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## what does negative waveform actually do?

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### #1tracegame  Members

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Posted 05 December 2012 - 11:09 AM

beginner to sound programming and theory.

just an odd idea hits me.

well,usually pcm file displayed as waveform in music softwares

they're all some kind of sine waves combination

if you just write a sine wave,say 500hz.

play it.you get your sine wave sound.

this is OK.but what if your just generate abs of your sine wave?

well,i did.and....it still sounds like sine wave sound....

negative part does the same thing

i guess other song may be the same(not tested yet)

so what does negative waveform actually do

what's the different between positive and negative?

and if there is no difference,can we just sample sound with one half axis?

i guess not??

Edited by tracegame, 05 December 2012 - 11:14 AM.

### #2Brother Bob  Moderators

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Posted 05 December 2012 - 11:19 AM

this is OK.but what if your just generate abs of your sine wave?

well,i did.and....it still sounds like sine wave sound....

Do you mean that the absolute value of the sine wave sounds like a sine wave, or that it sounds like the original sine wave?

Both are wrong though. Taking the absolute value of a sine wave introduces over-tones since the sine wave is no longer pure. What you get is a collection of multiple sine waves of which one, the fundamental frequency which is twice the original frequency, is the most dominant. If you take the absolute value of a 500 Hz sine wave, you get a fundamental frequency of 1000 Hz, as well as harmonics which are multiples of 1000 Hz.

The negative part of a sine wave is just as much part of the sine wave as the positive part of it. If you alter it, you no longer have a sine wave.

### #3bschmidt1962  Members

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Posted 05 December 2012 - 12:02 PM

If you are listening to a sine wave and an "Absolute value of sine wave" and they sound the same to you, then there's something wrong with your listening environment. They sound very different! For starters, the plain sine wave will sound an octave lower than the ABS(Sine wave). The ABS(Sine wave) will also sound much buzzier than the plain sine wave, which should sound very clean and pure.

What do you mean by "negative part" of sine wave? If by that you mean "take a sine wave, and take the negative of it", then what you end up with is exactly the same sine wave, but out of phase with the original. I.e. where the sine wave peaked at 1.0, the "negative sine wave" will be at its minimum of -1.0. However, if you mean "only take the part of the sine wave that is less than 0.0", then you will get a completely different wave.

I attached a quickie picture.
On the left is a sine wave (440Hz), and it's spectrum. You can see it's a single hump right at 440.
On the right is the Absloute Value of a sine wave and its' spectrum. You can see first that there is no component at 440; the lowest compoent is 880. You can also see that there are a lot more harmonics in this wave than the pure sine wave

Welcome to the world of digital audio!

Brian Schmidt
Brian Schmidt Studios
GameSoundCon

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### #4tracegame  Members

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Posted 05 December 2012 - 09:14 PM

yes,i was wrong,thank you.figure it out now.

it sounds like the same,i guess i just compare the abs of sine wave and the -abs.

anyway,i was totally wrong.

### #5GeneralQuery  Members

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Posted 11 December 2012 - 04:18 PM

A polarity flipped sine wave will sound exactly the same, assuming there are no other variables.

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