# Different frequency response despite having similar waveforms

I'm just comparing two pedals that generated square waves by using Audacity. The pedals are Homenoise Effects Serpent Cult silicon fuzz and Metasonix F1 distortion pedal.

First is the waveforms and frequency analysis of Homenoise Effects Serpent Cult silicon fuzz.

Next, we have the Metasonix F1 distortion pedal.

As you can see, both pedals are producing square waves but one of them the frequency spectrum of Homenoise Effects Serpent Cult didn't go to 22000 hz and beyond while the other did. Btw, I also generated 12544 hz square waves with Audacity and check its frequency analysis and it also extend to 22000 hz and beyond.

Can anyone explain why the frequency spectrum of Homenoise Effects Serpent Cult didn't extend to 22000 hz? I thought square waves have harmonics extended to ultrasonic frequency range. Is it because it have more higher pitched frequency extension or what? There are also other fuzz pedals I checked with Audacity in order to see the frequency analysis and they don't extend to 22000 hz but some of them did. Maybe due to sound quality again? Not sure.

• What is the sampling frequency? With 44.1 kHz, because of Nyquist, do not expect something "beyond" 22k ;)
– Tom
Commented Jul 14, 2020 at 4:53
• The project rate (same thing as sampling frequency, right?) is 44100 hz. The Metasonix frequency analysis did extend to 22000 hz, to be exact 22038 hz when I increase the size and axis to linear frequency. Commented Jul 14, 2020 at 5:11
• Yes, I will not be able to go further than half the sampling frequency : 44.1/2=22.05kHz
– Tom
Commented Jul 14, 2020 at 5:17

## 1 Answer

This is a guess, at one should look at the circuit to be sure.

Because of aliasing and the way we sample signals, you usually do not want significant power above 22kHz, because if you have, it will get folded back in the hearing range and sounds quite bad. This apply to synthesizers and actually all electronic devices dealing with audio.

In order to avoid that, manufacturers usually put a hard low pass filter on the output stage. The exact cutoff of this filter can be different, such as its attenuation by decade, it's up to the manufacturer. What you see here, is probably the manifestation of this filter, with different designs for the two manufacturers.

Hope it helps,

• So, the difference is due to how circuits are set up and the higher frequencies of the Serpent Cult fuzz is deliberately removed by the designer? Commented Jul 14, 2020 at 5:16
• That's likely. Anti aliasing filters are a must have for a circuit generating something as crunch as a square wave... Implementation though, are different. Serpent maybe wanted to be sure and thus removed more higher frequencies to have nothing above Nyquist frequency for standard sampling frequency.
– Tom
Commented Jul 14, 2020 at 6:11