[FFmpeg-user] Forced upmixing scheme with no way to disable

Paul B Mahol onemda at gmail.com
Thu Jun 23 19:58:03 EEST 2022

On Thu, Jun 23, 2022 at 6:45 PM Jeremy F <advanced4 at gmail.com> wrote:

> Hi I'm probably using ffmpeg in an edge-case way, but I simply want to take
> 2 channels/stereo, and end up with 6 channels of audio, where I can
> apply/modify/filter each channel as I please. And I figured out the ffmpeg
> magic to do this, but along my dive into this it appears that no matter the
> method used to upmix (of which there are many such as "-ac 6", "asplit",
> "pan", "channelsplit") it appears no matter what, I'm forced to use the
> upmixing scheme that always sets channel 4 to a lowpass filter of 128hz,
> rendering it useless for my case.
> There doesn't appear to be a way to disable this that I can find, and it
> seems like there should be
> My current command I'm using, which works great except for channel 4
> (labeled r2 in here). No matter what filter I apply, it always ends up
> being a lowpass of 128.
> -err_detect ignore_err -filter_complex
> [0:a]channelsplit[left][right];[left]asplit=3[l1][l2][l3];[l1]aecho=1.0:0.7:25|60:0.5|0.3[l1];[l2]lowpass=f=550[l2];[l3]adelay=1,aecho=1.0:0.7:25|60:0.5|0.3[l3];[right]asplit=3[r1][r2][r3];[r1]aecho=1.0:0.7:25|60:0.5|0.3[r1];[r2]lowpass=f=550[r2];[r3]adelay=1,aecho=1.0:0.7:25|60:0.5|0.3[r3];[l1][r1][l2][r2][l3][r3]amerge=inputs=6[a]
> -map 0:v -map [a] -acodec aac -c:v copy
> The code, at a glance, seems to corroborate this, but it's a bit beyond my
> depth to do anything further (and I don't want to compile / make a custom
> version of ffmpeg)
> https://github.com/FFmpeg/FFmpeg/blob/9aa20d28cdda0dcaa4daa2848670a6530c6ba26a/libavfilter/af_surround.c
> So, if possible, how can I disabled the upmixing scheme?

The surround filter have nothing to do with that filtergraph above.

And I really failed to understand what you are trying to do.

> Thanks!
> --
> Sincerely,
> Jeremy
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