Re: [Avid-L2] 5.1 Fold Down to Stereo Math Quiz for today?

Wild guess is that it is not time shifted back aka 'un'

Perhaps to have a richer sound on MS situations. (Where in downmix to mono you loose an entire mic with ambient, never understood why correlation meters don't go berserk on that.)

Bouke / edit 'B

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On 24 Sep 2024, at 21:36, John Moore <bigfish@pacbell.net> wrote:

[Edited Message Follows]

Nice article.  It's interesting that one of the choices involves a 90 degree phase shift to avoid undesirable signal cancellation.  I understand a 180 degree phase shift is reversing the polarity of the signal.  I can envision what a perfect sine wave shifted 90 degrees looks like compared to the original sine wave but a complex audio waveform with several sinusoidal esk waveforms is harder to picture.  Is the particular signal being time shifted relative to the other signal and then later un timeshifted? 

 

Here's part of the article:

 

Stereo downmix settings:

 

All the stereo downmix options except Direct Render are created by first creating a 5.1 downmix

from the 7.1 render of .atmos content, and then downmixing from 5.1 to 2.0 in the same manner as 2.0

consumer products.  The choice of the 5.1 downmix method above affects the subsequent 5.1 to 2.0 downmix.

 

Lo/Ro - default:  Downmixes from 5.1 to Stereo using the coefficients:

Lo = L + (–3 dB × C) + (–3 dB × Ls)

Ro = R + (–3 dB × C) + (–3 dB × Rs)

 

Lt/Rt (Pro Logic II):  Downmixes from 5.1 to Stereo using the coefficients:

Lt = L + (–3 dB × C) – (–1.2 dB × Ls) – (–6.2 dB × Rs)

Rt = R + (–3 dB × C) + (–6.2 dB × Ls) + (–1.2 dB × Rs)

 

Lt/Rt  (Pro Logic II) w/Phase 90:   Downmixes from 5.1 to Stereo using the coefficients:

Lt = L + (–3 dB × C) – (–1.2 dB × Ls) – (–6.2 dB × Rs)

Rt = R + (–3 dB × C) + (–6.2 dB × Ls) + (–1.2 dB × Rs)

Applies a 90 degree phase shift to Ls/Rs prior to feeding the downmix which reduces undesirable signal cancellation,

improves imaging, and enables proper matrix decoding. It is strongly recommended to use the 90-degree phase shift for

any Lt/Rt downmixes.