If there is a zero-referenced (non-differential) gain stage after the offset is added,
the zero crossings will still have been moved relative to the peaks and some of the
added harmonics will remain even after the offset is removed.
A lot depends on the type of gain stage.
"Flutter" sounds like you're generating subharmonics. Try raising the frequency
of the whole signal (record a few seconds and change the sample rate) to get the
subharmonics up into the range of the FFT processor.
D Smith
----- Original Message -----
From: "Bob Masta"
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: [AUDITORY] lopsided tones.
Date: Wed, 19 Aug 2009 09:08:27 -0400
On 17 Aug 2009 at 15:41, Ranjit Randhawa wrote:
> Dear List,
> I have been experimenting with a 100hz tone, where the positive
> half sinusoid of the period is larger than the negative, the
> phase is however is not changed. Speech seems to have this
> profile of larger positive pulses as compared to the negative,
> hence my interest. Applying fft to such a signal, I get an
> increase in magnitude for the 100hz component, and an increase in
> the dc component. What I hear however is the basic 100hz tone,
> and a flutter on top of it, not what fft seems to indicate. My
> assumption was that the increased dc component would not be
> heard, and I would hear an increase in loudness of the 100 hz.
> However, the base 100hz loudness does not seem to change as I
> increase the area under the positive sinusoid, but the flutter
> does. Any history or explanation would be most welcome.
> Thanks and regards,
> Randy Randhawa
Please go into more detail about your waveform and setup.
If you have simply added DC to the baseline, as Matt
mentions, you would see this in the FFT of the raw internal
signal, but the DC would normally be blocked from reaching
your headphones or speakers so would not be audible. (Not
that it would likely have been audible anyway, since
speakers and headphones won't go down to DC without exotic
treatments, like perfect air-tight seals to your head.)
If you had made the generated wave too positive, you would
get clipping that would be audible as harmonics and easily
visible in the spectrum.
So I'm wondering if you are using some sort of tone burst
or envelope system. If so, that might explain what you are
finding, since the DC would be modulated by the envelope
and would result in a component at the burst rate, which
would probably be described as flutter. You might not see
this flutter component in the FFT since it could be lost in
the lowest FFT bin with the DC. The flutter should vanish
if you get rid of the envelope and just leave the tone on.
Best regards,
Bob Masta
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