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Topic: 5.1 downmix algorithm in iOS Files app (Read 1116 times) previous topic - next topic
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5.1 downmix algorithm in iOS Files app

Hi everyone!
I'm experimenting with downmixing songs from 5.1 to 2.0 (original audio is in Dolby Atmos E-AC3-JOC). I feel that those versions sound less compressed and after downmixing I can play them basically anywhere.

I've tried using the regular formula (Dolby Lo/Ro):
Lo = L + 0.708 * C + 0.708 * LS
Ro = R + 0.708 * C + 0.708 * RS
And the Dolby Pro Logic II version (Lt/Rt) both with and without phase shifting LS/RS by 90° in different directions:
Lt = L + 0.708 * C - 0.871 * LS - 0.49 * RS
Rt = R + 0.708 * C + 0.49 * LS + 0.871 * RS

All three versions of the song come out too bass-heavy, though. And when I play the same m4a file in the iOS Files app (with earpods, so I get a regular non-binaural downmix), the vocal is clearer.
I wondered if Apple uses a custom downmixing formula and/or EQ, so I screen-recorded playing a full-spectrum sweep separately in L, LS, C and LFE channels. C and LFE versions were silent, L was played at original volume, and LS was played at -3db. So I assumed the formula was:
L = L + 0.708 * LS
R = R + 0.708 * RS
And the spectrum was uniform, so it's natural to assume there's no EQ either. But even if I use this formula to downmix, the iPhone's downmix still sounds clearer, I think.  ::)
Also, I can only hear it on my iPhone XS, not the 1st gen SE (which coincidentally doesn't support Dolby Audio playback). And they both use the earpods' DAC, so the difference is not due to the phones' DAC.

This question is a bit silly, but Is there a way to check which algorithm Apple uses for downmixing in the Files app?