Duplex Mode 3
This duplex mode allows a repeater (rather than app_rpt) to perform the repeat function. This is also known as in-cabinet repeat. Duplex Mode 3 is unique to repeaters and is not likely to be used on simplex, half-duplex nodes or hotspots. Setting duplex=3 in rpt.conf prevents Rx repeater audio from being sent to the repeater Tx. However, courtesy tones, morse code, voice messages and link audio are still sent to the repeater. PTT is continuously asserted upon a valid Rx signal.
Duplex mode 3 might be used with a repeater where in-cabinet repeat cannot be disabled. It has the advantage of no audio delay introduced from app_rpt. However, the disadvantage is reduced repeater control: time out timer, Tx on/off, CTCSS on/off and repeat audio level adjustment must be provided external to app_rpt.
The duplex mode 3 setting in an RTCM (menu 18) is complementary to app_rpt duplex mode 3. Rather than preventing a repeat audio path it provides one. In other words the RTCM performs in-cabinet repeat. This would be used to eliminate repeat audio delay. Duplex=3 must be set in app_rpt otherwise there would be two audio repeat paths and thus an echo.