CHANNEL VOCODER WITH REVERB

Thomas Carney 311107435

Digital Audio Systems, DESC9115, Semester 1 2012

Graduate Program in Audio and Acoustics Faculty of Architecture, Design and Planning, The University of Sydney

The digital signal processing (DSP) technology I have developed is a channel vocoder with reverberation. This product allows users to create vocoded sound sources and then place those sounds into a reverberant space. The advantage to this product is it allows the user a greater degree of creative control than just a simple vocoder. By placing the vocoded sound into a reverberant space, the user can simulate a room or “space” which best represents the music or project they are creating. Or, they can simply make weird and wonderful spatial effects to give their vocoded sound an extra degree of “strange” or otherworldliness (the vocoder is often associated with science fiction – so these reverb effects can be rather handy).

Vocoding is the cross synthesis of a musical instrument (often a synthesised tone) with voice. A vocoder (short for voice encoder) is a synthesis system, which was initially developed in the 1930s to reproduce human speech for telecommunications. In the 1970s vocoding took on a new form and use when artists such as Kraftwerk and engineers such as Robert Moog started experimenting with the devices in creative ways. The vocoder produced an effect that could make the human voice sound like a machine and became very popular in synthesised and electronic music, the technique eventually spreading and populating many forms of music including disco and prog rock.

The vocoder has two input signals. One of these signals usually comes from a microphone and is often a voice or an instrument (modulator signal) and another signal often comes from a synthesized tone, which is often a square, sawtooth or pulse wave (the carrier signal).

The vocoder takes the two signals, and using their spectral information, creates a third signal. The goal of the vocoder is to imprint the amplitude and frequency characteristics of the modulator signal onto the timbre of the carrier signal, whilst maintaining the pitch of the modulator signal.

It is a two-stage process. Traditionally, Source A (speech/modulator signal) travels through fixed frequency band pass filters. The output of each filter is connected to an envelope detector, which determines the amplitude of the signal. Source B’s signal travels through band pass filters identical to Source A, and then onto voltage controlled amplifiers (VCA). These VCAs are controlled by Source A’s envelope detectors (Source B’s signals will have the same amplitude as their corresponding Source A signal). The signals from Source B’s filters are then combined to form the output signal.

With the traditional analogue vocoder, filters would be used to separate the input signal into multiple band passes. Using computers, this is often performed by an FFT. The FFT (fast Fourier transform) determines the frequency and amplitude information of the modulator signal, and splits that signal into multiple bands that will eventually feed the output signal. This digital system is an almost exact replica of the original analogue vocoder system.

The resulting signal from the vocoder output is then used to generate reverberation. Reverberation is a property of concert halls and other performance spaces that greatly enhances the enjoyment of a musical performance. The on-stage performer generates sound waves that propagate directly to the listener's ear. However, sound waves also bounce off the floor, walls, ceiling, and back wall of the stage, creating myriad copies of the direct sound that are time-delayed and reduced in intensity. This product generates an electronic reverberation for the vocoded signal using filters and delays.

The reverberation gives the vocoded signal added depth and “space” and adds to the creative possibilities of this product. Many instruments or styles of music require a certain reverberation time to sound “correct” (this is often determined by the type of venue the music was originally designed for). This product allows for basic manipulation of this reverberant field.

The reverberator uses parallel comb filters followed by cascaded all-pass filters to produce an impulse response that closely resembles a physical reverberant environment.

You can specify the number of delays and the RT60 (the time it takes for the signal to decay 60dB). The reverb has four parallel comb filters followed by two all-pass filters. The parallel comb filters are used with different delay lengths (to simulate modes of a room, and sound reflecting between parallel walls) and all-pass filter to increase the reflection density (diffusion). The user can create a reverberant environment that replicates the real world, or something completely out of this world.

The advantage of this reverb is its simplicity. Simple to use and understand and achieve results quick and easily. Whilst the technology is simple to understand, it gives the user many parameters of control so they can be truly creative and experimental, with endless possibilities.

The parameters and settings of this technology allow for manipulation of many different areas of signal processing, giving the user ultimate control over their final creative outcomes. The user can determine the way the parameters manipulate the signal: • Chan – determines the length of the FFT (fast Fourier transform) – this will determine how many samples are taken of the input signals, a higher number generally results in a more accurate representation of the signal, but by using smaller FFTs the user can achieve interesting distortions (for the noise/sci fi enthusiasts). • Numband – determines how many bandwidths the signals will be split into after the FFT. This can also generate interesting distortions or sound effects by using fewer bands. • Overlap – determines the overlap between FFT samples, this will again result in some interesting results. A number closer to zero will be clearer whilst a number closer to one will be ‘muddier’. • Delay – determines the distance between samples for each delay. Smaller numbers will result in severe distortions. • RT60 – determines the length of the reverberation. This is essential for the reverb, as it determines how long it takes the reverberant signal to decay 60dB.

This product allows the user to create vocoded signals from virtually any sound source, and place that vocoded signal into a reverberant space, which can be given any desired characteristics to match different types of music and personal taste.

The vocoder has been used for voice synthesis for eighty years. It has found uses in music, other media industries such as film, television and games, telecommunications, defence, encryption and many other fields. The technology has been used in many other synthesiser applications and is quite important to the development of electronic music. This product allows greater flexibility and creative control with your channel vocoder. Try it today and discover the possibilities.