The MD823 and MD820 are both 30-pin dock adapters, which means they are compatible with devices that use the old 30-pin dock connector, such as the iPhone 4 and iPad 4. However, they are not compatible with newer devices that use the Lightning connector, such as the iPad Air, iPad Mini, and iPhone 5/6/7/8.

Here's a breakdown of the compatibility:

• Compatible:

  • iPhone 4
  • iPad 4
  • iPod with 30-pin dock connector

• Not compatible:

  • iPad Air
  • iPad Mini
  • iPhone 5/6/7/8
  • iPod touch (5th generation or later)

If you're looking for an adapter to connect your iPhone 4 or iPad 4 to a newer device with a Lightning connector, you'll need a different adapter, such as the Apple Lightning to 30-pin Adapter.

I hope this helps!