SCSI CD-ROM target devices with vendor unique commands not supported in the SCSI-2 standard require a SCSI-2 emulation filter. The emulation filter maps SCSI-2 commands received from the CD-ROM Device Manager to the vendor unique commands supported by the target device. This support is required to enable audio support on CD-ROM drives that adhere to the SCSI-1 standard. The SCSI-1 standard does not define a standard command set for audio control.

A SCSI-2 emulation filter is required for each vendor unique CD-ROM drive. Typically, a CD-ROM manufacturer uses the same vendor unique command set for all it's CD-ROM drives, therefore, one filter driver is required for each manufacturer.

The filter driver receives SCSI-2 commands from the CD-ROM Device Manager, converts the command to it's vendor unique equivalent, and routes the filtered command to the SCSI adapter device driver. If data returned with the command needs to be filtered, the filter driver regains control when the request is complete, converts the outgoing data to it's SCSI-2 equivalent, and then returns to the CD-ROM Device Manager. The filtering process is transparent to the Device Manager and to the adapter device drivers.

Filter drivers adhere to the adapter device driver interface previously defined. (See DASD, SCSI, and CD-ROM Device Manager Interface Specification.) The filter driver is loaded using the BASEDEV= statement in CONFIG.SYS.