Also known as: LTR, Logic Trunked Radio
LTR (Logic Trunked Radio) is a simple, low-cost trunking protocol from E.F. Johnson. Unlike systems with a dedicated control channel, LTR is distributed: the trunking data rides subaudibly on each voice channel, so every repeater carries its own low-speed signalling rather than one channel coordinating the rest.1 This makes LTR cheap to build but distinctive to monitor, since there is no single channel to watch for system activity.
Overview
Because there is no separate control channel, an LTR radio follows calls by reading a low-rate data burst embedded below the audio passband (a subaudible ~300 bit/s stream) on each channel. That data identifies the home repeater (the channel a given group is assigned to), the logical channel number in use, and a group/goto field telling radios which channel to move to for the next transmission. Radios in a group idle on their home repeater’s frequency; when a call starts, the subaudible data steers them to the active channel and back. This decentralised design means each repeater in a shared system operates semi-independently, which keeps hardware cheap and avoids a single point of failure, at the cost of the self-describing convenience a dedicated control channel provides. Later variants extended the idea: LTR-Net adds networking and richer features, and PassPort (from Trident/SmartTrunk lineage) layers on wide-area roaming and registration.
Technical characteristics
| Property | Value |
|---|---|
| Access | FDMA, distributed |
| Signalling | Subaudible (~300 bit/s) data burst on every channel |
| Data fields | Home repeater, logical channel number, group/goto |
| Voice | Analog FM |
| Variants | Standard LTR, LTR-Net, PassPort |
| Bands | VHF, UHF, 800/900 MHz |
History
LTR was introduced by E.F. Johnson and became a mainstay of low-cost business and SMR (Specialised Mobile Radio) trunking.12 Its distributed philosophy set it apart from the control-channel systems of Motorola and EDACS, targeting operators who wanted trunking economy over feature richness. Over time the family grew to include LTR-Net and the PassPort roaming variant, and LTR-style subaudible signalling influenced other economy trunking schemes.
Deployment
LTR is common in commercial and business shared systems — taxi and delivery fleets, utilities, tow companies, and community-repeater operators — especially across North America. Many of these systems are still on the air because they are inexpensive to maintain and meet modest fleet needs without a migration to digital.
Decoding it with GopherTrunk
Monitoring LTR is unusual because there is no control channel to lock to: a decoder must demodulate the subaudible data on each voice channel to learn the home-repeater and goto information, then piece together group activity across channels. Voice is plain analog FM, so no vocoder is required — the challenge is recovering the low-rate embedded signalling cleanly. See the Status page for GopherTrunk’s handling of LTR’s subaudible signalling and any supported variants.
Sources
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Logic Trunked Radio — Wikipedia, for the E.F. Johnson LTR distributed trunking scheme and its subaudible per-channel signalling. ↩ ↩2
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Logic Trunked Radio (LTR) — RadioReference Wiki, for home-repeater/goto signalling, logical channel numbering, and the LTR-Net and PassPort variants. ↩