audiomixer - reason not-negotiated (-4)

[bojan f]

I like to use the following pipeline (receving incoming stream and convert
it to webm) but I get error reason not-negotiated:

gst-launch-1.0.exe -v udpsrc address=127.0.0.1 port=63000 !
application/x-rtp,media=audio,payload=8,clock-rate=8000,encoding-name=PCMA
! rtppcmadepay ! alawdec ! audiomixer ! audioconvert ! opusenc ! webmmux !
filesink location=ab.webm
Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
/GstPipeline:pipeline0/GstCapsFilter:capsfilter0.GstPad:src: caps =
application/x-rtp, media=(string)audio, payload=(int)8,
clock-rate=(int)8000, encoding-name=(string)PCMA
ERROR: from element /GstPipeline:pipeline0/GstUDPSrc:udpsrc0: Internal data
stream error.
/GstPipeline:pipeline0/GstRtpPcmaDepay:rtppcmadepay0.GstPad:src: caps =
audio/x-alaw, channels=(int)1, rate=(int)8000
Additional debug info:
../libs/gst/base/gstbasesrc.c(3070): gst_base_src_loop ():
/GstPipeline:pipeline0/GstUDPSrc:udpsrc0:
streaming stopped, reason not-negotiated (-4)
/GstPipeline:pipeline0/GstALawDec:alawdec0.GstPad:sink: caps =
audio/x-alaw, channels=(int)1, rate=(int)8000
Execution ended after 0:00:00.043469200
Setting pipeline to NULL ...
Freeing pipeline ...

I know there is no sense to use audiomixer for one channel but this is the
minimize version where I can still reproduce the error. In real there are
two channel mixing on audiomixer.
If I use the same pipeline without audiomixer then it works ok.

Where is the problem with audiomixer or if I need to set some caps?
I tried also with that and audioconvert and audioresample before audiomixer
but without success.

I am testing this with gstreamer 1.17.2 because I should use this version.

BR, Bojan
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