"aloop" filter somehow gave misalignment in 48 KHz Stereo WAV

[Danny Weijermans]

when using the aloop audio filter in ffmpeg

to produce a looped audio section,
it works sample accurate in 44.1kHz mono or stereo,
and in 48kHz mono.

But for some strange reason,

it's off (probably by 1?) when using a 48kHz stereo file.

I didn't test other sample-rates yet..
I'm including a 48kHz stereo wav file containing a pure 480Hz sine,

so you can test yourself.

I'm using for testing:
ffmpeg -y -i /Users/disjt/Desktop/s.wav -filter_complex "aloop=loop=4:start=48000:size=48000" s.wav

[Danny Weijermans]

pure 480Hz Sinewave 16bits 48kHz stereo wav file

[MasterQuestionable]

#11060

͏----

͏    https://ffmpeg.org/ffmpeg-filters.html#aloop
͏    Caveat: Your sample file is of 192,001 samples.

͏    Also reproducible with:
͏    ffmpeg -y -v debug -hide_banner -nostdin -nostats -f lavfi -i "sine=480:0:48000:d=4,volume=+18.063 dB" -af "aloop=4:start=48000:size=48000" "!.wav"

͏    Glitch at Sample #96768 (2.016 s), #144000 (3 s), #192000 (4 s), #240000 (5 s), #288000 (6 s).
͏    Output message of interest:
͏    "Non-monotonic DTS; previous: 96128, current: 96000; changing to 96128. This may result in incorrect timestamps in the output file."

[Marton Balint]

No, this is a totally different issue, it has nothing to do with seeking. I will send a patch to fix this soon.

[MasterQuestionable]

͏    Looks somehow similar, nevertheless.

[Danny Weijermans]

thanks Marton!
are there anymore combinations of sample-rates and channel-counts to be tested?
I wouldn't expect this to behave different with a certain combination of S/R and channel-count..

[MasterQuestionable]

͏    I guess Mono alone would suffice. (doesn't seem Channel# related)
͏    I think it might be essentially seeking issue:
͏    So probably just try changing the loop start point..?

[Danny Weijermans]

I will investigate further, because I need it for some of my own code..
@Marton : you found the bug? ('I will send a patch to fix this soon.')

[Marton Balint]

These two patches should fix it:

​https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2024-November/335643.html
​https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2024-November/335646.html

[Danny Weijermans]

W O W !!!

[Danny Weijermans]

How will I know when it has been 'patched' ?

[Danny Weijermans]

thanks so much, Marton!
Would it be possible to also shine your light

on my other ticket:

​https://trac.ffmpeg.org/ticket/11281
would be great if you can find that 'bug' too..

[Marton Balint]

Fixed in b33a59416072ad31a5840f33f9975d88acf45add and fe18ed3f2a9221af0beaec7b04b7804849db1f2f.

[Danny Weijermans]

thanks a lot, Marton!!

[Danny Weijermans]

is the fixed version distributed already?

[Danny Weijermans]

would this also be automatically updated in ffplay? or is this the same code?

[MasterQuestionable]

͏    Likely the same bundle?
͏    (unsure MacOS's distribution handling)

͏    FF-series tools share significant portion of common codes.

[Danny Weijermans]

updated ffmpeg,

and bug is gone: THANKS a lot!!
but I think ffplay is not updated yet on OSX..
and it could be used with a filter to realtime-play seemless loops ;)

let me know when this will be updated in ffplay as well..
thanks again for all your work!
regards, Danny

[Danny Weijermans]

Dear Marton,
ffmpeg is working correct now.
I waited a while, but it's still not updated correctly in ffplay
(same filter can be used there as well..)
could you verify this?
thanks in advance!
regards,
Danny Weijermans

[Marton Balint]

The issue fixed was in the libavfilter library. FFmpeg and FFplay are both using this library, so unless you use a static build of ffmpeg/ffplay or some OSX magic so different binaries somehow use different version of libraries, I don't see how that can happen.

I'd start with comparing the versions reported by the ffmpeg and ffplay command.

[Danny Weijermans]

Hmmm,
ffmpeg = 7.1.3
ffplay = 7.1
would this be the problem?
how to upgrade ffplay?
isn't that part of the ffmpeg upgrade scheme?
I use 'brew ffmpeg upgrade' in Terminal..

[MasterQuestionable]

͏    Seems it was separate packages in MacOS.
͏    Try: brew ffplay upgrade

[Danny Weijermans]

yes, tried that, but doesn't work:

Unknown command..

even tried a reinstall of ffmpeg to no avail..
how to update ffplay correctly on osx??

[MasterQuestionable]

͏    Probably something peculiar with the distribution...
͏    Ask the packages manager?

[Danny Weijermans]

My bad:

I now upgraded to the latest ffmpeg
using: brew install ffmpeg --HEAD
(and removing any old installs)
and this also upgraded ffplay correctly
and now is working correctly as it should!

Yeah!!