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Week 1 · Tuning in

Hear the signal: AM vs FM

You found where a signal lives. Now for how it's shaped. Modulation is the second dial — get it wrong and a signal that's right in front of you turns to silence. Get it right and it snaps into view.

⏱ 45 min Gear: Flipper + a known remote Transmit: none

Goal

In Module 1 you read a frequency off the screen. But if you tried to capture a remote there and got nothing, this is almost always why: the frequency was right, but the modulation was wrong. This module makes modulation something you choose and immediately see the effect of — a signal appearing or vanishing as you flip a setting.

By the end you'll understand the difference between AM and FM in one sentence each, know what the four preset names mean, and have a simple habit — "try AM650, then sweep" — that solves the vast majority of "why won't this capture?" moments.

The carrier wave

Recall the carrier from Module 1: a pure, steady tone of radio energy at one frequency. On its own, a carrier says nothing — it's a held note. To carry information, the transmitter has to change the carrier in a pattern the receiver can read back. That deliberate changing is modulation: literally, "shaping" the wave.

There are only two knobs you can wiggle on a simple carrier:

  • Its strength (how loud) — change this and you're doing amplitude modulation.
  • Its pitch (how fast it wiggles) — change this and you're doing frequency modulation.

Cheap Sub-GHz remotes use the simplest possible version of each. That's the whole vocabulary you need, and it maps neatly onto two names you already half-know from car stereos: AM and FM.

AM vs FM, plainly

Here are the two, each in one sentence, with the analogy that makes them stick:

AM On-Off Keying (OOK)

The transmitter switches the carrier on and off — full blast for a "1," silent for a "0." It's Morse code with a flashlight: light on, light off.

Formally this is OOK (on-off keying), the bare-bones flavor of AM. Most doorbells, LED remotes, and cheap gate remotes use it because it's dead simple and cheap to build.

FM Frequency-Shift Keying (2-FSK)

The carrier stays on the whole time but shifts between two nearby pitches — one frequency means "1," another means "0." The flashlight never turns off; it just changes color.

Formally this is 2-FSK, the two-tone flavor of FM. It shrugs off certain kinds of interference better, so you'll meet it in some sensors, TPMS, and higher-end remotes.

The reason this matters practically: a receiver set for AM is watching for the carrier getting louder and quieter. Point it at an FM signal — which never changes loudness, only pitch — and it sees a steady, featureless tone and reports… nothing. The signal is right there; the receiver is just listening for the wrong kind of change.

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"AM" and "OOK", "FM" and "2-FSK"The Flipper's presets are named with AM/FM because that's the familiar family, but under the hood the everyday flavors are OOK and 2-FSK. For a beginner they're interchangeable: AM ≈ OOK (on/off), FM ≈ 2-FSK (two pitches). We'll use both names so you recognize either in the wild.

You cannot tell AM from FM by looking at a remote, and manufacturers rarely print it on the case. This is not a problem you solve by staring harder — it's one you solve by trying. The habit you'll build in the lab is exactly this: pick the frequency, then run through the modulation options until the signal reveals itself. The device already knows which one it uses; your job is only to ask the right question, and there are just four questions to ask.

There's a deeper reason AM dominates the cheap end. Amplitude is trivial to detect — you just measure "loud or quiet" — so an OOK receiver can be a couple of cents of components. FSK asks the receiver to distinguish two nearby pitches, which needs a slightly smarter (and pricier) front end. When a product's whole radio budget is a rounding error, OOK wins. When reliability earns its keep — a tire sensor that must be heard over engine noise, an alarm that can't afford a dropout — the extra cents for FSK start to make sense.

The four presets

The stock Flipper gives you four built-in modulation presets. Their names look cryptic but decode cleanly once you know the pattern: a modulation family (AM or FM) followed by a number.

