That's why we have modes like Dolby Digital EX (mono rear surround from any Dolby Digital 5.1 source - with our without EX encoding), DTS ES (mono rear surround from DTS ES sources), Pro Logic IIx (stereo rear surrounds from analog or PCM stereo, Dolby Digital 2.0, 5.1, and EX, or even in the case of the 990 DTS 5.1 sources), and DTS NEO:6 (mono rear surround from analog or PCM stereo and DTS 5.1 sources). Pro Logic IIx and NEO:6 of course have to also produce the center and side surround channels from those stereo sources.