Yaesu FTM-350AR audio interface: part one

I do a lot of support for horse endurance rides up at Imbil, and that means running a base for about 3 or 4 check-points. My go-to radio for this has been the Yaesu FTM-350AR, which is a good rig, but the head unit has woefully pathetic audio output.

The mono speaker is better, and you can get pretty good results plugging an un-amplified pair of speakers into the back — I use some old “SoundBLASTER” speakers from a SoundBLASTER 16 sound card. Works great if you’re in earshot of the radio, and much better than the built-in speaker in the radio body or the pathetic excuse for speakers in the head unit.

The radio can do cross-band, and this works pretty well, assuming you can find a pair of frequencies that don’t have interference. However, cross-band means you can’t use the radio directly as a standard radio, you must use a second radio to communicate on the cross-band link. In addition, it also means you can’t monitor more than one frequency at a time in base, or run packet from the same radio.

I’ve considered whether I make an audio interface that would link the radio to Mumble. Mumble is a VoIP solution for online gaming, and amongst other things, behaves much like a PTT-style radio in operation. The thinking was to interface the audio and PTT signals between the radio and the Mumble client, so that any transmit audio on the Mumble channel is fed to the radio, and vice versa.

Computer to radio is easy enough. Some years ago when I was doing the Brisbane to Gold Coast bicycle ride (net controller, in the back seat of the control vehicle), I had made an adaptor that let me plug in my DIN-5 headsets into the microphone port. I basically ignored most of the contacts, just paying attention to 0V (GND), EXTMIC and PTT.

Microphone pin-out for the FTM-350AR

An annoyance at the time is there was nowhere to plug headphones except the external speaker port, which is a little over-powered for headphones, so still had to rely on the radio’s speaker with my headset.

For a two-way radio interface, I’ll need to tap into that speaker audio. Now, an attenuator pad for the external speaker connector is a viable option, however it has the downside that it disables the internal speakers, meaning you need either a double-adaptor break-out, or you rely on the computer forwarding the audio somehow.

I contemplated opening up the head unit and adding a headphone jack, but then came to the conclusion that the audio is unlikely to be a digital signal being sent to the head unit — it must exist in analogue form on the 8-wire link between the head-unit and main-unit. Sure enough, I found some schematics, and there it is:

FTM-350AR head unit connector, showing left and right speaker connections

This goes off to a (rather anaemic) audio amplifier, so likely are line-level signals. Looks like I should be able to make a cable that taps off pins 2, 3 and 4 bringing those out to a 3.5mm jack which I can plug into an audio interface of my choosing. I have a Behringer U202 USB audio interface, seems like a good candidate for this experiment.

I lack the PTT signal on this connector, it is likely multiplexed in the TXD line, so unless I feel like reverse-engineering Yaesu’s protocols, the easiest bet is that I only tap received audio from here, and use the microphone socket for transmit audio (which I already know how to drive).

As for the software side, talKKonnect is one such option I could employ here.

Time to rummage through the junk box and see what I have on hand.