The optional A/D card can be purchased with the unit, or retrofitted at a later date. At the heart of the A/D card is a high quality 24bit 96KHz Delta Sigma converter running at 128KHz over-sampling for maximum conversion performance. Focusrite proprietary designs are used for all internal clocking and phase lock loop circuits to guarantee the absolute minimum jitter possible, whether free-running or locked to wordclock. The ISA 220's optional A/D is stereo and allows for inputs from internal (post-processing and external sources simultaneously giving a clean, protected, high-quality path to stereo digital media. Thus, with the Stereo A/D card installed, the ISA 220 can be used as a high quality mono/stereo tracking converter, or at mix down as a stereo converter channel for routing complete mixes to digital media. Two ISA 220 Session Packs can take full advantage of the one A/D converter, with one ISA 220 acting as the master and the other as the slave.