Adjust Audio Playback Speed - iOS

import Foundation
import IMGLYEngine

@MainActor
func createAudioAdjustSpeed(engine: Engine) async throws {
  let scene = try engine.scene.createVideo()
  let page = try engine.block.create(.page)
  try engine.block.appendChild(to: scene, child: page)
  try engine.block.setWidth(page, value: 1920)
  try engine.block.setHeight(page, value: 1080)

  let audioBlock = try engine.block.create(.audio)
  try engine.block.setString(
    audioBlock,
    property: "audio/fileURI",
    value: "https://cdn.img.ly/packages/imgly/cesdk-swift/1.75.1/assets/ly.img.audio/audios/far_from_home.m4a",
  )
  // Wait for the audio resource to load so duration and speed APIs work correctly.
  try await engine.block.forceLoadAVResource(audioBlock)

  // Slow Motion Audio (0.5x — half speed, doubles duration).
  let slowAudioBlock = try engine.block.duplicate(audioBlock)
  try engine.block.appendChild(to: page, child: slowAudioBlock)
  try engine.block.setTimeOffset(slowAudioBlock, offset: 0)
  try engine.block.setPlaybackSpeed(slowAudioBlock, speed: 0.5)

  // Normal Speed Audio (1.0x — original playback rate).
  let normalAudioBlock = try engine.block.duplicate(audioBlock)
  try engine.block.appendChild(to: page, child: normalAudioBlock)
  try engine.block.setTimeOffset(normalAudioBlock, offset: 5)
  try engine.block.setPlaybackSpeed(normalAudioBlock, speed: 1.0)

  // Query current speed to verify the change.
  let currentSpeed = try engine.block.getPlaybackSpeed(normalAudioBlock)
  print("Normal speed block set to: \(currentSpeed)x")

  // Maximum Speed Audio (3.0x — triple speed, reduces duration to 1/3).
  let maxSpeedAudioBlock = try engine.block.duplicate(audioBlock)
  try engine.block.appendChild(to: page, child: maxSpeedAudioBlock)
  try engine.block.setTimeOffset(maxSpeedAudioBlock, offset: 10)
  try engine.block.setPlaybackSpeed(maxSpeedAudioBlock, speed: 3.0)

  // Log duration changes to demonstrate the speed-duration relationship.
  let slowDuration = try engine.block.getDuration(slowAudioBlock)
  let normalDuration = try engine.block.getDuration(normalAudioBlock)
  let maxDuration = try engine.block.getDuration(maxSpeedAudioBlock)

  print(String(format: "Slow motion (0.5x) duration: %.2fs", slowDuration))
  print(String(format: "Normal speed (1.0x) duration: %.2fs", normalDuration))
  print(String(format: "Maximum speed (3.0x) duration: %.2fs", maxDuration))

  // Remove the original audio block (we only need the duplicates).
  try engine.block.destroy(audioBlock)

  let sceneContent = try await engine.scene.saveToString()

  _ = sceneContent
}

Control audio playback speed programmatically using CE.SDK’s headless engine, from quarter-speed (0.25x) to triple-speed (3.0x).

8 mins

estimated time

GitHub

Playback speed adjustment changes how fast or slow audio plays. A speed multiplier of 1.0 represents normal speed, values below 1.0 slow down playback, and values above 1.0 speed it up. This technique is commonly used for podcast speed controls, time-compressed narration, slow-motion audio effects, and accessibility features.

This guide covers how to adjust audio playback speed programmatically using the Engine API, understand speed constraints, and manage how speed changes affect block duration.

Understanding Speed Concepts#

CE.SDK supports playback speeds from 0.25x (quarter speed) to 3.0x (triple speed), with 1.0x as the default normal speed. Values below 1.0 slow down playback, values above 1.0 speed it up.

Speed and Duration: Adjusting speed automatically changes the block’s duration following an inverse relationship: perceived_duration = original_duration / speed_multiplier. A 10-second clip at 2.0x speed plays in 5 seconds; at 0.5x speed it takes 20 seconds. This automatic adjustment maintains synchronization when coordinating audio with other elements.

Common use cases: Podcast playback controls (1.5x–2.0x), accessibility features (0.75x for easier comprehension), time-compressed narration, dramatic slow-motion effects (0.25x–0.5x), transcription work, and music tempo adjustments.

Setting Up the Engine#

Audio blocks require a scene with timeline support. Create a video scene and add a page to host the audio blocks.

let scene = try engine.scene.createVideo()
let page = try engine.block.create(.page)
try engine.block.appendChild(to: scene, child: page)
try engine.block.setWidth(page, value: 1920)
try engine.block.setHeight(page, value: 1080)

Setting Up Audio for Speed Adjustment#

Loading Audio Files#

Create an audio block and load an audio file by setting its audio/fileURI property.

let audioBlock = try engine.block.create(.audio)
try engine.block.setString(
  audioBlock,
  property: "audio/fileURI",
  value: "https://cdn.img.ly/packages/imgly/cesdk-swift/1.75.1/assets/ly.img.audio/audios/far_from_home.m4a",
)
// Wait for the audio resource to load so duration and speed APIs work correctly.
try await engine.block.forceLoadAVResource(audioBlock)

Unlike video or image blocks that use fills, audio blocks store the file URI directly on the block itself via the audio/fileURI property. Awaiting forceLoadAVResource ensures CE.SDK has downloaded the audio file and loaded its metadata, which is essential for accurate duration information and playback speed control.

