VFX 1 - Introduction

VFX Article, Chapter 1

1. Introduction

It is not uncommon today to hear and see special effects on radio and TV. Often a radio announcer will emphasize a point by adding special effects with a harmonizer which shifts the pitch of the voice up or down. We will show you how to build our VFX processor which you can use to generate the same effects for fun while you're learning about DSP techniques. You will also learn about and listen to other effects such as echo and reverb. The cost of this project is much less than the cost of any one of these effects if purchased individually. The basic algorithms to perform these DSP techniques are clearly explained and the heart of the VFX hardware, the Analog Devices-2105 Digital Signal Processing Microcomputer is examined. Before we describe the details of the hardware and software, lets look at what the VFX processor does. The VFX processor accepts audio signals, processes the information digitally in one of three ways set by you and sends the output through a small audio amplifier to a speaker or a pair of headphones. All you need to get started in addition to the VFX processor is a microphone, a pair of headphones and a 9 volt DC power source, all of which are available from the source listed at the end of this article. Plug in the mic, the headphones and the power supply and start talking. The first VFX effect is the Harmonizer-it changes the pitch of your voice so you can sound like you're breathing helium or a baritone singer. The LED readout displays the current pitch change, 0 is maximum down shift (-203 Hz) and 10 is maximum upshift (+305 Hz). An LED readout of 4 indicates no shift at all but the processor still goes through the same processing except that the pitch shift is left out. The VFX board starts out at 4 after power up and reset. (This mode can be useful for measurements of the bandwidth and delay of the VFX Harmonizer). By pressing the IRQ2 (interrupt 2) button you can step through the range of pitch shifts, after 9 the processor returns to 0 (max downshift). Each pitch shift increment is approximately 51 Hz (the reason for this will be explained later). To change the VFX to the Echo effect, set the dip switch as follows, SW1-1:ON, SW1-2:OFF, SW1-3:ON, SW1-4:ON and press the reset button for 2 seconds. You can decrease the Echo delay time by pressing the Interrupt button. The LED will display from 9 to 0 for a delay time of .63 to 0 seconds. Each button press decreases the delay by 70 milliseconds (msec). The next effect is Reverb and it is activated by setting the dip switch as follows, SW1-1:ON, SW1-2:ON, SW1-3:OFF, SW1-4:ON and pressing the reset button. The Reverb effect is similar to the ECHO effect; the difference being that the delay time is fixed at 78 msec and the amplitude of the feedback signal is adjustable from .5 to 0 with the interrupt button. This effect is more subtle than the ECHO effect and simulates the acoustics of a large room.