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Stereo to 2.1 "upmix" on Linux - part 1


I recently upgraded my set of speakers after my old Braun boxes finally reached their end of life. The "surrounds" of the speakers were degraded too far after almost 30 years. Given that the speaker system was from Braun's budget series of speakers (still relatively expensive at the time) it does not make sense to refurbish it. So I replaced the front speakers with a set of Nubert nuJubilee 35 (comparable to nuLine 32) speakers. If you are a fan of the 70th classic music sound like me: give Nubert speaker systems a try. At least the nuLine series is quite close to the sound of medium to large Brauns, though slightly more modern (i.e., more bass.)

The center, rear and bass speakers were from a Teufel Magnum E system, which barely worked well enough with the Braun speakers, but does not fit to the new speakers. The center was always far too small, so I replaced it with a nuLine WS-12. Could have a bit more bass, however as I can only mount the center hanging on a shelf and everything larger would be too heavy, this has to do. And it sounds great when watching movies! Alas, the sound of the bass box doesn't fit anymore, and the rears were also too limited, thus I ended up with two new rear speakers (nuLine DS-22) and a bass box (nuJubilee 35), completing the upgrade.

The new system sounds exactly the way I want it to sound. Probably way too dry for other people, but perfect for me. The only thing that buggers me is the AV receiver: With stereo signals, I can either have stereo output, but that won't feed the bass. Or I can have a Dolby ProLogic II upmix which does weird things. Luckily, the receiver can be configured to NOT send a sum signal to the center speaker in that mode, but it still has some phase shift and delay on the rears. This is usually okay for pop music, but I do not want to hear the trombone from almost-rear right when listening to an opera... I cannot get any analog line out signal from a digital source, either. Said opera recording, even though a digital production, was mastered with vinyl records in mind, so it doesn't have any frequencies low enough for the bass box to be needed, but I do have some recordings that would benefit from a 2.1 "upmix". Or even 4.1 with just the unmodified stereo signal sent to the rears.

Since I'm playing all recordings I own from a openSUSE 11.4 system via TOSLINK / SPDIF / IEC958, how about creating the upmix on the machine, encode it as AC3 and use SPDIF pass-through to send it to the AV receiver? Sounds simple, it even is, but information about how to do it is hard to find, unless you know that instead of googling for "alsa ac3", which gives you tons of information on how to split up a 5.1 signal to analog output, you have to google for... Nope, won't tell you yet. ;-)


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