Unable to use line-in/mic on Audio Shield

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doodlezucc

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Hi, I'm trying to build a small audio interface using Teensy 4.0 and the accompanying Audio Shield. As a software guy this is pretty new territory so please bear with me.

I've managed to join and solder both parts together and successfully played audio from my PC to the board's output headphone jack. What I actually want to achieve is reversing this connection by feeding an electric guitar signal (or any audio signal for that matter) into the Audio Shield, then using the USB connection to pass it on to my computer.
As noted in this forum thread, it should be possible to simply hook up a guitar to the Audio Shield's line-in. So that's what I tried:

patch_cable_connection.jpg

This might be really naive and please correct me if I'm wrong, but connecting GND to the sleeve and line-in L to the tip of an instrument cable is all I need to do to involve my guitar signal in the electrical circuit, right?
Theoretically I should have some audio signal flowing in the left channel of my I2S input at this point. In order to confirm this, I tried multiple Teensy audio examples (PeakMeterStereo, PassThroughStereo) with audioShield.lineInLevel and audioShield.micGain at different values (partly using line-in L, partly MIC), I tried wiring line-in L directly to 3.3V. Nothing yielded any satisfying result, though. There's not even any noise. The peak meter indicates no input on both channels and when passed to my PC and recorded through Audacity it shows a flatline.

Does anyone know what Teensy pins are used for I2S inputs? Maybe my soldering's just a bit unstable at places.

Cheers
 
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And your code? You may have forgotten something like initializing the SGTL5000

BTW guitar picks are high impedance and inductive, thus requiring high impedance inputs to prevent loss of high frequencies.

The inputs are DC blocked so putting 3.3V on line in does nothing useful.
 
I mostly used Examples > Audio > Analysis > PeakMeterStereo up to this point, which does call enable() on the SGTL5000, along with selecting the line-in input. It's probably not a problem with the code.

Another thing I've tried just now is using a basic guitar amp in between the instrument and my Teensy. Didn't solve it unfortunately, there's still no input from line-in or mic.
 
Two problems. The connections from the jack to the line-in pins seem to be just touching rather than being soldered. They will not be making good electrical contact.
Many of the solder joints on the audio board itself do not look good. There should be a clean, small blob of solder all the way around each hole.
See Paul's soldering guide

Pete
 
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