The SPDIF input on the FireStudio Mobile is RCA format, to use this with the Focusrite ISA One with the Digital Output option, you'll need to get a format converter to go from the SPDIF TOSLink Optical Port to the SPDIF RCA Input on the FireStudio Mobile. Neither Focusrite nor PreSonus makes such a product. However one can be obtained from a your music retailer. Just Google "Toslink to RCA" to find one. Or contact Focusrite to see which one they recommend. You need to look for something like this.