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# formatDescriptions

The format descriptions of the media samples that a track references.

```
static var formatDescriptions: AVAsyncProperty<Root, [CMFormatDescription]> { get }
```

## Discussion

Use the [`load(_:isolation:)`](/documentation/AVFoundation/AVAsynchronousKeyValueLoading/load(_:isolation:)) method to retrieve the property value.

The array contains <doc://com.apple.documentation/documentation/CoreMedia/CMFormatDescription> objects that indicate the format of media samples the track references.

Asset tracks typically present uniform media (for example, media that uses the same encoding settings) and contain a single format description. However, in some cases, an asset track may contain multiple format descriptions. For example, an H.264-encoded video track may have some segments that use the Main profile and others that use the High profile. Also, an individual [`AVCompositionTrack`](/documentation/AVFoundation/AVCompositionTrack), which subclasses [`AVAssetTrack`](/documentation/AVFoundation/AVAssetTrack), may contain audio or video segments using different codecs.

You can use <doc://com.apple.documentation/documentation/CoreMedia/CMFormatDescription> to access low-level details about the media the track references. For example, you can retrieve the details of track’s media type and subtype as the code below shows:

```swift
extension AVAssetTrack {
    var mediaFormat: String {
        get async throws {
            var format = ""
            let descriptions = try await load(.formatDescriptions)
            for (index, formatDesc) in descriptions.enumerated() {
                // Get a string representation of the media type.
                let type = CMFormatDescriptionGetMediaType(formatDesc).toString()
                // Get a string representation of the media subtype.
                let subType = CMFormatDescriptionGetMediaSubType(formatDesc).toString()
                // Format the string as type/subType, such as vide/avc1 or soun/aac.
                format += "\(type)/\(subType)"
                // Comma-separate if there's more than one format description.
                if index < descriptions.count - 1 {
                    format += ","
                }
            }
            return format
        }
    }
}
 
extension FourCharCode {
    // Create a string representation of a FourCC.
    func toString() -> String {
        let bytes: [CChar] = [
            CChar((self >> 24) & 0xff),
            CChar((self >> 16) & 0xff),
            CChar((self >> 8) & 0xff),
            CChar(self & 0xff),
            0
        ]
        let result = String(cString: bytes)
        let characterSet = CharacterSet.whitespaces
        return result.trimmingCharacters(in: characterSet)
    }
}
```

---

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