SDR Squelch Mode

Funny thing about it, if you...

1. Set your system input to a non-Bluetooth microphone or line-in, instead of a virtual device (eg. Soundflower)
2. Set your system output to your Bluetooth speaker
3. Run ./macintercom --sdr

HFP won't switch on just like it doesn't switch on for Soundflower; it only does for a Bluetooth speaker & its integral mic at once - the Media-Aware & Standalone modes.

In --sdr, non-Bluetooth inputs will form a PA with your Bluetooth speaker, with Voice Activity Detection on by default (unless you debugflag it false to switch to the legacy envelope follower). Although. there is a slight but noticeable digital noise floor, for lack of a better vocabulary, with VAD. Haven't re-compiled to see how a legacy envelope follower PA sounds yet.

PA Mode was roadmapped for v0.2, but maybe it can just be documented here as a side-benefit of --sdr rather than adding a mostly-redundant --pa argument across the codebase, to also switch off the Bluetooth mic (HFP) in favor of A2DP.

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The method to my madness of adding SDR Squelch Mode in an intercom app?

So that the user can filter a raw, unsquelched in any way, radio frequency initially demodulated by their actual SDR (which may not have a CTCSS squelch unless forked into the code) and hear it through their system output with either VAD/legacy envelope follower, or -tone squelch.

This is the partial-inverse of PA Mode as originally conceived; instead of the user broadcasting their own microphone signal out any selected output, a non-Bluetooth radio transmitter becomes the PA's wireless microphone and the computer cross-band repeats it, from any analog modulated frequency supported by the SDR, to Bluetooth's 2.4 GHz ISM frequencies.

If you want to listen for a specific person or group's radio's transmissions, you can narrow it down if you match the CTCSS frequency in their radio when you run ./macintercom --sdr -tone <frequency>. You may hear unrelated people transmitting with the same one, but no transmissions with different ones/none at all.

This may also enable resurrection of some old wireless intercoms & transmitters without receivers, if you can find their published operating frequencies, or determine them via RF capture and near-field frequency finding.