Reduce Noise in Pedal-Pi

ES_Team
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6 years 10 months ago - 6 years 9 months ago #888 by ES_Team

Reduce Noise in Pedal-Pi was created by ES_Team

As digital guitar pedal, pedal-pi is subject to several sources of noise, usually a background hiss that could be reduced in different ways. These are our find outs so far:

Software Noise Reduction:

The easiest way because you dont need to "touch" the circuit:

Make sure that the wireless service is not working: it takes a big amount of resources and makes a big impact on the sound to have it activated:
- If your Pi Zero is not a "W" (Wireless) model: Just unplug the usb Wireless Adapter, it makes a huge difference.
If you have a "W" Raspberry Model (integrated Wireless): In headless mode, login in parallel with putty (linux supports several users at the same time) and just type "ifconfig wlan0 down"

Try to remove all the superfluous services from Raspbian, some of them can slow the system and affect the sound. There is plenty of info in Google about speed up Rasbian, overclocking, etc.. so far I did not find much difference in sound disabling services

Hardware Noise Reduction:

Power Supply: The USB 5V power supply could introduce noise into the power supply rails. For me an Apple adapter like this is one of the bests but there is plenty of nice USB power supplies out there. If you have more than one at home, try with different ones and check if you can feel the difference.
Low Pass Filters in the Circuit: The digital noise is usually high frequency noise, in order to deal with that there are 3 low pass filters in the input stage (R2&C2, R4&C5 and C4) and a 3dr order Sallen and Key filter in the output stage.
In my experience, changing the input stage filters do no make much difference. In the other hand, changing the filters in the output stage reduces the noise at the expense of also cutting some highs of the signal. Changing C6 for any bigger value (10nF, 27nF, 33nF) will definitely reduce the noise but muting the sound too.

Last edit: 6 years 9 months ago by Ray.

The following user(s) said Thank You: Mickey

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BowDown
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6 years 8 months ago #941 by BowDown

Replied by BowDown on topic Reduce Noise in Pedal-Pi

Hi! Can we use this filter in pedalshield DUE?
I am getting a ton of noise when connected via USB.
If yes, the two connections that touch the PWM to the PI, are connected instead to the output of DAC0 and DAC1?
Are the values of the resistors the same?

Also, could you tell a bit more about this filter? What is its cutoff frequency?

Thanks!

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Ray
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6 years 8 months ago #945 by Ray

Replied by Ray on topic Reduce Noise in Pedal-Pi

Hi! Can we use this filter in pedalshield DUE?

Which filter are you referring to?

Are the values of the resistors the same?

No, they are not the same:
Pedal-Pi: 2x6bits PWM: Resistor1=4K7, Resistor 2= 4K7x2^6=301K
pedalSHIELD DUE: uses 2 DACs in parallel, so no need for blending resistors.

Also, could you tell a bit more about this filter? What is its cutoff frequency?

About the filter in Pedal-Pi, have a look at here:
www.electrosmash.com/forum/pedal-pi/206-...-pi-circuit-analysis
About the filter in pedalSHIELD DUE is again a 3rd order low pass filter with similar cutting frequency (around 5-6KHz), we use this freq because many other commercial pedals use it too, and it is a good balance between sound and noise. You can further reduce the noise if you make the caps bigger

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