Analog FM: the veteran
Turn on a traditional two-way radio, and the voice you hear has been carried by FM (frequency modulation): the carrier frequency shifts up and down following the speaker’s audio, and the receiver converts that shift back into sound. Compared to AM, FM is less sensitive to static and pulse noise at handheld power levels—subjectively “cleaner”—which is why it dominated analog radio for decades.
Channel spacing was historically 25 kHz. As spectrum grew scarce, regulators pushed toward 12.5 kHz and narrower—fitting more channels into the same band, but demanding tighter filters and frequency stability.
Squelch and tone codes: selection, not encryption
When nobody is talking, squelch mutes the receiver so you don’t hear constant hiss. Many systems also add CTCSS (a sub-audible tone) or DCS (a digital code)—the transmitter piggybacks a low-frequency tone or data word, and the receiver only opens audio when it detects a match. This reduces interference from other users on the same frequency, but it is not encryption: anyone using the same tone setting can listen in.
Why go digital
Analog FM works, but hits three ceilings:
- Spectrum efficiency—digital vocoders maintain intelligible speech in narrower bandwidths and free up capacity for short messages, location data, etc.
- Network management & security—digital frames natively carry terminal IDs, group numbers, priority flags, and encryption indicators; dispatch and recording systems plug in cleanly.
- Fade resilience—interleaving and error-correction coding can rescue signals in multipath environments. Though in extremely weak fields, digital systems “cliff” suddenly (metallic artifacts or lost words) rather than gradually getting noisier like analog.
Major digital PMR standards at a glance
| Standard | Body | Multiple access | Typical use |
|---|---|---|---|
| DMR | ETSI | TDMA dual-slot / 12.5 kHz | Commercial buildings, logistics, energy, multinational enterprises |
| TETRA | ETSI | TDMA four-slot | European public safety, metro, airports |
| P25 | TIA / APCO | FDMA → Phase II TDMA | North American law enforcement, federal agencies |
| NXDN | Vendor alliance | FDMA narrowband | Commercial/industrial, parts of Asia-Pacific |
| dPMR | ETSI | FDMA narrowband | European license-free / light commercial |
Note: “Same standard” ≠ “plug and play.” Interoperability also requires matching frequency plans, color codes / network IDs, group addresses, and encryption policies. Cross-brand interop relies on IOP certification or vendor declarations—always field-test before deployment.
Half-duplex PTT: analog or digital
Regardless of whether the underlying layer is analog FM or digital frames, the interaction stays the same: press to talk, release to listen. Digital systems can layer talk-permission management, preemption, and multi-group logic on top, but the user experience remains “push and speak.”
Cellular or internet-based PTT follows a different path—signals travel over IP packet networks rather than narrowband RF channels. See Volume 5 · Network PTT.
Further reading
Transmit frequencies, power, and equipment types are subject to local radio regulations and type approval.