[mythtv-users] Can mythtv (or mplayer/xine through mythtv) upmix 5.1 sound in 6.1 or 7.1?

Eric Sharkey eric at lisaneric.org
Sat Oct 5 14:40:46 UTC 2013


On Sat, Oct 5, 2013 at 1:35 AM, Marc MERLIN <marc_mtv at merlins.org> wrote:
> On Tue, Oct 01, 2013 at 08:32:24PM -0400, Eric Sharkey wrote:
>> How are you feeding the audio signal into your receiver?  As analog
>> 2-channel stereo or a digital multichannel signal?
>
> Digital. Otherwise 5.1 wouldn't work right either :)

No, that's not the case.  That's precisely how "Pro Logic" surround
sound works.  The audio from multiple channels are downmixed to only 2
channels, with various phase shifts and then the receiver upmixes back
to a larger number of channels.  You can derive 5+ channels of audio
from a 2 channel analog input.  Sort of.  If you understand the math,
you'll know that this mixing/unmixing process can't be done perfectly.

My first surround sound receiver which I got in 1993 worked in this
way and this way only.  It has 5.0 output and only takes 2 channels of
analog input.  There's a reason I have this unit sitting unplugged in
my basement.

>> If you're looking for satisfying surround sound, I'd steer away from
>> matrix encoded solutions.
>
> So you're saying that upmixing 5.1 to 6.1 or 7.1 in hardware sucks?

There's no distinction in quality between hardware and software.  Any
modern CPU will be able to keep up with the job.  I'm saying that
upmixing surround sound in general sucks and the more upmixing you
need to do the more it sucks.  "Matrix encoded solutions" are cases
that involve first downmixing before upmixing.  That's what you really
want to avoid.  If you've got a pure unmixed 5.1 signal and want to go
to 7.1 or 6.1 that's not nearly as bad as when you're starting with
something that was first folded down to 2.0.

Eric


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