The A2320 is a video deinterlacer board originally designed for the A2000. It is essentially the motherboard deinterlacer circuitry from the A3000 on a board. Based on the Amber chip used in the A3000, the board is often referred to as the Amber board. Physically, the board is built to fit into the video slot of an A2000. Electronically, it works fine in an A4000.
Why would you need a separate deinterlacer board when the A4000 already has AGA circuitry that can scan-double? If you have a VGA or multisync monitor, there are two main reasons:
A modified "slot cover" can be attached to the back panel of the Amber board to allow it to be mounted securely in an A4000 slot. You'll also need to trim a bit off the "top" of the Amber's metal panel to allow clearance for the A4000 case (a nibbling tool is useful here). The board will only fill part of A4000 video slot; it looks funny this way, but it works.
Don't remove the enable/disable switch! The Amber gets confused by some of the "doubled" screen modes, and rather than passing them through, tries to double them to 55 kHz or above! On these modes, you'll need the disable switch to force the board to pass the video through. (Productivity mode is passed through correctly, since it was part of the ECS chip set that was around when the Amber board first came out.)
The Amber board was designed before AGA came out, and doesn't really
understand AGA. As noted above, some modes are not passed through properly
unless the board is disabled with the switch. According to Scott Hood,
the designer of the A2320, it samples 12 bits for each color. On the A4000,
this is the upper 12 bits of the 24-bit AGA information. So AGA screens
with more than 32 colors or HAM-6 will have the colors quantized to a certain
degree. This hasn't been a problem so far, although it can be seen on things
like ImageFX preview screens. Games that use the AGA color abilities but
don't allow for promoting their screens to doubled modes are the only likely
sources for this trouble.