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OT: Paging any audio engineers, why does my waveform look like this?
Hi guys, I know this is a little off topic but it's sound/music related and I know there is a wealth of knowledge in this forum so I figured I'd ask it here. A little background, I recently bought an acoustic electric guitar from my local music shop (it's a Seagull S6 Classic M-450T in case that matters), and I love how it feels and sounds. I then decided to hook it up to my audio interface to record it into Logic and that's when I noticed something strange about the waveform.
The top track is my acoustic guitar plugged in direct to the audio interface. The 2nd track is my acoustic guitar recorded through a microphone and the 3rd is a vocal through a microphone. I noticed that my acoustic guitar doesn't record a "symmetrical" waveform when recorded direct and I'm not sure what exactly that means. I also tried two other guitars and neither of those show this issue. Can someone provide any insight on what is happening? Is there something wrong with the pickups in my acoustic? Is there anything actually wrong at all? I'm debating whether or not it's worth it to take it back to my local shop where I bought it and let them know, but before I do I want to understand what exactly is causing this. Note, I already tried putting in a brand new battery and that did not change the outcome. Any insight would be appreciated, thanks guys!
Comments
D.C. Offset i suspect
Oh, and I'm not an audio engineer. But I DID stay at a Holiday Inn Express last night
Not D.C. offset. That would show up as the whole waveform shifted up or down from center. I’ve seen something like this before but it sounded fine so no problem. Some apps export a waveform like that. Never bothered to find out why since it always sounded right.
Ahh...that's right. It's been a while since seeing that.
https://www.soundonsound.com/sound-advice/q-why-do-waveforms-sometimes-look-lop-sided
Ya just looks like an asymmetrical wave. Not unusual for some things to look like that, like pulse waves, but it is odd to see an acoustic guitar look like that. Tried to look at really high or low frequencies that you may not be hearing? What are you monitoring on?
Looks like the signal is unable to reach as large a negative value as it is a positive value compared to the zero point, or ground. As to what caused it, I have no idea, but perhaps if you turn down the output of the guitar it might reduce the positive side of the signal more than it affects the negative side and thereby even up the two. You can boost the signal afterwards in the recording app.
This thread has some interesting comments on the subject.
https://groupdiy.com/index.php?topic=63967.0
So according to that SOS article, it’s “a naturally occurring asymmetrical waveform, built from the linear sum of a cosine fundamental and its first four harmonics”.
Simple!
Have you tried with a different cable? I don’t know if that could cause something like that, but it’s an easy thing to try.
My guess is it’s the pickup, but that if it sounds OK it’s nothing to worry about.
I think it looks weird. I think it could be a bad component in the preamp, in the guitar. A bad opamp or transistor, or misbiased gain stage. One component in a pair that does a push-pull isn't pulling as much as its twin is pushing.
Does it sound screwed up, like overdriven? Does it sound kind of bad? A lot of problems with analog audio electronics manifest themselves in ways that don't stop the thing from making sound, it just makes it sound crummy.
One test a tech could do on the preamp is disconnect the pickup and inject a test signal, and check the symmetry of the test signal at the preamp output.
What kind of pickup does your acoustic guitar have?
Does it require phantom power from the audio interface, or have you left it switched on accidentally?
One other thing to test is the 1/4" ins on your interface. Those have a different little preamp section than the mic/xlr inputs. If you have multiple inputs, check if the asymmetrical thing happens on more than one input. Could also play a synth or other simple test input, like a trangle wave or sine wave (not square wave) at varying volumes into the 1/4" input and check that wave is symmetrical.
That's exactly what a TB-303 signal looks like. Not necessarily something to worry about... those signals can exist in the wild without stuff being broken
As a bass player that has done a decent amount of remote session work— I’ve also seen plenty of waveforms like this which pose no practical problems.
This pretty much says it all.
The 303 and other famous, lopsided synth waves (like the Roland GR-300!) are electronically generated, though, not acoustic. To make such a lopsided wave form naturally, wouldn't the guitar string have to be physically swinging away from its resting place, much wider in one direction, than the other, for the pickup and electronics to be reproducing it accurately?
Good point, but in the 303 it's also not the waveform itself that's lopsided, but the effect of a bunch of wonky-coupled capacitors (which behave like a high-pass filter of sorts). I don't know what's happening in this case, in the signal chain between the string and the recording so I could be way off
Luckily, most music listeners enjoy our music using loudspeakers and headphones instead of oscilloscopes
I am an oscilloscope unfortunately.
Well that certainly does extend the market for waveform-reshaped music compositions
When I record trombone the wave often looks like that. If it sounds good then it sounds good.
In conclusion, these type of recordings still sound good, but rob you of some headroom. That was my take when I got to the bottom of that SOS article that @dougdi posted.