MCPX
The MCPX is the southbridge chip of the Xbox chipset made by Nvidia. It contains the sound processors (APU and ACI) as well as the USB, PCI, IDE, etc, controllers [1], [2].
Dolby Laboratories anounced in April 2001, The xbox will feature a new "Dolby Interactive Content Encoder" of which the Xbox is the first console to get this.[3][4]
ROM
The MCPX is home to the secret MCPX ROM.
Pin L21: PC Speaker
The MCPX has a PC Speaker pin which can be controlled using the standard PC Speaker interface. However, no actual speaker is connected to the pin, so while the signal exists, there will be no audible sound on a stock Xbox.
A speaker can be soldered to this pin in order to make the signal audible [5][6]
The original Microsoft code does not drive the PC Speaker at all, so this otherwise unused pin can also be used for inaudible forms of unidirectional communication.
