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Thread: Audio noise, audio shield, effectiveness of isolation

  1. #1

    Audio noise, audio shield, effectiveness of isolation

    Hello all,

    I've been trying to track down the source of artifacts in wavetables I've been importing and am becoming increasingly convinced its the signal to noise ratio. I have read the multitude of posts concerning ground loops and usb noise. I was considering purchasing these objects but in all objectiveness this project is starting to exceed it's cost and I'm hesistating shelling out 45 bucks for an unsatisfactory solution. I have the T4 directly atop the audio shield with clean solder joints, but I am picking up 129 mV of noise regardless of whether I power it via usb or a dedicated supply. It goes down slightly with the PS but not enough to matter. Anyways long story short, I have 23 mV noise coming from my Tinkerboard 3.5 jack... vs 129 on the T4. How effective can I hope isolation to be? Is it feasible to approach PC line out noise levels? The signal distortion is enough that it basically makes any real polyphonic synthesis quickly sound like well, not good enough for professional music applications lets just say that. I'm half tempted to focus my efforts on a JUCE synth for the tinkerboard to provide the same functionality.

    Any advice would be appreciated.

  2. #2
    Senior Member Blackaddr's Avatar
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    I spent a lot of effort designing my Teensy audio shield to be low noise, hi quality. It's a very difficult task to keep mixed signal (digital and analog) designs noise free on the analog side. You need to create a separate filtered power supply for the analog stuff and be very careful in your power and ground design on the PCB. The PJRC Teensy audio board is designed to be inexpensive, accessible and easy to use. For such a low price, you can't expect hi-fi audio quality.

    My audio board specifically targets guitarists who have specials needs for high input impedance as well but you can look at the freely available schematic and study the power circuits to create clean/stable analog power from the noisy USB based digital supplies.
    https://www.tindie.com/products/Blac...-audio-shield/

  3. #3
    I will definitely check that out, thanks for the reply!

  4. #4
    You know your board looks very tempting actually, I was toying with attempting a POG style clone... What does the noise look like on the output on this bad boy? I didn't see it in the specs but I probably just missed it. Considering the price of a good usb isolator it may make more sense just to order one of your boards.

  5. #5
    Quote Originally Posted by RABB17 View Post
    Hello all,

    I have the T4 directly atop the audio shield with clean solder joints, but I am picking up 129 mV of noise
    I think there was an issue with HP of SGTL5000 HP filter, if you disable it you'll get much less noise.


    http://openaudio.blogspot.com/2017/0...elf-noise.html

  6. #6
    very interesting read! I will try these suggestions out, thanks for such clear documentation of your process

  7. #7
    Ok so that just dropped the 129 mv noise to 18.7 mv I owe you a beer!

  8. #8
    In other news, dropping the noise allowed me to properly scope the signal. It looks as if the wavetables are being recreated with fidelity it's just injecting transient spikes into the wave form at 1024th sample. Weird.

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