These two get mixed up all the time in GNU Radio because they sound similar but solve slightly different problems.

I’ll explain them intuitively first, then a bit more technically, and finish with a couple of “when do I use which?” rules of thumb.

Big picture (one-liner version)

• Selector block: Choose which input stream goes to the output
• Switch block: Choose whether data flows through at all (and from where)

Think selector = multiplexer, switch = gate / traffic controller.

Selector block

What it does

A Selector has:

• Multiple inputs
• Usually one output
• A control value (integer)

It forwards exactly one input stream to the output, based on the selector index.

Mental model

Like a rotary switch or audio mixer channel selector:

“Give me input 0… no, now input 1… now input 2…”

Example

You have:

• Input 0 → raw antenna
• Input 1 → filtered signal
• Input 2 → simulated test tone

Set selector value to:

• 0 → antenna
• 1 → filtered
• 2 → test tone

Everything else is ignored.

Key properties

• Control input is an integer
• Switching happens on the fly
• Other inputs still exist but are not forwarded
• Very commonly used for:
  • Debug paths
  • Mode switching
  • A/B comparisons
  • Choosing antennas, demodulators, or signal sources

Switch block

What it does

A Switch routes data based on a boolean condition.

Depending on the variant:

• It may have:
  • Two inputs → one output
  • One input → two outputs
• Controlled by a true/false (or nonzero/zero) signal

Mental model

Like a valve or gate:

“If condition is true, let data through this path; otherwise, use the other path or block it.”

Common variants

• 2-to-1 switch: choose between two inputs
• 1-to-2 switch: route one input to one of two outputs
• Enabled / disabled path (effectively mute/unmute)

Key properties

• Control signal is usually boolean
• Often used for:
  • Gating data
  • Threshold-based decisions
  • Conditional routing
  • Turning processing on/off dynamically

Selector vs Switch — side-by-side

GNU Radio gotchas (important)

1. Switching is not sample-accurate

Changing selector/switch values mid-stream can:

• Drop samples
• Repeat samples
• Cause brief discontinuities

That’s normal. If you need phase-continuous switching (e.g. interferometry), you usually:

• Align streams
• Or switch at block boundaries
• Or cross-fade instead of hard switching

2. Unselected inputs still run

Selector blocks do not stop upstream blocks from running — they just don’t forward their data.

If you need to actually stop computation, you’ll want:

• Enable blocks
• Or separate flowgraphs
• Or message-based control

Practical radio examples (your world 😉)

Use a Selector when:

• Choosing between antennas
• Switching between real SDR input and a test signal
• Selecting different filter chains
• Debugging stages of a receiver

Use a Switch when:

• Muting output below an SNR threshold
• Gating data when lock is achieved
• Routing signal only when a detector fires
• Turning recording on/off based on power

Quick rule of thumb

If you’re choosing which stream, use a Selector.
If you’re choosing whether data flows or which path it takes, use a Switch.

Next is a sketch a small GNU Radio Companion example for:

• Antenna selection
• Threshold-triggered recording
• An interferometry-friendly approach that avoids phase jumps

Perfect — let’s walk through all three, step-by-step, in a way that maps cleanly onto GNU Radio Companion and real radio work. I’ll keep it practical and slightly opinionated.

1️⃣ Antenna / Source selection (Selector block)

Use case

You want to switch between:

• Antenna A
• Antenna B
• A signal generator (test tone / noise source)

…without rewiring the flowgraph.

GRC structure

[Antenna A] ─┐

             ├──► Selector ───► Rest of receiver

[Antenna B] ─┤

             │

[Test Src ] ─┘

Key blocks

• Selector
  • Number of inputs = 3
  • Output index = variable (e.g. src_sel)
• QT GUI Range or Variable
  • src_sel = 0, 1, 2

How it behaves

• src_sel = 0 → Antenna A
• src_sel = 1 → Antenna B
• src_sel = 2 → Test source

Upstream blocks keep running, but only one stream reaches the receiver.

Pro tip

If you’re doing interferometry or phase-sensitive work, do not switch rapidly — change only when stopped or during dead time. Selector switching is a hard cut.

2️⃣ Threshold-triggered recording (Switch block)

Use case

Record data only when signal power exceeds a threshold
(e.g. Jovian bursts, meteor scatter, RFI detection)

GRC structure

[Signal] ───►┬──────────────► Switch ───► File Sink

             │

             └──► Power ─► Compare ─► Control

Key blocks

• Complex to Mag² (or RMS)
• Moving Average
• Compare (e.g. > threshold)
• Switch
  • Data input 0 = signal
  • Data input 1 = zeros (or null source)
  • Control = compare output

Logic

• Power > threshold → control = 1 → signal flows
• Power ≤ threshold → control = 0 → muted

This avoids filling disks with noise.

Gotcha

The switch is not sample-perfect. Expect:

• Slight truncation at start/end
• That’s fine for detection, not for phase science

Upgrade path

For cleaner edges:

• Gate with a burst tagger
• Or record continuously and post-filter offline

3️⃣ Phase-safe switching for interferometry (important one)

Use case

You want to:

• Select between signal paths
• Enable/disable correlation
• Change modes

…but without destroying phase continuity.

Hard switches will ruin fringes.

❌ What NOT to do

• Selector mid-stream
• Switch based on a boolean
• Stop/start streams independently

These introduce phase jumps.

