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Audio Frequency Triggered Digital Envelope Generator

IP.com Disclosure Number: IPCOM000050574D
Original Publication Date: 1982-Nov-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Gruber, PA: AUTHOR

Abstract

This circuit creates an Amplitude Envelope of Audio Frequencies and allows total control of the envelope by digitally dividing it into any number of discrete steps. All parameters of the envelope may be modified and the circuit is self-triggering by the audio frequency signal.

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Audio Frequency Triggered Digital Envelope Generator

This circuit creates an Amplitude Envelope of Audio Frequencies and allows total control of the envelope by digitally dividing it into any number of discrete steps. All parameters of the envelope may be modified and the circuit is self- triggering by the audio frequency signal.

The circuit function is illustrated by an application to music synthesis. All musical instruments derive their distinctive sound in two ways: 1. frequency spectrum (primary and harmonics)

of a certain note and

2. the Amplitude Envelope that modulates the frequencies. This circuit provides digital control over the Amplitude Envelope. A musical Amplitude Envelope contains three parts: (1) the attack, (2) sustain, and (3) decay. An illustration is provided in Fig. 1. Each of these parts varies in time and intensity for every type of musical instrument. This circuit, as shown in Fig. 2, can synthesize any naturally occurring Amplitude Envelope as well as an infinite variety of others. If white or pink noise is used in conjunction with audio frequencies, speech synthesis is a possible application.

Referring to Fig. 2 showing an overview of the circuit, the basic operation is as follows: A digital circuit is used to modify the audio frequency by using a virtual-ground or current to voltage transducer feedback amplifier with parallel variable resistors in the feedback path. While any number of parallel feedback resistors may be used, 25 are shown in Fig. 3 with a detailed view in Fig. 4.

These gain-determining resistors are used sequentially by digitally-enabled analog switches in series with the feedback resistors. Gain for the Envelope Generator Amplifier Block is simply R(F)/R(in). Since She 16 analog switches are enabled by a 4-to-16 bit demultiplexer which is fed by a 4-bit binary counter, they can be sequenced in an orderly fashion to produce a 16-step variable gain envelope, each step equaling one cycle of the clock frequency. The Amplitude Envelope can be divided into as many or few steps as desired, depending on the counter and demultiplexer. The clock rate determines the time for each step (1 cycle) as well as the time for ...