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

Jeremy F advanced4 at gmail.com
Fri Jun 24 02:46:51 EEST 2022 

The join filter appears to work - specifying a 6.0 layout does indeed
output what I want, but: the backend video player (AVPro) in the game I'm
playing does not respect/acknowledge 6.0 (seemingly) so that doesn't work.
Also the webplayer for the CDN I'm using also does not work (flv.js). Lucky
me!

These of course are not ffmpeg's fault, but would anyone have advice for,
perhaps setting the AAC channel layout bit to say it's 5.1 instead? (I
*think* that might fix it?). I don't think that's unreasonable, my layout
very well may be true to 5.1, but ffmpeg forcing a 128hz filter is not
always desired. I should be able to override that filter but retain a 5.1
channel layout designation.

On Thu, Jun 23, 2022 at 5:22 PM Paul B Mahol <onemda at gmail.com> wrote:

> On Thu, Jun 23, 2022 at 10:30 PM Jeremy F <advanced4 at gmail.com> wrote:
>
> > Unfortunately due to platform limitations, AAC is the only
> > mutli-channel audio codec supported in my use case. However, I do not
> see,
> > at a glance, where the AAC specification mandates that if there are 6
> > channels, that channel 4 must be a 128hz LFE channel. AAC also supports
> the
> > 6.0 channel layout for example, which does not include LFE. Ffmpeg
> appears
> > to be making assumptions at channel layout, as well as doing
> corresponding
> > upmixing schemes, with no way to override or disable. This is does not
> > appear to be an AAC problem
> >
>
> Then use join filter instead of amerge and specify 6.0 layout,
>
> Good luck.
>
> >
> > On Thu, Jun 23, 2022 at 4:04 PM Paul B Mahol <onemda at gmail.com> wrote:
> >
> > > On Thu, Jun 23, 2022 at 8:43 PM Jeremy F <advanced4 at gmail.com> wrote:
> > >
> > > > That's what I thought! However, here are some commands showing this.
> I
> > > set
> > > > the lowpass to be 10khz on channel4 just to prevent any confusion (it
> > > > should clearly _not_ sound like any kind of real low pass filter).
> With
> > > an
> > > > input of "vid.mp4" that has standard stereo, run the following:
> > > >
> > > > ffmpeg -i vid.mp4 -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=10000[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 vid1.mp4
> > > >
> > > > Then to extract channel 4 (since its zero based it's labeled 3):
> > > >
> > > > ffmpeg -i vid1.mp4 -map_channel 0.1.3 ch4.mp3
> > > >
> > > > ch4.mp3 will be a 128hz lowpass filtered version of the audio,
> despite
> > me
> > > > never specifying that.
> > > >
> > > >
> > > Stop transcoding to aac.
> > >
> > >
> > >
> > > >
> > > > On Thu, Jun 23, 2022 at 1:48 PM Paul B Mahol <onemda at gmail.com>
> wrote:
> > > >
> > > > > On Thu, Jun 23, 2022 at 7:09 PM Jeremy F <advanced4 at gmail.com>
> > wrote:
> > > > >
> > > > > > A more succinct way of putting it: if I channelsplit (or any
> other
> > > type
> > > > > of
> > > > > > upmixing as far as I'm aware) into 6 channels, channel 4 always
> > has a
> > > > > 128hz
> > > > > > low pass filter applied to it. I do not want this
> > > > > >
> > > > > > My particular use case is I play a game that supports
> multi-channel
> > > > live
> > > > > > audio streams, but no filters on any audio. So I apply filters
> > myself
> > > > and
> > > > > > position them as appropriate in the game (i.e. a reverb/echo
> effect
> > > in
> > > > a
> > > > > > hallway, a 550hz lowpass outside the main area, etc.). More
> > > generally,
> > > > > > ffmpeg should allow me to apply any effect to any channel, but it
> > > would
> > > > > > appear ffmpeg sees me using 6 channels, and decides to use the
> > > default
> > > > > > 5.1(side) upmixing scheme. I could find little documentation on
> > this
> > > > > > behaviour, but this page very briefly mentions this behaviour
> about
> > > > > halfway
> > > > > > down https://trac.ffmpeg.org/wiki/AudioChannelManipulation
> > > > > >
> > > > > > Also my provided command was a bit verbose. I could simply
> > > channelsplit
> > > > > L &
> > > > > > R into 3 channels each without any modification to any individual
> > > > > channels.
> > > > > > All channels will be duplicates of its original L or R channel,
> but
> > > > > channel
> > > > > > 4 will have a 128hz low pass on it.
> > > > > >
> > > > >
> > > > > That directly contradicts with reality.
> > > > >
> > > > > FFmpeg never applies lowpass to LFE channels, unless you call
> lowpass
> > > > > filter or similar filter on it.
> > > > >
> > > > >
> > > > > >
> > > > > > On Thu, Jun 23, 2022 at 12:55 PM Paul B Mahol <onemda at gmail.com>
> > > > wrote:
> > > > > >
> > > > > > > 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
> > > > > > > > _______________________________________________
> > > > > > > > ffmpeg-user mailing list
> > > > > > > > ffmpeg-user at ffmpeg.org
> > > > > > > > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> > > > > > > >
> > > > > > > > To unsubscribe, visit link above, or email
> > > > > > > > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> > > > > > > >
> > > > > > > _______________________________________________
> > > > > > > ffmpeg-user mailing list
> > > > > > > ffmpeg-user at ffmpeg.org
> > > > > > > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> > > > > > >
> > > > > > > To unsubscribe, visit link above, or email
> > > > > > > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> > > > > > >
> > > > > >
> > > > > >
> > > > > > --
> > > > > > Sincerely,
> > > > > >
> > > > > > Jeremy
> > > > > > _______________________________________________
> > > > > > ffmpeg-user mailing list
> > > > > > ffmpeg-user at ffmpeg.org
> > > > > > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> > > > > >
> > > > > > To unsubscribe, visit link above, or email
> > > > > > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> > > > > >
> > > > > _______________________________________________
> > > > > ffmpeg-user mailing list
> > > > > ffmpeg-user at ffmpeg.org
> > > > > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> > > > >
> > > > > To unsubscribe, visit link above, or email
> > > > > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> > > > >
> > > >
> > > >
> > > > --
> > > > Sincerely,
> > > >
> > > > Jeremy
> > > > _______________________________________________
> > > > ffmpeg-user mailing list
> > > > ffmpeg-user at ffmpeg.org
> > > > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> > > >
> > > > To unsubscribe, visit link above, or email
> > > > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> > > >
> > > _______________________________________________
> > > ffmpeg-user mailing list
> > > ffmpeg-user at ffmpeg.org
> > > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> > >
> > > To unsubscribe, visit link above, or email
> > > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> > >
> >
> >
> > --
> > Sincerely,
> >
> > Jeremy
> > _______________________________________________
> > ffmpeg-user mailing list
> > ffmpeg-user at ffmpeg.org
> > https://ffmpeg.org/mailman/listinfo/ffmpeg-user
> >
> > To unsubscribe, visit link above, or email
> > ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
> >
> _______________________________________________
> ffmpeg-user mailing list
> ffmpeg-user at ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-user
>
> To unsubscribe, visit link above, or email
> ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
>


-- 
Sincerely,

Jeremy

More information about the ffmpeg-user mailing list