North Suburban HAMMOND ORGAN Society

principle of digital pitch shifting 1

Notice that when the pickup rotates in the opposite direction that the output frequency increases; furthermore, when the pickup rotates at the same rate as the buffer, the frequency of the output signal is twice that of the input. From looking at this we may infer that the rotational speed difference must be very small to create the much more subtle pitch changes that we would generally use as for example in making a celeste or a pitch changing vibrato effect this way.

If we rotate the point where we pick off the signal from the circular buffer in the direction opposite to that in which the buffer runs, the frequency of the output will increase. In the example above, the rotation of both the circular buffer and the pickup are at the same rate, but in opposite directions. This causes a doubling of the frequency of the output. It's important to emphasize that nothing physical rotates here; the diagram simply illustrates how picking up signals sequentially from different parts of the buffer results in a frequency change of the output. Thus, a digital signal processor can change the frequency of a signal in real time. Although I have used an analog sinewave to represent the signals at various stages of the device, we are in fact dealing with a signal consisting of digital information. However, the conversion of the digital information in the output of the above arrangement does yield an analog signal whose frequency has, in this example, been doubled.


 Previous Page   Page 6.    Next page