[sdiy] Additive Synthesis - phase shifts important??

[Tom Wiltshire]

On 14 Jul 2008, at 07:30, Florian E. Teply wrote:

> Hiho SDIYers,
>
> currently i'm fiddling with the thought of some additive synthesis  
> based on DDS.
> Now i'm wondering if a phase shift of some harmonics would alter  
> the sound.

I remember we had quite a debate about whether the human ear can hear  
phase or not fairly recently. It looks to be a question that needs  
some further research, although the summary of the earlier  
discussions would be that the usual statement that "The ear cannot  
detect constant phase" is a simplification.

I can think of two situations in which changing the phase of a  
harmonic in a waveform has a notable effect:

Altering the phase of a harmonic can have a significant effect on the  
peak amplitude of a waveform. This can be significant if you are  
trying to normalise amplitudes between various waveforms in a synth -  
it can help to move the phases about a bit.

Another example of where phase is important is FM. Lower sidebands  
often have negative frequencies, refecting around 0Hz and becoming  
inverted in the process. These reflected sidebands can then cancel  
other harmonics. Changing the modulator phase changes the way these  
interact and consequently the harmonic structure.


T.