PresetFamilyThe number is roughly…Reach for it when
AM270 AMOOK (on/off)a narrower receive window (~270)weaker or cleaner OOK signals; less noise let in
AM650 AMOOK (on/off)a wider receive window (~650)your default first try — catches most OOK remotes
FM238 FM2-FSK (two pitches)a smaller deviation (~2.38 kHz)FSK devices with a small pitch shift
FM476 FM2-FSK (two pitches)a larger deviation (~47.6 kHz)FSK devices with a larger pitch shift
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About that numberThe number means something slightly different for AM and FM, so don't over-read it. For the AM presets it's roughly the receiver bandwidth in kHz — how wide a slice of spectrum the receiver listens to (AM650 = wide, AM270 = narrow). For the FM presets it's the frequency deviation — the offset of a tone from the centre frequency (FM476 = a bigger shift, FM238 = a smaller one), not the bandwidth. Both FM presets use approximately 270 kHz receive bandwidth. FM238 uses about 2.38 kHz deviation; FM476 about 47.6 kHz. Deviation is not signal strength.

Official firmware and compatible forks support custom presets beyond these four, loaded from a settings file on the SD card — handy for oddball devices later. For now, the four built-ins cover almost everything, and AM650 alone covers most of your junk-drawer remotes.

Watch modulation happen

Reading about on/off versus two-pitches only gets you so far. In the Wave Lab, type a short bit pattern and watch it ride a carrier — flip between OOK and FSK and see the very shapes from the diagram above build themselves from your bits.

Encode a little message and you'll earn the Wave Rider badge.

Two independent dials

Here's the mental model that prevents the most common beginner frustration. Capturing a signal means getting two separate settings right at once:

Dial 1 · Frequency where

Which channel — 315, 433.92, 868, 915 MHz. Set it with the Analyzer (Module 1) or by knowing the device.

Dial 2 · Modulation how

Which shape — AM/OOK or FM/FSK, and how wide a window. Set it with the preset.

These dials are independent. Right frequency + wrong modulation = silence. Right modulation + wrong frequency = silence. You need both right, at the same time, before a signal appears. That's why "I found the frequency but nothing captures" is so common — one dial was correct and the other wasn't.

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The habit: lock the frequency, then sweep modulationOnce you know a device's frequency, fix it, then try the presets in order: AM650 first (it catches most things), then AM270, then FM476, then FM238. One of them will usually make the signal jump out. This "sweep" takes under a minute and answers the modulation question by experiment instead of guesswork.
How do manufacturers decide AM or FM anyway?

Cost and robustness, mostly. OOK is the cheapest thing that works, so the budget end of the market — doorbells, toy remotes, LED strips — leans OOK/AM. FSK costs a little more but resists certain interference and amplitude fades, so it shows up where reliability matters more than pennies: some sensors, tire-pressure monitors, and pricier remotes. You don't have to predict it; you just have to be willing to sweep both families.

Lab 2: sweep the presets

Lab 2.1

Find the winning combo

Gear: Flipper + one remote whose frequency you already know (from Lab 1)

  1. Open Sub-GHz → Read and set the frequency to the value you recorded for this remote in Module 1.
  2. Set the modulation preset to AM650.
  3. Press and hold the remote's button close to the Flipper. Does Read react — a decoded name, or clear activity?
  4. If nothing, change only the preset and try again, in order: AM270, then FM476, then FM238. Keep the frequency fixed.
  5. When the signal jumps out, write down the winning frequency + modulation pair for that device. That combo is its fingerprint.
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Still nothing on any preset?If a remote is silent across all four presets at the right frequency, it may be a rolling-code device or need Read RAW — both are Module 5 territory, not a failure on your part. Note it as "needs RAW" and move on; you're not stuck.
✓You've got it when: for at least one remote you can state its full combo out loud — "this doorbell is 433.92 MHz, AM650" — and you understand that if either half were wrong, you'd have seen nothing.
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The "it was there all along" momentThe first time you sweep from a dead-silent preset to the right one and a decoded remote suddenly fills the screen, it feels like you turned on a light in a dark room. The signal never changed — your ears did. That's modulation.

Check yourself

Verify the details

Read the primary sources: Flipper: reading signals and presets. Course review: 10 September 2026.