **WSPR transmission on 12m** appears to be _noise_ or "hash".
- Other bands (10 m, 15 m, etc.) appear cleaner under similar conditions.
- The effect is much stronger on **Raspberry Pi 4** than on older Pis.
- No LPF or passive filtering is present; probing is directly on **GPIO 4**.

This behavior is **expected** given how the signal is generated and the differences in Pi 4 hardware.

(2-why-testtone-is-clean-but-wspr-looks-noisy)=
## 2. Why Test‑Tone Is Clean but WSPR Looks Noisy

(testtone-mode)=
### Test‑Tone Mode

- Generates a **single, constant frequency**.
- No fractional scheduling.
- No rapid switching between tones.
- GPIO outputs a steady divider → narrow spectrum (aside from harmonics).

(wspr-mode)=
### WSPR Mode

- Uses **four closely spaced tones**.
- Each symbol duration is generated by **fractional switching** between two adjacent divider values (f0 / f1) to hit the exact desired frequency.
- This results in _many rapid GPIO transitions_ within each symbol.

Without filtering, these transitions appear as:

- Broadband energy on a scope
- "Noise‑like" waveform instead of a clean sinusoid

(3-why-12m-is-worse-than-other-bands)=
## 3. Why 12m is Worse Than Other Bands

12m sits in a **bad corner case**:

- High enough frequency that GPIO edges are extremely sharp in phase terms.
- Small divider differences → **fine‑grained fractional dithering**.
- Fractional scheduling creates very short segments:
  - A few PWM clocks of f0
  - Followed by a few PWM clocks of f1
- These tiny segments behave like **phase noise / wideband FM**.

Other bands:

- Larger divider gaps or different ratios.
- Fractional error spreads more benignly.
- Less visible broadband hash on a scope.

(4-why-raspberry-pi-4-is-much-worse-than-pi-123)=
## 4. Why Raspberry Pi 4 Is Much Worse Than Pi 1/2/3

(gpio-hardware)=
### GPIO Hardware

- Pi 4 GPIO edges are **much faster** (sub‑2 ns rise time).
- Faster edges → more high‑frequency content → more visible noise.

(clocking-architecture)=
### Clocking Architecture

- Pi 4 uses multiple high‑speed PLL domains:
  - PLLD_PER
  - PWM clock
  - Core / VPU clocks
- Fractional scheduling errors are no longer "blurred" by slow hardware.

(pwm-clock-rate)=
### PWM Clock Rate

- Pi 4 PWM clock ≈ **500 MHz** domain.
- Tiny timing errors become real RF events.
- Older Pis had slower clocks and unintentionally smoothed things out.

**Net effect:**  
Pi 4 exposes exactly what the math is doing.

(5-what-was-changed-in-the-software)=
## 5. What Was Changed in the Software

The original scheduler:

- Used round() each iteration.
- Produced unpredictable tiny segments.
- Randomized iteration length.
- Generated high‑rate, irregular tone switching.

(improvements-made)=
### Improvements Made

- **Fixed‑Point Bresenham Scheduler**
  - Error‑accumulator style scheduling.
  - Exact long‑term frequency ratio.
  - Deterministic behavior.
- **Predictable Toggle Cadence**
  - Fixed block size (PWM_CLOCKS_PER_ITER_NOMINAL).
  - No random jitter.
- **Tail Clamp**
  - If the remaining clocks are small, send only **one tone**.
  - Prevents end‑of‑symbol micro‑toggles.
- **Tiny‑Segment Merge**
  - If n_f0 or n_f1 < threshold:
    - Merge the entire block into one tone.
  - Eliminates 2-10‑clock fragments.
- **Extensive Debug Instrumentation**
  - Counts actual switches.
  - Logs tiny / zero‑length segments.
  - Proves why 12m is noisy before.

These changes **significantly reduce broadband hash**, especially on Pi 4.

(6-is-this-fixed-without-an-lpf)=
## 6. Is This "Fixed" Without an LPF?

No - and that's expected.

(important-reality-check)=
### Important Reality Check

- GPIO RF output is **not a DAC**.
- It is a fast square‑wave source.
- Software can reduce _unnecessary modulation artifacts_, but cannot remove harmonics.

A **low‑pass filter is mandatory** for:

- Legal operation
- Spectral cleanliness
- Making Pi 4 look as good as Pi 1/2/3 did "bare."

With a proper LPF:

- The 12m WSPR signal will look clean.
- The remaining differences between bands largely disappear.

(7-final-takeaway)=
## 7. Final Takeaway

- 12m looks noisy because **fractional dithering + fast GPIO edges** were fully exposed on Pi 4.
- The issue is **not** a bad frequency, band bug, or calibration error.
- Software changes addressed _avoidable_ noise:
  - Tiny segments
  - Randomized timing
  - Unpredictable switching
- Pi 4 is simply more honest - and less forgiving - than older Pis.
- With the current scheduler **and** a proper LPF, the signal is correct.

**In short:**  
Nothing is "wrong" with 12m. The Pi 4 just stopped hiding the physics.