Controlling Backchannels in Streamable Full-duplex Models

Maike Züfle, Peter Polák, Sefik Emre Eskimez, Jan Niehues, Peter Bell +1 more
9/24/2026
cs.CLcs.HC

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Abstract

Backchannels, brief acknowledgements like "uh-huh" produced while the other party may still be talking, are central to natural conversation, but full-duplex spoken dialogue models rarely model them explicitly. We introduce a lightweight backchannel head that predicts, from a full-duplex model's own hidden states, when a backchannel should begin. Once this probability crosses a tunable threshold, a backchannel is force-decoded. Attached to both a 7B (PersonaPlex) and a 1B (F-Actor) model, it generalizes across scale. Probing confirms the hidden states anticipate real human timing, and generation evaluation shows more frequent, better-timed backchannels. Human raters judge the resulting backchannels on par with real ones.

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@article{zfle2026controlling,
  title  = {Controlling Backchannels in Streamable Full-duplex Models},
  author = {Maike Züfle and Peter Polák and Sefik Emre Eskimez and Jan Niehues and Peter Bell and Ondřej Klejch},
  year   = {2026},
  eprint = {2609.29418},
  archivePrefix = {arXiv},
  url    = {http://arxiv.org/abs/2609.29418v1}
}

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