Monday, December 25, 2017

More TS-440S hijinx, or "ok, what if you wanna homebrew a digital hookup?"

I've been homebrewing digital hookups between my amateur radios (HF, VHF, UHF) and a FreeBSD PC. It all ... mostly works. There's one or two FreeBSD hiccups though, which are summarised thusly:

The default package selection for audio paths is .. suboptimal. Some can be configured to use OSS and that is nice. Some provide ALSA but FreeBSD's "ALSA" implementation doesn't provide full ALSA device emulation so we don't get a list of ALSA devices by default. You have to put your devices into asound.conf with names for things. However ..

.. FreeBSD doesn't currently make it easy to hard-code say, USB device paths to serial port names or sound devices to something predictable. So every time I reboot or mess with the setup it goes pear shaped.

Then there's a bug where I can run three USB audio devices, but I can't do mic input on the last one. Output works fine. That's going to be amusing to diagnose.

So I do have my TS-440S, TS-711A and TS-811E all doing digital modes. They're just all .. subtly different.

The TS-711A and TS-811E have an accessory jack (ACC2) that has input and output. There's a PTT control line and a mic mute line. The line levels of those signals is a couple hundred millivolts, so it's good enough to build a little resistor divider with a potentiometer to get the computer output down to the right level. I'm using mic input on the USB audio devices, so that also works fine at a couple hundred millivolts.

The TS-440S also has a similar accessory jack, however the audio input in that jack seems to be quite a big higher than a couple hundred millivolts. It looks like it needs to be around 4-5v peak-to-peak for it to be at the right level internally on the IF board. The ACC2 path has a couple of resistor attenuators so it looks like this was intentional - after asking around it looks like it's expecting professional line audio output levels (~4v peak-to-peak) instead of consumer grade levels (~1.5v peak-to-peak.) I'll go dig into it some more. This path bypasses the microphone pre-amplifier entirely and goes straight into the Mic Gain control pot.

The TS-440S also has AFSK input/output RCA jacks on the back. The audio output is at the same level as the ACC2 jack, however the audio input side is routed via the microphone input side so it gets preamp'ed and processed appropriately. That's what I've been using for digital modes - I can divide down the input side to a couple hundred millivolts to keep it all kosher. However - and here's the really annoying part - the mic mute input on ACC2 also mutes the AFSK input line.

Then there's what happens if you leave the microphone connected. If you do leave it connected, even in a quiet room, it seems to present some load that requires a lot more signal on AFSK input to do its thing. If you tune it all up to the right signal levels and then disconnect the microphone, you'll be really overdriving the RF section. Ugh.

So - I don't have to do any of this for the TS-711 and TS-811 - their input values are a lot lower and grounding the mic line actually just quietens the mic input.

If I can score a slightly different radio - like a TS-680S for example - then I can do this stuff with a much lower level line input value. It'll be tricky to get it down to the TS-680S level (it wants it at 10mV!) but at least I can do that with passive, well shielded bits.

On the plus side - yes, this means I at least can do digital modes on my TS-440S. I just have to keep unplugging the microphone line for now. What I may end up doing for now though is adding another switch to the desktop microphone I have to turn /its/ microphone input off so it is fully disconnected. Hopefully that'll be enough to do digital modes without constantly screwing and unscrewing things.

If you're at all curious -

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