In recent years, dynamic mixing has achieved broader adoption in video games. Real-time parameter controls, mix events, and mixer snapshots are frequently used to control audio focus. Now, the use of a powerful tool for achieving clarity in games—'side-chaining'–is also starting to emerge.
Side-chaining consists of monitoring the level of an audio signal and using it to manipulate another audio signal. A concrete example of side-chaining occurs in radio broadcasting where a DJ's voice automatically ducks the music volume. In music production, side-chaining is often used to control the energy in the low end of the frequency spectrum when the bass drum quickly ducks the bass volume.
For games, side-chaining is a great tool for controlling the player's focus and for reducing the cacophony when all hell breaks loose. It also helps to prioritise and clean the mix for objects within the same categories.
Garry Taylor, audio manager for Sony Computer Entertainment Europe, summarised the advantages of side-chaining for games in an interview stating that "Side-chaining is all about control. It enables us to select the most important sounds or sets of sounds and to automatically ride the volumes of other sounds so that the important ones cut through the mix. Unlike event based ducking, side-chaining will take into account any transients within the sounds to be focussed on, and not just reduce the volume of a group of sounds for a set duration".
The key here is that, when transients from the important sounds are played, side-chaining reduces the volume of less important sounds by following the transient shapes.
Figure 1 - Transients from the first signal duck the volume (represented by the red line) of the second signal.
A rule of thumb created for certain games is to set a priority system first among objects of the same categories and then between the categories. For example, following this rule, a game can decide that the playing character’s (PC) weapon sounds are more important than the non-playing character’s (NPC) weapon sounds. Side-chaining can then be setup so that PC weapon sounds duck the volume of the NPC weapon sounds. The weapon sounds between the PC and the NPC are quite similar, but, in this case, the system ensures that the PC sounds are always the main focus for the player.
By extrapolating this idea, a game could decide that nearby explosions should be the main focus over PC and NPC weapon sounds or that critical dialogue should rule over any SFX sounds, including weapons and explosions.
The following graph shows a hierarchical representation of such a system.
Figure 2 - Side-chaining in a hierarchy of busses.
Setting up Side-chaining in Wwise
It’s easy to set up side-chaining in Wwise using real time parameter control (RTPC) curves and the Wwise Meter effect, introduced in Wwise 2010.2. The following example demonstrates how signals going through the PC Weapon audio bus will automatically compress the volume of the NPC Weapon audio bus in a three step operation.
Step 1 - Create a Game Parameter
The first operation consists of creating a game parameter (i.e. PC_Weapon_Volume) and providing a range from -48 to 0, which represents a plausible dynamic range for a game. This game parameter will operate as a communication channel between the busses.
Figure 3 - Game Parameter property editor
Step 2 - Insert Wwise Meter Effect on a Bus
The second step consists of inserting the Wwise Meter effect on the bus whose audibility is to be ensured, in this case, the PC_Weapon, and selecting the output game parameter.
Figure 4 - Wwise Meter effect editor
The Wwise Meter effect monitors the input audio signal. When the effect is used for side-chaining, the Mode, Attack, Release, and Hold parameters are used to slow down the response speed of the output signal and to send smoothed values to a game parameter, in this case, to the PC_Weapon_Volume.
Step 3 - Create a RTPC Curve
The third and final step consists of creating a RTPC curve on the bus to be attenuated (i.e. NPC Weapon). In this example, the bus volume is attached to the PC_Weapon_Volume game parameter and then an attenuation curve is created. The 'x' axis represents the calculated RMS signal from the Wwise Meter effect, and the 'y' axis represents how much the volume of the NPC Weapon audio bus will be attenuated.
Figure 5 - RTPC curve editor
These are the three simple steps for setting up side-chaining in Wwise. To build a hierarchy of busses using side-chaining, simply repeat these operations.
Since the Wwise Meter effect feeds a generic game parameter, any property that can be attached to an RTPC can be driven by side-chaining.
For example, side-chaining can ride the gain of an EQ band to notch a certain frequency band. Other examples include driving the Threshold value of a compressor effect, modifying the LFO frequency of a flanger effect, or amplifying the distortion intensity of FutzBox lo-fi effect.
Wwise side-chaining is remarkably flexible and can be used for various applications. Above all, it's a powerful tool for controlling what a player should focus on in all situations.