<!--
{
  "availability" : [
    "iOS: 6.0.0 - 18.0.0",
    "iPadOS: 6.0.0 - 18.0.0",
    "macCatalyst: 13.1.0 - 18.0.0",
    "macOS: 10.8.0 - 15.0.0",
    "tvOS: 9.0.0 - 18.0.0",
    "watchOS: 1.0.0 - 11.0.0"
  ],
  "documentType" : "symbol",
  "framework" : "AVFoundation",
  "identifier" : "/documentation/AVFoundation/AVPlayer/masterClock",
  "metadataVersion" : "0.1.0",
  "role" : "Instance Property",
  "symbol" : {
    "kind" : "Instance Property",
    "modules" : [
      "AVFoundation"
    ],
    "preciseIdentifier" : "c:objc(cs)AVPlayer(py)masterClock"
  },
  "title" : "masterClock"
}
-->

# masterClock

The host clock for item time bases.

```
nonisolated var masterClock: CMClock? { get set }
```

## Discussion

The default value of this property is `NULL`, which means that the host clock is the automatic choice. When non-`NULL`, this property overrides the automatic choice of host clock for item time bases. This is most useful when you’re synchronizing video-only movies with audio from another source.

> Important:
> If you specify a host clock other than the appropriate audio device’s clock, the audio may drift out of sync.

---

Copyright &copy; 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)