# AUInputSamplesInOutputCallback Called by the system when an audio unit has provided a buffer of output samples. ``` typealias AUInputSamplesInOutputCallback = (UnsafeMutableRawPointer, UnsafePointer, Float64, Float64) -> Void ``` ## Parameters `inRefCon` Custom data that you provided when registering your callback with the audio unit. `inOutputTimeStamp` The time stamp that corresponds to the first sample of audio data produced in AudioUnitRender (its output data). `inInputSample` The sample number of the input that is represented in the first sample of that output time stamp. `inNumberInputSamples` The number of input samples that are represented in an output buffer. ## Return Value A result code. ## Discussion If you named your callback function `MyAUInputSamplesInOutputCallback`, you would declare it like this: ### Discussion When your application uses a *varispeed* or pitch-shifting audio unit, it may not be clear which input samples are represented in a buffer of output samples. This callback function addresses this issue by providing the input sample number corresponding to the first sample in an output buffer. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)