Teensy 4.0: Audible noise with external delay.

DragonSF

Well-known member
As soon, as I activate the external delay, I can hear some 375Hz noise at the output of the Audio Shield.
The noise doesn't come from the delay itself it seems to be inducted directly to the output (via DAC?).
noise1.png
Is this a known issue or are there ways to suppress this annoying noise? The internal delay works well, but I want to use stereo (ping pong) delay.
the setup is:
delay.png
Any help is appreciated.
 
Usually strange noises are due to ground loops.

The first step in troubleshooting a ground loop is trying an alternate connection where GND connects to nothing but the USB cable, which usually means listening with headphones. If you use the audio shield, that's easy... just plug headphones in and listen. If you're using different hardware, what exactly you would do depends on the specifics of your hardware. Sometimes just running from a battery rather than external power is enough for testing.

But it's also possible the problem is happening in the software. 375 is pretty close to the 344 Hz rate the audio library updates.
 
Usually strange noises are due to ground loops.

The first step in troubleshooting a ground loop is trying an alternate connection where GND connects to nothing but the USB cable, which usually means listening with headphones. If you use the audio shield, that's easy... just plug headphones in and listen. If you're using different hardware, what exactly you would do depends on the specifics of your hardware. Sometimes just running from a battery rather than external power is enough for testing.

But it's also possible the problem is happening in the software. 375 is pretty close to the 344 Hz rate the audio library updates.

But how can this be related to the external delay? When I don't use the delay (or the internal one), this doesn't happen. I checked the phone suggestion, it's even more prominent there.
Maybe a power supply problem? It seems so, I tried a split USB cable (one for power, one for data) and the problem is gone.
 
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is there somewhere a USB mains power adapter involved? Seems like many of them tend to have this 375Hz tone
 
Just counting the period from those graphs confirms this is a 344Hz, ie once-per-audio-block pulsed noise, not 375Hz, not ground loop issues, this is a software issue, perhaps not enough AudioMemory?
 
This is probably DROPOUT UNDERRUN. Try to increase the BLOCK size in the AudioStream.h file. It looks like this <#define AUDIO_BLOCK_SAMPLES 128 > Add 32 to it until the dropout goes away but this increases latency. There is also a relationship between system clock and DROPOUT. The faster the system clock, the more frequent the buffer is refreshed at the expense of power and chip temperature.
 
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