✅ Correct approach: keep streams running, select later

Pattern A — Parallel processing + Selector after correlation

[Antenna 1] ─┬─► Corr ─┬─► Selector ─► Display / Save

[Antenna 2] ─┘         │

                        └─► Alternate Corr

All signal paths:

• Run continuously
• Stay phase-locked
• Only results are switched

Pattern B — Cross-fade instead of switch (advanced, very clean)

Instead of switching:

• Multiply Path A by (1 - α)
• Multiply Path B by α
• Add them

Where α ramps smoothly 0 → 1.

Path A ─► Multiply (1-α) ─┐

                          ├──► Add ─► Output

Path B ─► Multiply (α) ───┘

This:

• Preserves phase
• Avoids spectral splatter
• Is gold-standard for precision work

Pattern C — Message-based enable/disable

If you want to pause recording or processing without breaking phase:

• Keep DSP running
• Use Enable blocks or message-controlled sinks
• Ignore data instead of stopping it

Summary table (radio-astronomy mindset)

Next we will:

• Build a full GRC interferometer flowgraph
• Add real-time fringe stopping
• Or design a selector-driven test harness for debugging SDR chains

Alright — let’s build all three properly, as if you were about to drop them straight into GNU Radio Companion. I’ll give you exact block lists, variables, and wiring, plus a few “radio-astronomer sanity notes” along the way 📡

I’ll assume complex baseband everywhere unless stated otherwise.

1️⃣ Antenna / Source Selection (Selector block)

Goal

Select between multiple signal sources (e.g. two antennas + test source) without rewiring.

Blocks you need

Sources

• Osmocom Source (Antenna A)
• Osmocom Source (Antenna B)
  (or second channel if using RSPduo)
• Signal Source (test tone or noise)

Control

• QT GUI Range
  • ID: src_sel
  • Min: 0
  • Max: 2
  • Step: 1

Routing

• Selector
  • Input type: Complex
  • Number of inputs: 3
  • Output index: src_sel

Wiring

Antenna A  ─┐

            ├─► Selector ──► Filter ─► Demod ─► Sink

Antenna B  ─┤

            │

Test Src   ─┘

Behaviour

• src_sel = 0 → Antenna A
• src_sel = 1 → Antenna B
• src_sel = 2 → Test source

💡 Important
All sources continue running. Only the output changes.

📡 Astronomy note
Do not change src_sel mid-observation if phase matters. Stop, switch, restart.

2️⃣ Threshold-Triggered Recording (Switch block)

Goal

Record data only when signal power exceeds a threshold
(e.g. Jovian bursts, meteors, RFI spikes)

Blocks you need

Signal path

• Complex to Mag²
• Moving Average
  • Length: e.g. 1024
  • Scale: 1/1024

Threshold logic

• QT GUI Range
  • ID: pwr_thresh
• Compare
  • Operation: >
  • Input A: averaged power
  • Input B: pwr_thresh

Routing

• Switch
  • Data input 0: signal
  • Data input 1: Null Source (or zeros)
  • Control: output of Compare

Sink

• File Sink (complex)

Wiring

Signal ──────────────┬────────► Switch ─► File Sink

                     │

                     └─► Mag² ─► Avg ─► Compare ─► Control

Behaviour

• Power > threshold → recording ON
• Power ≤ threshold → recording muted

📌 Expected artefacts

• First few samples clipped
• Slight truncation on drop-out

📡 Astronomy note This is detection-grade, not correlation-grade.
Great for burst hunting, not phase science.

3️⃣ Phase-Safe Interferometry Switching (the “right” way)

This is the one that actually matters for you.

❌ What we are avoiding

• Selector on raw antenna signals
• Switches before correlation
• Stop/start data streams

All of these break phase.

✅ Correct pattern: parallel DSP, select results

3A️⃣ Parallel correlators + Selector

Goal

Choose which correlation result you view or save — without touching the RF streams.

Blocks

Inputs

• Osmocom Source (Ch 0)
• Osmocom Source (Ch 1)

DSP

• FFT / Delay / Fringe Stop (as required)
• Complex Multiply Conjugate (correlator)

Selector

• Number of inputs: e.g. 2
• Output index: corr_sel

Wiring

Ant 1 ─┬─► DSP ─► Corr A ─┐

       │                  ├─► Selector ─► Display / File

Ant 2 ─┘                  │

                          └─► Corr B

All paths run continuously.

Switching affects only what you look at, not the coherence.

3B️⃣ Cross-fade switching (ultra-clean, recommended)

Goal

Transition between two paths without phase jumps or spectral splatter

Blocks

• Multiply Const ×2
• Add
• QT GUI Range
  • ID: alpha
  • Range: 0 → 1
  • Smooth ramp (slow slider or scripted)

Wiring

Path A ─► × (1 - α) ─┐

                     ├─► Add ─► Output

Path B ─► × (α) ─────┘

When:

• α = 0 → Path A only
• α = 1 → Path B only

🎯 This preserves

• Phase
• Amplitude continuity
• Fringe integrity

📡 This is interferometry gold

3C️⃣ Enable/Disable recording without phase loss

Instead of switching signals:

• Keep DSP alive
• Gate only the sink

Use:

• Tagged Stream Blocks
• Message Strobe
• File Sink (message-controlled)

Data keeps flowing → you just decide whether to store it.

Final decision table (bookmark-worthy)