SPM4 provides the ability to mix the midrange and tweeter channels coming from an amplifier and supply one single mid-high channel. The Ch1(L/R) channels feature two inputs, one features an inverting input compared to Ch2 (L/R) channels; the mixing process often gives better results when one of the two channels undergoes a 180 phase shift. SPM4 is especially recommended for extending the number of inputs of Audison digital processors or amplifiers with built-in DSP because they feature a De-equalizer (De-Eq). In this case, any possible non-linear acoustic emissions due to the mixing of the pre-existing filtered channels are duly compensated for.