Hello,

This is my first post to this list, and while I am not a circuit builder (as of yet), I am interested in sound design and after looking over this site and forum, I chose to post here because there are so many intelligent people and the field of synthesis and circuitry/electronics go hand in hand. If my questions are too unrelated, please forgive me, and let me know, and I will limit future posts to more on-topic subjects. Thanks!

Now for my question...

I am experimenting with a Pulse wave, starting out with it as a symetrical square and decreasing it's duty cycle ratio (1:2, 1:3, 1:4, etc) making it resemble a smaller pulse.

While monitoring the wave through a spectrograph and an O-scope. I notice as the duty cycle decreases, the Fundamental frequency attenuates very drastically.

According to the texts I have read, decreased duty cycles should increase harmonic content... so why does the fundamental attenuate so rapidly? Does this phenomena have a name?

If anyone can reccomend textbooks that cover this sort of topic, I would be greatfull for title/author reccomendations.

Thank you much!
Andrew Mylko
Eugene, OR, USA

 

Hi,

While monitoring the wave through a spectrograph and an O-scope. I notice as the duty cycle decreases, the Fundamental frequency attenuates very drastically.

According to the texts I have read, decreased duty cycles should increase harmonic content... so why does the fundamental attenuate so rapidly?

'cause you put less energy into each period

Does this phenomena have a name?

The Second Law of Thermodynamics

 

Good common sense, thank you for bringing me back to the ground, Soren!

That's the first time I have heard the laws of thermodynamics cited in sound design, but it also makes good sense

Still, I would like to see more about this...maybe some literature about PWM... including some numbers/formulas/and observations relating to harmonics, amplitude and maybe how this can be used down the road for other purposes.

Can you reccomend some books about PWM and it's affects on sound?

Thanks again,
Andrew

 

I read an article once about design of music synthesiser. One of the topic in that article was exactly this. It analyses different harmonic contents of different synthesised waveforms and how each of them sounds. Like sinewave is pure, square wave is holllow/woody, sawtooth is rich, etc. It also has theory on the harmonic contents of different duty cycles of pulses. I think it states that for x:y mark to space ratio, the y*n harmonic would be missing, i.e. for D=50% (1:2) the re is no even harmonics (2n). It also treats the amplitude of harmonics, like 1/p curve, etc.

It was a pdf on a webpage somewhere, I've tried to find it again but couldn't. Maybe you could try searching for it, if it interests you.