Real-Time Digital Signal Processing from MATLAB to C with the TMS320C6x DSPs, Third Edition, 3rd Edition by Michael G. Morrow & Cameron H.G. Wright & Thad B. Welch

Author:Michael G. Morrow & Cameron H.G. Wright & Thad B. Welch [Michael G. Morrow]
Language: eng
Format: epub
Tags: Engineering
Publisher: CRC Press
Published: 2016-12-18T22:00:00+00:00

Figure 10.11: A block diagram of the flanging effect using a single comb filter.

β[ n ]=R2[ 1−cos⁡(2πf0Fsn) ].

In the equation above, R is the maximum number of sample delays, f0 is a relatively low frequency (often less than 1 Hz), and Fs is the sample frequency. This equation results in a slowly changing sinusoidally varying delay that ranges from 0 to R samples. Note that, as an alternative to sinusoidally varying the delay, musicians sometimes choose to use a triangle, sawtooth, or even an exponential function for β[n] in order to obtain a somewhat different sound.

A block diagram of the chorus effect is shown in Figure 10.12. To generate a chorus effect that makes one musician sound similar to four musicians playing the same notes, three separate chorus signals (identical to flanging except having longer delay times) are summed with the original signal. For the best sound, each of the β’s and α’s should be independent.

The phasing effect can be achieved in various ways. One method uses the output of an allpass filter with a slowly varying delay that is added back to the original signal; due to the phase shift, some frequencies will tend to cancel (creating notches). This effect is very similar to the comb filter used in the flanger. Another method, which is easier to fine-tune to get exactly the sound you desire, uses the output of a notch filter with a slowly varying notch frequency that is added back to the original signal. Because it is easy to independently control the notch frequency and the notch width, this second method has many advocates.

Two block diagrams that can implement the phasing effect are shown in Figure 10.13. To generate a rich phasing sound, more than one separately phased signal can be summed with the original signal. For the best sound, it is recommended that the notch frequencies not be evenly spaced or harmonically related; experiment with all the parameters to obtain the sound you want.

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