How to Use Playback States to Control Audio Flow

Wed, 12 Jun 2024 00:00:00 +0000

When implementing dynamic audio in Unity using FMOD, you often need to know exactly what an audio event is doing at any given moment. Is it playing? Is it stopping? Has it already stopped?

This is where Playback States come in.

In this tutorial, we will explore what FMOD Playback States are, how to access them in your C# scripts, and how to use them to fix a common audio bug: the “stuttering” sound effect on UI sliders.

What is a Playback State?

A Playback State is a tool from FMOD’s API that allows us to check the status of an Event Instance. It returns the current status of the audio as an Enumerator (Enum).

The five main states returned by the API are:

PLAYING
STARTING
STOPPING
STOPPED
SUSTAINING

By accessing these states, we can write logic that prevents sounds from overlapping or triggering incorrectly.

Part 1: Setting Up the Variable

To use Playback States, you first need to define a variable to store the state information. Open the MusicPlayer script.

Inside your class, declare a variable of type FMOD.Studio.PLAYBACK_STATE:

private FMOD.Studio.PLAYBACK_STATE musicPlayState;

Note: You can omit FMOD.Studio if you are already using that namespace at the top of your script.

Part 2: Retrieving the State

To see what your audio is doing, you need to use the .getPlaybackState() command. This command queries the event instance and outputs the result into the variable we just created.

If you want to track the state in real-time, place this logic in your Update() function:

void Update()
{
 // Assuming '_musicInst' is your FMOD Event Instance
 _musicInst.getPlaybackState(out musicPlayState);

 // Debug to the console to see the state change
 Debug.Log(musicPlayState);
}

How it works:

While Playing: The console will log PLAYING.
Entering a Portal/Stopping: If you trigger a stop command, it switches to STOPPING.
Finished: Once the audio fades out completely, it switches to STOPPED.

Important: getPlaybackState only checks the state on the specific frame it is called. If you only put this in Start(), it will likely only ever say STARTING because it never updates again.

Part 3: A Practical Example (The UI Slider Fix)

One of the most useful applications for Playback States is preventing audio spam.

Imagine you have a volume slider in your Settings menu. You want a sound effect to play when the user moves the slider so they can hear the volume level. However, if you simply tell the sound to Start() every time the slider value changes, the sound will restart every single frame the mouse moves. This creates a messy “machine gun” stuttering effect.

We can use Playback States to ensure the sound only starts if it isn’t already playing.

Step 1: The Setup

In FMOD: Create a 2D Event (e.g., LevelTest) in a “UI” folder.
1. Add some test sounds into a Multi Instrument.
2. Ensure the event is sent to the SFX bus.
In Unity: Open your F_BusControl script.

Step 2: Modify the Script

// ... Your existing using statements ...

public class F_BusControl_class : MonoBehaviour
{
 // .. . Your existing variables ...
 EventInstance levelTestEvent;

 // Start is called once before the first execution of Update after the MonoBehaviour is created
 void Start()
 {
 // ... Your existing bus setup code ...

 // only assign the event if its actually an SFX
 if(busPath == "SFX")
 {
 levelTestEvent = RuntimeManager.CreateInstance("event:/SFX/UI/LevelTest");
 } else
 {
 levelTestEvent.release();
 }
 }

 public void VolumeLevel(float newSliderValue)
 {
 bus.setVolume(newSliderValue);
 levelTestEvent.start();
 }
}

You can hear that the sound stutters when you drag the slider quickly. We need to fix it by telling it to only start if th sound is not already playing.

We can use playback states to fix this issue:

// ... Your existing using statements ...

public class F_BusControl_class : MonoBehaviour
{
 // .. . Your existing variables ...
 EventInstance levelTestEvent;
 PLAYBACK_STATE pbState;

 // Start is called once before the first execution of Update after the MonoBehaviour is created
 void Start()
 {
 // ... Your existing bus setup code ...

 // only assign the event if its actually an SFX
 if(busPath == "SFX")
 {
 levelTestEvent = RuntimeManager.CreateInstance("event:/SFX/UI/LevelTest");
 } else
 {
 levelTestEvent.release();
 }
 }

 public void VolumeLevel(float newSliderValue)
 {
 bus.setVolume(newSliderValue);
 if(busPath == "SFX")
 {
 // Check the playback state before starting the event
 levelTestEvent.getPlaybackState(out pbState);

 if (pbState != PLAYBACK_STATE.PLAYING)
 {
 levelTestEvent.start();
 }
 }
 }
}

Now, when you drag the slider, the sound will play once. It will only play again after the previous instance has finished, resulting in a smooth, non-stuttering audio experience.

Summary

By using Playback States, you gain precise control over when your audio triggers. You are no longer just telling FMOD to “Play”; you are asking FMOD “Are you busy?” and acting accordingly.

This simple check is essential for creating polished UI sounds, managing dynamic music transitions, and ensuring your game’s audio sounds professional and clean.

Playback States on DAD 424 Sound Design for Games