# AudioConverterPrimeInfo Specifies priming information for an audio converter. ``` struct AudioConverterPrimeInfo ``` ## Overview Some audio data format conversions, particularly those involving sample-rate conversion, yield higher quality output when leading or trailing frames are available to the converter. The appropriate number of these so-called *priming frames* depends on the input audio data format. You specify leading or trailing frames in the `leadingFrames` and `trailingFrames` fields of an `AudioConverterPrimeInfo` structure. You then configure your audio converter by calling the [`AudioConverterSetProperty(_:_:_:_:)`](/documentation/AudioToolbox/AudioConverterSetProperty(_:_:_:_:)) function for the [`kAudioConverterPrimeInfo`](/documentation/AudioToolbox/kAudioConverterPrimeInfo) property, specifying this structure as the property value. You also indicate to the converter which priming option to use by setting the [`kAudioConverterPrimeMethod`](/documentation/AudioToolbox/kAudioConverterPrimeMethod) property. When you configure an audio converter by specifying leading or trailing frames, you alter the behavior of the [`AudioConverterFillComplexBuffer(_:_:_:_:_:_:)`](/documentation/AudioToolbox/AudioConverterFillComplexBuffer(_:_:_:_:_:_:)) function. The very first call to that function, or the first call after calling [`AudioConverterReset(_:)`](/documentation/AudioToolbox/AudioConverterReset(_:)), results in a request for additional input frames when the converter object invokes your [`AudioConverterComplexInputDataProc`](/documentation/AudioToolbox/AudioConverterComplexInputDataProc) callback. The number of priming frames requested depends on the priming option you specify, as shown in below. |Priming option |Number of priming frames (approximate)| |------------------------------|--------------------------------------| |`kConverterPrimeMethod_Pre` |`leadingFrames` + `trailingFrames` | |`kConverterPrimeMethod_Normal`|`trailingFrames` | |`kConverterPrimeMethod_None` |`0` | The default priming option is that specified by the `kConverterPrimeMethod_Normal` constant. This option requires no preseeking of the input stream. It generates `trailingFrames` of latency at output. The `kConverterPrimeMethod_Pre` priming option results in a first invocation of your callback that asks for approximately `leadingFrames` + `trailingFrames` priming frames. Latency at output will correspond, approximately, to this duration. The `kConverterPrimeMethod_None` priming constant is especially useful in real-time applications that process live input. This option minimizes output latency, but is appropriate only for formats that do not require priming frames. In a digital audio workstation, the `kConverterPrimeMethod_Pre` option may be preferable for real-time applications. This is because your application may be able to provide priming frames by preloading them from disk or memory, avoiding latency while still supporting audio formats that require priming. ## Relationships ### Conforms To [`Sendable`](/documentation/Swift/Sendable) [`BitwiseCopyable`](/documentation/Swift/BitwiseCopyable) --- 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)