Coherent output modes (Coh1–Coh4)
These are listening modes, not a decoder. Using the carrier phase recovered from a narrow filter, Linrad can route the in-phase and quadrature parts of a signal to the two ears in several ways — synchronous AM, or a binaural presentation that separates amplitude from phase. They share the coherent machinery of coherent CW (the narrow carrier filter and phase extraction) but produce audio for the operator instead of decoded Morse.
The Coh button on the baseband graph cycles bg_coherent from 0 (Off) through 4; the neighbouring Rat value (bg.coh_factor) sets how much narrower the carrier filter is than the audio passband. The operator chooses which carrier to lock onto by placing the receive passband on it.
The modes#
| Button | What you hear | Output |
|---|---|---|
Off (Coh0) |
normal demodulation, no coherent processing | mono/stereo per mode |
| Coh1 | the signal in one ear, the recovered carrier in the other | stereo |
| Coh2 | binaural I/Q: amplitude (AM) in one ear, phase (FM) in the other — the "3D" soundscape | stereo |
| Coh3 | synchronous: the in-phase component (I) to both ears | mono |
| Coh4 | synchronous: the quadrature component (Q) to both ears | mono |
In Coh3 and Coh4 the recovered carrier phase rotates the signal so the wanted modulation lands on one axis. With a BFO in use (CW/SSB) only the positive half is passed; for AM/FM the DC (carrier) level is removed instead — synchronous AM detection, which suppresses the noise and fading on the quadrature axis. Coh3 and Coh4 are mono (the same audio to both ears) and force single-channel baseband processing; Coh1 and Coh2 are two-channel and cannot be combined with the two-polarization or synthetic-stereo-by-delay outputs.
In the source
The output is built by the switch (bg_coherent) in fft3_mix2() — mix2.c:1774: signal + carrier mix2.c:1843 (Coh1); I/Q binaural mix2.c:1884 (Coh2); I to both ears mix2.c:1918 (Coh3, positive-only when use_bfo, else DC removed); Q to both ears mix2.c:1954 (Coh4). The coherently detected I/Q it reads (baseb[]) is produced upstream by the coherent function. Toggling: BG_TOGGLE_COHERENT increments bg_coherent (baseb_graph.c:2577), wrapping past 4 to 0 and forcing single-channel processing for modes ≥ 3 (baseb_graph.c:3086, baseb_graph.c:3150); the button text Coh%d is drawn at screen.c:2971 and the Rat (coh_factor) button at screen.c:2830. The narrow carrier filter is extracted in the second mixer mix2.c:246.
Related#
The carrier filter and phase extraction come from the second mixer and the same coherent code as coherent CW and Morse decoding; locking onto a drifting carrier relies on AFC. The synchronous output can be inspected on the coherent graph's oscilloscope.