RE: Audacity-nyquist digest, Vol 1 #73 - 2 msgs

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rbd
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RE: Audacity-nyquist digest, Vol 1 #73 - 2 msgs

rbd
David,
        DC bias is the average value of the signal. Some reasons to get rid
of DC bias are that people cannot hear DC and audio systems cannot reproduce
it. In some cases, audio systems will try to reproduce it resulting in
running current through bass speaker coils, generating heat and also pushing
the speaker cone from the "neutral" position, which might limit the
speaker's ability to produce normal audio. When a signal with DC bias is
started, the onset of the DC bias is essentially a "thump" that's audible
and annoying.
        Signals are not ordinarily symmetric about zero, so if you offset
the signal so that positive and negative peaks are equal, you will probably
*introduce* DC bias rather than remove it.
        In practice, what you really want to get rid of (in my opinion) is
very low, inaudible frequencies, not just the "zero Hertz" DC bias. E.g. if
there were a sizeable component at 0.1 Hz, your DC bias could be zero, but
the signal would alternate between positive bias and negative bias every 10
seconds. I'm not sure how this would happen in real recordings, but I've
seen it happen in digital audio effects in Nyquist. The high-pass filter is
by definition the way to get rid of the low frequency terms.

        -Roger
 



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Re: RE: Audacity-nyquist digest, Vol 1 #73 - 2 msgs

edgar-rft
Hello Roger,

the problem is not of audible nature, the problem is that David needs the
Nyquist peak function to normalize the signal at the end because he cannot
see the waveform on the screen (but I think you know).

Now the problem is that the Nyquist peak function can only work right if
the audio signal is symmetrical to zero. With a highpass filter in the
first few samples there remains a peak because of the sudden change of
the dc bias caused by David's rectifier effect and the Nyquist peak
function tries to keep this peak even though the rest of the audio is
of a much lower volume so this function becomes useless in this case.

I send a modified plugin with an offset compensator added to the list.
It is something like (but not a real) two pass plugin. You may have a
look at it if you like (there I have explained in more detail).

Thank you,

- edgar




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Re: dc bias and hp filters

David R. Sky
In reply to this post by rbd
Thanks Roger. To my ears it sounded best when I used a highpass filter at
20Hz, and Edgar confirmed this by what he saw on the Audacity screen.

Thanks

David

On Sun, 4 Dec 2005, Roger Dannenberg wrote:

> In practice, what you really want to get rid of (in my opinion) is
> very low, inaudible frequencies, not just the "zero Hertz" DC bias. E.g. if
> there were a sizeable component at 0.1 Hz, your DC bias could be zero, but
> the signal would alternate between positive bias and negative bias every 10
> seconds. I'm not sure how this would happen in real recordings, but I've
> seen it happen in digital audio effects in Nyquist. The high-pass filter is
> by definition the way to get rid of the low frequency terms.
>
> -Roger



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Re: dc bias and hp filter

David R. Sky
In reply to this post by edgar-rft
Hi Edgar,

You're both right, and for different reasons. As you confirmed on screen
with my ears, it did sound best to use an hp filter at 20Hz.

I'll be posting the rectifier plug hopefully in a few minutes, I hope the
commas work instead of the dots for decimals...

David

On Mon, 5 Dec 2005, [hidden email] wrote:

> Hello Roger,
>
> the problem is not of audible nature, the problem is that David needs the
> Nyquist peak function to normalize the signal at the end because he cannot
> see the waveform on the screen (but I think you know).
>
> Now the problem is that the Nyquist peak function can only work right if
> the audio signal is symmetrical to zero. With a highpass filter in the
> first few samples there remains a peak because of the sudden change of
> the dc bias caused by David's rectifier effect and the Nyquist peak
> function tries to keep this peak even though the rest of the audio is
> of a much lower volume so this function becomes useless in this case.
>
> I send a modified plugin with an offset compensator added to the list.
> It is something like (but not a real) two pass plugin. You may have a
> look at it if you like (there I have explained in more detail).
>
> Thank you,
>
> - edgar



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