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# activeExternalSyncVideoFrameDuration

The receiver’s external sync frame duration (the reciprocal of its frame rate) when being driven by an external sync device.

```
var activeExternalSyncVideoFrameDuration: CMTime { get }
```

## Discussion

Set up your input to follow an external sync device by calling [`follow(_:videoFrameDuration:delegate:)`](/documentation/AVFoundation/AVCaptureDeviceInput/follow(_:videoFrameDuration:delegate:)).

> Note: The value of this readonly property is `kCMTimeInvalid` unless the ``doc://com.apple.avfoundation/documentation/AVFoundation/AVExternalSyncDevice`` is actively driving the ``doc://com.apple.avfoundation/documentation/AVFoundation/AVCaptureDeviceInput``. This is reflected by the ``doc://com.apple.avfoundation/documentation/AVFoundation/AVExternalSyncDevice/status`` being either ``AVExternalSyncDeviceStatusActiveSync`` or ``AVExternalSyncDeviceStatusFreeRunSync``.

---

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