SRS Labs uses their multi-patented Sound Retrieval System (SRS) technology in the Pro 220 to enhance the three-dimensional spatial information present in all stereo audio mixes. Stereo sound recordings are made up of two components: Mono and Difference signals. The Mono or Center channel signal is expressed algebraically as the left channel added to the right channel or L+R while the Difference signals, which provides the aural cues to our ears as to the recording space or ambience, is expressed as the left channel minus the right channel PLUS the right channel minus the left channel or (L-R)+(R-L).

The Pro 220 allows the remix of these two components which are normally "locked" within a stereo mix. If you raise or lower the level of the mono component, all information in the center of the mix will increase or decrease. If you want to hear the vocals louder on a CD (which are mixed in the center), increase the mono component. This is controlled on the 220 with the Center control. The Difference component, adjusted by the Space control, comprises all spatial and directional information like reverb, room sound and effects and by increasing this level, your CDs will have a new wider and more spacious sound.

I liked the Pro 220 for a wider CD sound and in the studio to widen narrow, individual stereo instruments. It works great on pads, quirky synths, strings, and for background vocals to make them "wrap around" the outside of the mix. The Pro 220 also has a 3D Mono section with a separate Mono input and left and right outputs for converting strictly monaural sounds into quasi-stereo sounds. I found this good for individual noises in my mixes and since it is totally separate from the SRS section of the unit, you can have both processes running at the same time on different sound sources.