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I don't know that much about electronics and stuff but I converted some videos from Youtube to MP3 where the videos involved some guys putting their fuzz pedals, which are Zvex Fuzz Factory and a DIY Calculator Fuzz, on the oscilloscope in order to see the waveforms. The waveforms are square waves as you can see in the pictures below.

zvex fuzz factory through a scope zvex fuzz factory

calculator fuzz through a scope calculator fuzz

But when I used the Audacity to see the waveforms after I converted the videos to MP3, the waveforms are different and the waveforms are not square waves. Notice the time I captured the waveforms, both are taken at the same time. For the Zvex Fuzz Factory, it was at 0:45 and for the Calculator Fuzz it was at 0:38, same goes for the waveforms in the Audacity.

zvex fuzz factory waveforms on Audacity zvex fuzz factory waveforms on Audacity

calculator fuzz waveforms on Audacity calculator fuzz waveforms on Audacity

Can someone explain why the differences? Is it possible that different brands of oscilloscope generate different waveforms from each other?

Community
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SnoopyKid
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  • I think the important thing is for you to view the waveform as produced by either the final amp or the speakers themselves. in the latter case you have to deal with your microphone's slew rate capability too, so be careful. – Carl Witthoft Jun 29 '20 at 13:22

1 Answers1

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A guess:

What is plotted on the oscilloscope on these video is the sound wave actually coming out of the pedal. On the other hand it seems that the audio is captured using the camera (a phone maybe) as you can hear what is going on in the room (clicks).

That said, it is very likely that this poor recording device, added to youtube conversion and the fact that the pitch is already quite high, have removed a good part of the higher frequencies which are forming the square wave.

Also note that all frequencies above half the sampling frequency of the recorder cannot be captured by it. This is why the square looks a bit more like a sin: the higher frequencies have not been recorded.

What you are seeing on Audacity is the wave, modified by the recording process, conversions etc… In order words, the oscilloscope and Audacity are not plotting the same thing.

Tom
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    There's the additional factor that [wherever they grabbed the audio to the oscilloscope, presumably off the direct out] no speaker can repro a square wave, there will be slew, lots of slew. So no-one will ever hear what the scope is reading. I'm actually surprised they managed to get an analog pedal that square in the first place. – Tetsujin Jun 29 '20 at 09:49
  • @Tetsujin I agree, but I guess the bandwidth is even a stronger effect… It would have been nice to have the oscilloscope settings: time_div and to increase the vertical gain to have a clear view of the slew rate… – Tom Jun 29 '20 at 09:52
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    Good answer. The OP is comparing apples to oranges, and not aware. I am not familiar with Audacity but could there also be triggering and sampling limits that can be user adjusted. I would think that with an AD converter one could sample the direct out from the pedal and replicate the scope reading. –  Jun 29 '20 at 13:54
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    **MP3 is a lossy compression scheme! This may cause waveforms to appreciably be different!** (Don't forget that Youtube is a black box and does whatever Google wants, including lossy compression.) – Xunie Jun 29 '20 at 16:23
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    @Xunie at least it's possible to see which codecs are used by YouTube by right-clicking the video and selecting "Stats for nerds". Though AFAIK, [YouTube always uses lossy compression](https://appuals.com/why-converting-youtube-to-320kbps-mp3-is-a-waste-of-time/), so it's not really a mystery... – Andrew T. Jun 29 '20 at 17:58
  • The whole point here is: what you see (on the scope) is _not_ what you get (in the end of the complete sound system) ;). – Tom Jun 29 '20 at 18:37
  • In addition to the points made by the answer and by Xunie's comment, there are elements in the signal chain that will create phase shifts of the various harmonics relative to one another. For example, if it's been through a speaker, the speaker has a cross-over filter that will affect phases (as well as sending highs and lows to different drivers). If you change the phases of the harmonics, it will make a square wave not look at all like a square wave anymore, but it will sound the same, because the ear is essentially deaf to phase. –  Jun 29 '20 at 22:53