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+# G3RUH FSK — CRDOP P1 design spec
+
+**Scope:** 9600–**19200** baud direct FSK. **Hard maximum 19200** — nothing above.
+
+**Prerequisite:** P0 on-air (1200 AFSK) stable. **Not** CB FM mic path.
+
+See [ROADMAP.md](../ROADMAP.md) P1 milestone (stack scheduling: **DEV-Level 4**).
+
+---
+
+## PHY difference from P0
+
+| | 1200 AFSK (P0) | G3RUH FSK (P1) |
+|---|----------------|----------------|
+| Modulation | Audio tones 1200/2200 Hz | Scrambled baseband FSK |
+| RF path | Mic / speaker / line | **Direct** varactor + discriminator |
+| CB FM voice channel | ✅ | ❌ |
+| VHF backbone | Access only at 1200 | ✅ at 9600–19200 |
+
+---
+
+## Parameters (normative)
+
+| Baud | Peak deviation | RF bandwidth (~-60 dB) |
+|------|----------------|------------------------|
+| 4800 | ±1.5 kHz | ~8 kHz |
+| **9600** | **±3 kHz** | **~20 kHz** |
+| **19200** | **±6 kHz** | **~30 kHz** |
+
+CRDOP implements **up to 19200** only.
+
+---
+
+## Radio interface requirements
+
+| Requirement | Detail |
+|-------------|--------|
+| TX | Direct modulation input (varactor), not acoustic |
+| RX | Discriminator / data output, wide IF |
+| Filter | Flat amplitude 10 Hz–5 kHz, constant group delay |
+| Scrambler | G3RUH compatible — interoperate with standard 9600 peers |
+
+Standard narrow FM voice audio **fails** eye-diagram tests at 9600+.
+
+---
+
+## Software modules (planned)
+
+| Module | Path | Status |
+|--------|------|--------|
+| Scrambler | `stacks/crdop/lib/g3ruh_scrambler.py` | Not started |
+| FSK modulator | `stacks/crdop/lib/g3ruh_modulator.py` | Not started |
+| FSK demodulator | `stacks/crdop/lib/g3ruh_demodulator.py` | Not started |
+| HDLC/framing | Reuse `hdlc_codec.py` | Done |
+
+Host protocol unchanged — same M25/KISS TCP; baud selected via INI `speed_baud` (future).
+
+---
+
+## P1 exit criteria
+
+1. Loopback with recorded baseband fixtures at 9600.
+2. Over-the-air exchange with reference 9600 FSK peer.
+3. 19200 optional stretch after 9600 stable.
+
+---
+
+## Out of scope
+
+- 38400+ satellite/backbone rates
+- Acoustic coupling at 9600+
+- CB 27 MHz deployment