[78-L] waveforms

[DAVID BURNHAM]

Sorry folks, but I really must challenge what David Burnham has written.

Joe: When you say waveform, what you actually mean is you are looking at a
few cycles of the dominant frequency. If you scan through the waveform
you'll find it changes shape within milliseconds. Imagine if you had some
mains hum on the audio (say 50Hz 'cos the maths is easier). You'd need to be
looking at at least 20 milliseconds (the wavelength of a single 50Hz cycle)
to be able to see the effect of the hum, which would be a rise and fall in
the centre point of the dominant frequency waveform. The human voice is very
complex and contains a very
 wide range of frequencies including breathing
and chest resonance, so looking at the dominant frequency isn't the full
picture.

David: Energy doesn't have positive and negative. If you push or pull
something you are always using energy. Perhaps force is a better
description? But to continue with your terminology - the positive and
negative half cycles don't necessarily contain equal "energy" on a per cycle
basis. The only thing that we can say is that the sum of the "energy" in the
two parts of the cycle approaches zero as they are summed over a long
period. As a mathematician might say it TENDS to zero as time TENDS to
infinity.

.. wes 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hi Wes

I assumed Joe was looking at a lissajous waveform on a scope.  I don't know whether the correct term is energy or force, (Michael Biel, for whom I have huge respect, leaves me licking my wounds if I use the term "phase" when I should have said "polarity"), but electronic audio waveforms can, for any number of reasons, have a DC offset, but this always indicates a problem in audio, (not in video), but I don't think this is what Joe is referring to.  On a scope with horizontal sweep, the baseline would move up or down from the centre if there was a DC element;  a lissajous pattern will also move up or down along the mono axis.  But after seeing exactly what Joe is talking about on scopes for so many years I believe this is just the natural appearance of complex voice and music audio waveforms.  When I say that the positive half of the waves contain more of the high frequency components, I'm referring to the fact that you'll see sharper higher peaks in
 the positive half of the wave compared to the more rounded features of the negative half.  In nature, most if not all sounds begin with a compression of air.  In speech explosive consonants such as t p b k d also start with a compression.  Some sounds like brass, (expecially muted trumpet or trombone), have a very sharp positive peak many dB higher than the waveform overall.  I don't want to start repeating myself but I think it's recogniized that positive compressions of air travel much more efficiently than negative rarefactions.  Going back to the birthday candles, if you aim a short compression of air at a candle, the flame will flicker at a distance of 15 feet or more, (after a delay);  aim a rarefaction of air, (a short inhale), and you'll have trouble disturbing a flame a foot away.

db