general dolby, aften question

[BabaG]

in the olden days dolby was associated with noise reduction
through audio compression techniques. older versions also
achieved surround with phasing techniques. are any of these
processes still employed with 5.1 endoding to ac3? or is the
current thinking that digital production pretty much takes care
of any noise and that it's easy enough now to fold discreet
audio channels into a single file, making the phase relation
of the channels less relevant to the positioning process? i'm
prompted to wonder this last by the suggestion of a particular
order for files when using wavewizard to create a six channel
wav file.

thanks,
BabaG

[tebasuna51]

in the olden days dolby was associated with noise reduction through audio compression techniques. older versions also achieved surround with phasing techniques. are any of these processes still employed with 5.1 endoding to ac3?
Only in ac3 2.0 there are a flag in header to inform the decoder if the two channels are encoded with this phasing techniques. The decoder can apply automatically a Dolby ProLogic decoder to this 2 physical channels to obtain 4/5 logical channels.
In Aften TODO list there are downmix/upmix functions related with this techniques.
or is the current thinking that digital production pretty much takes care of any noise and that it's easy enough now to fold discreet audio channels into a single file, making the phase relation of the channels less relevant to the positioning process?
The phase relation between the channels is important, but I can't related with "positioning process" after read the next comment:
i'm prompted to wonder this last by the suggestion of a particular order for files when using wavewizard to create a six channel wav file.
The channel order in files is only a convention and is not related with spatial distribution or phase relation.
Old style ac3 encoders (Sonic Foundry SoftEncode, ac3enc.dll, ffmpeg, ...) need a input wav with channels ordered L-C-R-sL-sR-LFE to obtain a ac3 well played by standard ac3 players.
But a wav with this order is played incorrectly by standard wav players because a multichannel wav is expected with L-R-C-LFE-sL-sR.

This problem is solved if the encoder know/accept the correct input format and Aften is the first free encoder to do. If you have a correct wav 6 channels you don't need WaveWizard to encode with Aften, if you want encode a correct wav6 with HeadAc3he-ac3enc, ffmpeg, ... you need remap the channels.

[BabaG]

thanks again tebasuna51 for your generous help. what i meant
by the phase/positioning comment was that, in the past, dolby
determined what was sent to the surrounds (back when it was
just l, c, r, s) by what was out of phase. if i wanted something
in the surrounds, i reversed the phase and it went to the back.
seems like the more current way of working is just to have the
surround have its own channel. that would make that kind of
out-of-phase relationship less important as it wouldn't be used
to send a sound to the surround. is this right? has dolby, in this
sense, become less of an electronic process (of reading and
creating phase relations for the purpose of position - front/back)
and more of a standard?

[Mug Funky]

pretty much.

however, when a multichannel file is played back to 2 speakers, the ac3 standard says there's a few ways the 6 channels can be mixed together, most of which involve the matrixed stereo that you refer to.

so if you have a pro logic (or pro logic II) decoder, it can take a 6ch ac3 that's mixed down to stereo, then bump that up to 6 channels again. you wouldn't want to do it that way though... it's provided as a sort of backward compatibility.

dolby own loads of standards - there's the various NR techniques involving companding etc, and there's the ac-3 standard which is a digital multichannel audio compression scheme (that you'll find on DVDs, cinema prints and newer Laserdiscs). then there's standards for upmixing stereo audio into multichannel (pro logic II is actually very heavily based on the Tate II SQ quadrophonic decoder, so as a bonus, if you play back quad LPs into a pro logic II amplifier you get a very good "quadrophonic plus centre" mix).