... It is a combination of the MPU-401AT card and SCB-55 daughterboard. The SCB-55 daughterboard (commonly referred to by the package name, SCD-15) is the same in function as the external SC-55mkII module (with a 16-bit instead of 18-bit digital-to-analog converter (DAC)). ... A few were for General MIDI Sound Canvas's but I forget which since they always work on my SCC-1). ... The SCM-15AT replaces the SCC-1(B). ... Again the SCD-15(SCB-55) has the same GM/GS sound "engine" as the SC-55mkII. ...