2020
The Bilinear transform is an established design method for producing digital representations of analog filters. But due to frequency warping inherent in the transform, adequate representation in the digital domain is limited for analog filters designed near or above the Nyquist frequency. Previous work has demonstrated a method for surpassing this design limitation in the case of parametric filters. This paper is an extension of that previous work, and proposes a design methodology for digitally representing any second order system with a center frequency near or above the Nyquist frequency. By repositioning the system poles, this method compensates for the effects of frequency warping, and allows for a more exact digital replication of an analog system’s magnitude response.
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2019
This paper presents a digital grey box model of a late 1960s era Shin-ei Uni-Vibe analog effects foot pedal. As an early phase shifter, it achieved wide success in popular music as a unique musical effect, noteworthy for its pulsating and throbbing modulation sounds. The Uni-Vibe is an early series all-pass phaser effect, where each first-order section is a discrete component phase splitter (no operational amplifiers). The dynamic sweeping movement of the effect arises from a single LFO-driven incandescent lamp opto-coupled to the light dependent resistors (LDRs) of each stage. The proposed method combines digital circuit models with measured LDR characteristics for the four phase shift stages of an original Uni-Vibe unit, resulting in an efficient emulation that preserves the character of the Uni-Vibe. In modeling this iconic effect, we also aim to offer some historical and technical insight into the exact nature of its unique sound.
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