uBITX + Teensy audio interface - ground loops (?), RX/TX audio bleed? #ubitx #teensy

Rob French (KC4UPR)


So, I've put together a rudimentary audio interface and DSP filter using a Teensy 3.2 and the Teensy audio board.  Currently, it's on a breadboard.  I have three 1:1 audio transformers to interface the Teensy to the uBITX:

RX audio: Vol-H => 1:1 => Teensy audio in (L) => (DSP processing) => Teensy audio out (L) => 1:1 => high-side of volume pot
TX audio: Mic => preamp => Teensy audio in (R) => (DSP processing) => Teensy audio out (R) => 1:1 => 10uF blocking cap => uBITX mic in

(I actually haven't hooked up the mic/preamp yet, but I'm actually using USB audio.)

So that's the other piece--I've put together a sketch for the Teensy that allows switching between audio inputs/outputs, enabling/disabling filters, etc.  So I've been doing FT8, and the Teensy sketch actually takes USB audio in, and sends that to the Teensy audio out (R) for transmit.  Likewise, I take the audio in (L) and send that to the computer via the Teensy USB audio.

So, it IS working... I've just made a few FT8 contacts with my terrible attic antenna, and they seem like about what I've been getting when I was using a simple USB audio adaptor.  I will eventually need to do some kind of assessment of the transmitted audio, make sure that's up to snuff (especially make sure I'm not transmitting noise/hum).

Which brings me to my issues:

(1) When I just have the Teensy plugged into the computer via USB, I get terrible hum.  Nearly unusable.  But, when I also plug in the Raduino to the computer via USB for CAT control, the hum goes away!  So I'm guessing this is some kind of ground loop caused by the USB cables and the computer??? Why would it go away when I plug in the second USB device?  And why aren't my transformers "protecting" me from the hum?  As far as I can tell, the ground between Teensy and the computer is isolated from the input and output audio grounds.  (That said, I will note that for ease of use on my breadboard, the uBITX side of the transformers are all tied together, both the input transformers and the output... maybe that's an issue?)

(2) Whenever I transmit, I also get the transmitted audio in my uBITX headphones.  So, I'm FAIRLY certain I've got my sketch configured to use the left channel for all RX audio, and the right channel for all TX audio.  Likewise, I tell WSJTX to do input on the right channel, and output on the left channel.  This transmitted audio also has pretty significant hum on it.  I'm wondering if it's coupling between the left and right channel output transformers?  (They're about an inch apart.)  It is pretty loud... I wouldn't expect that if it were coupled, but I'm really not smart on this.  


(FYI, I've attached the audio layout of my sketch.  As noted, L channel is always intended to be RX, and R channel is always intended to be TX.  I use the rxAudioMixer and txAudioMixer as switches to select which input to pass through.  I probably need to eliminate some paths that I put in when I was testing the sketch... e.g. there will never be RX audio input coming from the PC, nor should there ever be TX audio output going to the PC (unless I want to do it for monitoring).  The txAudioSwitch and rxAudioSwitch are used to determine if I send unfiltered audio to the PC, or filter it first.)

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