[docs](trx-rs): add noise blanker section to MANUAL.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

MANUAL.md

@@ -0,0 +1,67 @@
+# trx-rs Manual
+
+## SDR Noise Blanker
+
+The noise blanker suppresses impulse noise (clicks, pops, ignition interference)
+on raw IQ samples before any mixing or filtering takes place.  It works by
+tracking a running RMS level of the signal and replacing any sample whose
+magnitude exceeds **threshold x RMS** with the last known clean sample.
+
+### Configuration (server-side)
+
+Add a `[sdr.noise_blanker]` section to your `trx-server.toml`:
+
+```toml
+[sdr.noise_blanker]
+enabled = true
+threshold = 10.0     # 1 – 100; lower = more aggressive blanking
+```
+
+| Field       | Type  | Default | Range   | Description |
+|-------------|-------|---------|---------|-------------|
+| `enabled`   | bool  | false   | —       | Turn the noise blanker on or off. |
+| `threshold` | float | 10.0    | 1 – 100 | Multiplier applied to the running RMS. A sample whose magnitude exceeds this multiple is replaced. Lower values blank more aggressively; higher values only catch strong impulses. |
+
+The noise blanker is off by default.  A threshold of **10** is a good starting
+point for typical HF impulse noise.  Reduce toward **3–5** for heavy QRM;
+increase toward **20–30** if weak signals are being clipped.
+
+### Web UI
+
+When the server reports noise-blanker support, two controls appear in the
+**SDR Settings** row of the web interface:
+
+- **Noise Blanker** checkbox — enables or disables the blanker in real time.
+- **NB Threshold** number input (1–100) with a **Set** button — adjusts the
+  detection threshold.  Press Enter or click Set to apply.
+
+Both controls stay hidden until the server sends filter state containing NB
+fields, so they only appear when connected to an SDR backend.
+
+### HTTP API
+
+```
+POST /set_sdr_noise_blanker?enabled=true&threshold=10
+```
+
+| Parameter   | Type   | Required | Description |
+|-------------|--------|----------|-------------|
+| `enabled`   | bool   | yes      | `true` or `false` |
+| `threshold` | float  | yes      | Value between 1 and 100 |
+
+### How it works
+
+The blanker runs on every IQ block (4096 samples) *before* the mixer stage in
+the DSP pipeline:
+
+1. For each sample, compute magnitude² (`re² + im²`).
+2. Compare against `threshold² × mean_sq` (the exponentially-smoothed running
+   mean of magnitude²).
+3. If the sample exceeds the threshold, replace it with the previous clean
+   sample.
+4. Otherwise, update the running mean with smoothing factor α = 1/128 and store
+   the sample as the last clean value.
+
+Because the blanker operates on raw IQ before frequency translation, it removes
+impulse noise across the entire captured bandwidth regardless of the tuned
+channel offset.