Producing Interactive Audio

Case Study: The GEX Project
 
Producing Interactive Audio
Introduction

The Rules of Interactive Sound Design

Creating an Adaptive Audio Engine

Case Study: The GEX Project

Only the Beginning

Tools of the Trade

Getting More Info about the IASIG

Most of my work this year has centered around the interactive sound design for our upcoming PC and PlayStation title, GEX: ENTER THE GECKO. (The PlayStation version will be the first title to use our new sound driver.) This game is the second installment of the GEX franchise and has undergone a complete technological overhaul by some of the best programmers and designers in the business. The game will include at least eight worlds thematically based upon TV and movie parodies, as well as secret, bonus, and boss levels - fertile ground for a sound designer indeed.

I'll talk briefly about what we're doing currently in the game's audio. For clarity's sake, I've focused on the treatment of a variable called numberofenemies and a small number of related variables. These few examples are by no means all that we plan to do in the game, but they illustrate my main points.

One aspect of the new GEX title is the game's fast and efficient engine. It allows us to have more enemies on screen moving at a faster rate. Since this is one of the features that really makes the product stand out, we on the audio team designed the interactive audio to work tightly with the new engine functionality. We set up a 7-bit variable called numberofenemies that reflects the number of enemies on the screen and is updated by the game continually. This variable is read and used by the sound driver to adjust the game audio.

Here's a breakdown of the GEX audio, by game level:

THE PRIMAL ZONE. The control track of the MIDI sequence has logic built into it. When the numberofenemies register is 0 and kills is 1, the sequence is paused, and playback of all tracks is branched to a short stinger of the GEX theme song orchestrated with the instrument palette of the Primal Zone level. When the stinger is over, the main sequence is resumed.

We keep a location register that allows the sound driver to see if Gex has entered a new area of the map that has different music. The control track checks every four beats to see if the value of location has changed. If it has, a short transition drum fill plays, and then the MIDI sequence that is matched with value of location is started on the beat.

SCREAM TV. Scream TV is the "horror" level of ENTER THE GECKO. For this level, I used eerie chamber music laid over some ambient loops of slow ragged breathing and strange heartbeats. The same numberofenemies register is used in this level, but instead of muting and unmuting, the value of the register affects the harmonic center of the piece. For this type of transformation, we use a technique called "pitch mapping." A pitch map in our system is a mechanism to remap the pitches of individual notes as they come from the MIDI sequence on their way to the physical voice. Our pitch maps are built so that the transformation can be kept within the prevailing harmony of the piece. In other words, for a scary piece such as this, the pitch table will confine all of the note remappings to a diminished scale. The numberofenemies value is used continuously to set the number of scale degrees upwards to transpose the harmonic instruments. This creates a very smooth, subtle, intensifying effect as more and more enemies are on the screen and danger is increasing.

CIRCUIT CENTRAL. The Circuit Central level looks like the inside of a CPU. The opening music is weird, ambient analog electronica with a slow, trip-hop beat. After some exploring, it becomes clear to the player that there are jumps and expanses that cannot be passed in Gex's normal state. Within this time, the player should also discover a number of chargers set out in various locations. When Gex steps into a charger, he starts to glow in a green light with orbiting electrons. This state lasts for 15 seconds and gives Gex the ability to use embedded "chips" in the floor to do super jumps and turn on "data bridges" that span the formerly uncrossable chasms.

The state of being charged up is stored as a 1 in a chargedup register, and location keeps track of the seven different areas within the main level. When the control track in the main sequence sees a 1 in chargedup, it checks location and matches the area to one of seven different 15-second-long high-energy pieces of audio.

While most often the game affects the sound design, this relationship can also work the other way. In Circuit Central, we set a register at every beat of the music. Since each measure has four beats, I set the beats variable with a 1 on the down beat followed by 2, 3, and 4 on the remaining three beats and then cycle back to 1. This timing information will synchronize the lighting effects in this world.
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