DRM interface overview

The key element of the digital rights management (DRM) system is the DRM manager.

A reference in the PTMediaPlayer to the DRM manager object that implements the DRM subsystem:
```
@property (readonly, nonatomic) DRMManager *drmManager
```

The PSDK issues a PTMediaPlayerItemDRMMetadataChanged notification when DRM metadata changes.

If the DRM-protected stream is multiple bit-rate (MBR) encoded, the DRM metadata used for the variant playlist should be the same as the one used in all the bit-rate streams.