Splitting a TRS input to use with two piezos

psxjpma

Hi I am trying to use a stereo splitter to get two mono signal inputs from a 6.35mm TRS input into Bela.
Although this fritzing diagram does not show the splitter, the wiring is the same. I'm connecting one leg of each 1M resistor to each signal leg of the TRS and then connecting the other legs from the resistors to 3.3V. The piezos are then connected via 6.35mm mono jacks:

However, whenever I want to trigger a signal with a single piezo (adc₁), it also triggers the other signal from adc₂. I don't think my Pd patch is what's causing the problem:

I think it's the wiring. I've seen other posts where multiple piezos are used, but these are connected each to an analog input rather than using the audio inputs, so I'm not really sure what went wrong here.

giuliomoro

When connecting to the audio inputs you don't need any pull up resistors. Wire the black wires of the piezos together into the ground pin, then one piezo's red wire to the left input and the other piezo's red wire to the right input. If you do get cross talk in this configuration, it may be that the signal from the piezos is too hot and is saturating the codec's preamp, causing some weird crosstalk in its analog stage. If that's the case, you may need to reduce the input gain. If that's not enough, you may need to add a resistor divider, e.g.:

red----|
       |
       R1
       |
       +--audio in
       |
      R2
       |
black  _ --gnd

Start with R2 == R1 == 47k and then adjust R1, making it smaller to increase gain or larger to further reduce it.

giuliomoro

Of course you need the piezos to be mechanically isolated so that the vibrations affecting one don't affect also the other one.

Regarding the patch, I don't see [bonk~] in the [adc~]` signal path, but that may be inside pd bass-string ?

BTW, what GUI are you using?

psxjpma

Thanks! I'll give that a go.
The bass-string subpatch is running Bela's native karplus strong patch, hence why there's no bonk there.
I'm using plug data 🙂