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Either on linux or mac when I adjust my pcm percentage to over 80% I begin to hear static during audio playback.

I have the asus p7p55-m motherboard which has the VIA VT1708S audio chipset (identified as intel hda in linux).

And plain budget $30 Altec Lansing speakers:

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Is the issue cause by high definition audio being converted to crappy speakers or something else?

I had a similar problem on a laptop running linux.

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2 Answers

It's clipping. In short, amplifiers have a maximum output volume they can deal with; if a signal's peaks are above that limit, they get "clipped" to the limit. This introduces harmonics which you hear as static. This Wikipedia article gives some more details, including an image of an oscilloscope showing a clipped waveform.

The physics behind this is somewhat complex; I'll try to be brief without being too abstruse. Any waveform is, ultimately, made up of a combination of simple sine waves. (If you've ever heard of the Fast Fourier Transform, it's a programming algorithm that resolves random waveforms into their component sine waves.) Waves whose tops are flat — like square waves, or clipped audio — resolve to the original waveform plus bursts of waves whose frequencies are odd-numbered harmonics (multiples) of the original waveform; for example, a clipped sound at 443 Hz (a common A-above-middle-C, in at least some standards) will have bursts of sound at 1329 Hz (443 * 3), 2215 Hz (443 * 5), etc., whose volumes slowly decrease as the frequency increases. We hear these as static, a jumbled mishmash of frequencies the ear can't resolve properly, superimposed on the original sound.

The only fixes for this are to reduce the output volume from the computer (amplifier input) or the speakers' volume (amplifier output), or get a better amplifier. Computer speakers don't, in general, have very high quality amplifiers built into them.

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Can my speakers get damaged if I keep the PCM at 100%? –  Absolute0 Mar 7 '11 at 19:19
    
Potentially, but I don't think most sound cards can overdrive any but the absolutely cheapest speakers that badly. –  geekosaur Mar 7 '11 at 19:23
    
thanks! you seem to know what you're talking about. My physics is lacking, unfortunately. –  Absolute0 Mar 7 '11 at 19:25
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To add to Geekosaur's answer, digital clipping can also happen inside the software before getting out to the sound card. If you have a music player with an EQ bass boost, for example, that could easily translate into clipping that will be carried away even if the rest of the chain (player->PCM->Master->Speakers) is not at 100%. So try to lower your music player's volume when you hear such artifacts, especially if you use EQ, normalize/gain or if you downmix channels (such as playing surround movies in stereo).

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