Also known as: DMO, Direct mode operation, talkaround, simplex operation
Direct mode operation (DMO) is a mode in which radios talk peer-to-peer, handset to handset, bypassing the infrastructure entirely — no repeater and no base station in the path.1 It is the counterpart to normal trunked operation, where every call is routed through a network of sites; in DMO the radios simply transmit and receive on a shared channel the way a walkie-talkie does. The same idea is called talkaround or simplex in conventional land-mobile parlance.2
How it works
In DMO the two (or more) radios agree on a single simplex frequency and share it: one transmits while the others listen, keyed by push-to-talk, with no repeater retransmitting the signal on a second frequency. Because nothing amplifies or re-radiates the transmission from a tall site, range is strictly radio-to-radio line of sight — typically a few kilometres on the ground, far less than the wide-area coverage a trunked site provides. The trade is deliberate: DMO keeps working when the network cannot.
Different standards give the mode different names but the concept is identical. TETRA draws the sharpest line, contrasting DMO with TMO (trunked mode operation) as two formally specified modes a radio switches between. DMR calls it direct mode, where two radios share a channel using the same two-slot TDMA structure without a repeater. P25 and analog conventional systems call it direct or talkaround — a button that drops the repeater and transmits on the output frequency directly.
Switching to DMO means giving up the trunking machinery. There is no control channel, so there are no channel grants assigning a traffic frequency per call, no wide-area roaming between sites, and no centralised affiliation of a talkgroup. Users pick a channel manually and stay on it. This makes DMO closer in spirit to conventional radio than to trunking, even on a radio that is otherwise a trunked subscriber unit.
Where it is used
DMO is the fallback and the local-work mode. It is used when radios are out of network coverage — inside a building, down in a valley, or beyond the last site — and for on-scene traffic where a crew working together does not need to burden the network, such as firefighters on a fireground talking among themselves. It is distinct from failsoft, the degraded mode a trunked site falls back to when it loses its controller; DMO removes the infrastructure from the path entirely rather than running a crippled version of it.
Many systems allow a DMO gateway or DMO repeater: a radio or fixed unit that bridges direct-mode traffic back into the trunked network, so a crew working simplex on scene can still reach dispatch. The gateway listens on the direct channel and relays onto the infrastructure, extending DMO’s short range without every handset needing a network link.
Relevance to SDR
For an SDR listener, DMO traffic looks different from trunked traffic on the waterfall: there is no control channel to follow and no grant to chase, so calls appear as intermittent simplex transmissions on fixed frequencies rather than as a managed hop across a site’s channel pool. GopherTrunk’s trunking follower keys on the control channel and channel grants, so purely direct-mode conversations fall outside that flow and are monitored like conventional channels — parked on the known simplex frequency. Recognising that a system has dropped to DMO explains why expected grant activity on the control channel goes quiet while voice is still on the air nearby.
Sources
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Terrestrial Trunked Radio — Wikipedia, for TETRA’s DMO versus TMO distinction and the loss of trunking features off-network. ↩
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Talkaround — Wikipedia, for the conventional/simplex “talkaround” equivalent of direct mode. ↩