[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: lopsided tones.

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
            D A Q A R T A
Data AcQuisition And Real-Time Analysis
Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!