Adjusting Playback Speed#

Setting Normal Speed#

By default, audio plays at normal speed (1.0x). Set it explicitly to ensure consistent baseline behavior, and call getPlaybackSpeed to read the current multiplier.

// Normal Speed Audio (1.0x — original playback rate).
let normalAudioBlock = try engine.block.duplicate(audioBlock)
try engine.block.appendChild(to: page, child: normalAudioBlock)
try engine.block.setTimeOffset(normalAudioBlock, offset: 5)
try engine.block.setPlaybackSpeed(normalAudioBlock, speed: 1.0)

// Query current speed to verify the change.
let currentSpeed = try engine.block.getPlaybackSpeed(normalAudioBlock)
print("Normal speed block set to: \(currentSpeed)x")

Setting speed to 1.0 ensures the audio plays at its original recorded rate. This is useful after experimenting with different speeds and wanting to return to normal, or when initializing audio blocks programmatically to ensure consistent starting states. Reading back the current speed is handy for populating UI controls or validating relative adjustments.

Common Speed Presets#

Slow Motion Audio (0.5x)#

Slowing audio to half speed creates a slow-motion effect that’s useful for careful listening or transcription.

// Slow Motion Audio (0.5x — half speed, doubles duration).
let slowAudioBlock = try engine.block.duplicate(audioBlock)
try engine.block.appendChild(to: page, child: slowAudioBlock)
try engine.block.setTimeOffset(slowAudioBlock, offset: 0)
try engine.block.setPlaybackSpeed(slowAudioBlock, speed: 0.5)

At 0.5x speed, a 10-second audio clip will take 20 seconds to play. This slower pace makes it easier to catch details, transcribe speech accurately, or create dramatic slow-motion audio effects in creative projects.

Maximum Speed (3.0x)#

The maximum supported speed is 3.0x, three times normal playback rate.

// Maximum Speed Audio (3.0x — triple speed, reduces duration to 1/3).
let maxSpeedAudioBlock = try engine.block.duplicate(audioBlock)
try engine.block.appendChild(to: page, child: maxSpeedAudioBlock)
try engine.block.setTimeOffset(maxSpeedAudioBlock, offset: 10)
try engine.block.setPlaybackSpeed(maxSpeedAudioBlock, speed: 3.0)

At maximum speed, audio plays very quickly — a 10-second clip finishes in just 3.33 seconds. This extreme speed is useful for rapidly skimming through content to find specific moments, though comprehension becomes challenging at this rate.

Speed and Block Duration#

Understanding Duration Changes#

When you change playback speed, CE.SDK automatically adjusts the block’s duration to reflect the new playback time.

// Log duration changes to demonstrate the speed-duration relationship.
let slowDuration = try engine.block.getDuration(slowAudioBlock)
let normalDuration = try engine.block.getDuration(normalAudioBlock)
let maxDuration = try engine.block.getDuration(maxSpeedAudioBlock)

print(String(format: "Slow motion (0.5x) duration: %.2fs", slowDuration))
print(String(format: "Normal speed (1.0x) duration: %.2fs", normalDuration))
print(String(format: "Maximum speed (3.0x) duration: %.2fs", maxDuration))

The logged durations illustrate the inverse relationship: the 0.5x block reports roughly double the source duration, the 1.0x block reports the source duration, and the 3.0x block reports about one-third. The audio content is identical across all three blocks — only the playback rate differs, and CE.SDK shrinks or extends the block’s duration accordingly so the timeline stays synchronized.

Exporting Results#

After adjusting audio speeds, serialize the scene to preserve your work. engine.scene.saveToString() captures the entire scene, including every audio block with its speed setting.

let sceneContent = try await engine.scene.saveToString()

The returned .scene string can be loaded later for further editing or used as a template for batch processing workflows.

API Reference#

Method	Description	Parameters	Returns
`engine.block.create(.audio)`	Creates a new audio block	—	`DesignBlockID`
`engine.block.setString(_:property:value:)`	Sets a string property on a block	`id: DesignBlockID, property: String, value: String`	`Void`
`engine.block.forceLoadAVResource(_:)`	Forces loading of audio resource metadata	`id: DesignBlockID`	`async throws`
`engine.block.duplicate(_:attachToParent:)`	Duplicates a block including its children	`id: DesignBlockID, attachToParent: Bool = true`	`DesignBlockID`
`engine.block.setTimeOffset(_:offset:)`	Sets the block’s time offset on the timeline	`id: DesignBlockID, offset: Double`	`Void`
`engine.block.setPlaybackSpeed(_:speed:)`	Sets the playback speed multiplier. Valid range is [0.25, 3.0] for audio blocks. Also adjusts duration	`id: DesignBlockID, speed: Float`	`Void`
`engine.block.getPlaybackSpeed(_:)`	Gets the current playback speed multiplier	`id: DesignBlockID`	`Float`
`engine.block.getDuration(_:)`	Gets the playback duration of a block in seconds	`id: DesignBlockID`	`Double`
`engine.block.destroy(_:)`	Destroys a block	`id: DesignBlockID`	`Void`
`engine.scene.saveToString()`	Serializes the current scene into a string	—	`async throws -> String`