rfg@hound.UUCP (R.GRANTGES) (05/08/85)
[] THere is a simple experiment that you can do yourself if you can borrow several sine wave oscillators of the radio shop type, having an oscilloscope is helpful...maybe essential, but it doesn't have to be fancy. Neither do the oscillators. Hook the outputs of all the oscillators in parallel. Hook up the scope to let you look at the combined signal. Hook up your phono so you can hear the combined signal. In other words hook up everything to the same point. Now set one oscillator for some convenient musical tone at some reasonable frequency. Lets say you set it to 400 hz which is just below "A" on the musical scale (A=440hz).(Use 440 if you want, or any other pitch). Next adjust a second oscillator to exactly twice the frequency of the first. You can tell when they are exactly synchronized (in synch) because the scope pattern will stand still when they are exactly in synch. The pattern will slowly change if they are not exactly synched. If you have a third oscillator, tune this to three times the original frequency or 1200hz and synch its signal with the other two. If you don't have a third oscillator just use two, it is enough. Now listen carefully and watch the scope while you diddle with one of the oscillators. Set it just out of synch. Notice the effect on the redsulting wave shape on the scope and how the sound varies in a cyclical manner. You will note that as the oscillator signal drifts slowly with respect to the fundamental (you can think of it as having its phase slowly varying), the effect is VERY audible and EASILY SEEN on the scope, some of the waveforms having VERY unusual looking shapes. Now - and this is the important part - try nudging the oscillator in and out of synch. When it is in synch, the pattern will move. When it is in synch the pattern will stand still. When the pattern stands still with the tone in different phase positions, the scope patterns will look VERY different each time. You can think of these positions as being like the original sound one of the frequencies phase shifted from what it was (or phase distorted). NOTE: ALL of the positions in which the pattern is synched will SOUND EXACTLY ALIKE. THey will look different on the scope, but they will sound the same. Every time the pattern starts to move, you will instantly be able to hear the change. Implication: Relative phase of harmonics can not be heard even though the wave shapes are mightily affected. Dynamic changes of phase are VERY audible. THis experiment is easy for folks with synthesizers or electronic organs provided they can get at the tuning. This experiment does not prove that all forms of phase distortion are inaudible on all signals. It does seem to demonstrate that any of zillionbs of forms of phase distortion are inaudible on steady tones. It might be, for example, that it did make a difference on transients (like drum beats) or something else. However, it certainly tends to make a skeptic of you. -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg