[sdiy] harmonic generator & megaloopers

[KHeck73 at aol.com]

In a message dated 2/28/2002 9:34:42 PM Eastern Standard Time, 
jhaible at debitel.net writes:




> the harmonic contents of the window function will directly reflect
> the amount of artefacts in the gated sine signal.
> So a triangle (or trapezoid) will produce less "click" than a rectangle, and
> I guess
> a raised cosine would be even smoother.
> 
> JH.
> 

An example of this in application concerns continuous wave radio transmission 
(CW, morse code). The shape of the keyed pulse is important because sharp 
edges on either the leading or tail edge of a sine carrier wave pulse will 
create spurious emissions and harmonics off the carrier frequency (ie, 
interference on other frequencies). In radio, the oscillator pulse envelope 
edges should be shaped away from having sharp corners to prevent the 
generation of key clicks and harmonics. If the oscillator output is well 
rounded, but the amplification of the pulse is not linear, then the envelope 
shape might still be altered to sharper attack and decay. Driving audio 
modulation of a carrier to clipping is similar, and creates what is called 
'splatter' in radio lingo. I think the same sine function math is involved 
(fuzz?).

A similar form of distortion results if an amplifier is pulsed into a 
momentary parasitic oscillation by a sharp envelope. Then, clicks form in 
groups on both sides of the sine carrier at multiples related to the 
parasitic oscillation. There's a "gibbs function" (?) that might describe the 
ringing at the edge of a sharp transition, but I can't remember the details.

If you have a shortwave receiver (switchable between AM mode and SSB) you can 
listen to morse code and hear differences in how different transmitted signal 
pulses are shaped. Ranging from woody thumps (mellow), to sharp clicks and 
pops (harsh). 

-Karl.
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