How to convert a byte stream not from the beginning into raw video frames?

[sonntex]

As you can see I already used videoparse and it did not work as I want. So,
avimux and avidemux can not parse stream from any position too:

home grabber # gst-launch-1.0 -qe v4l2src device=/dev/video0
do-timestamp=true ! video/x-raw,width=640,height=480,framerate=30/1 !
videoconvert ! video/x-raw,format=I420 ! avimux ! fdsink > fanout0 &
[1] 14174
home grabber # cat fanout0 | gst-launch-1.0 -qe fdsrc do-timestamp=true !
avidemux ! videoparse format=i420 width=640 height=480 ! videoconvert !
jpegenc quality=50 ! multipartmux boundary=boundary ! fdsink >
video.multipart
ERROR: from element /GstPipeline:pipeline0/GstAviDemux:avidemux0: The
stream is of a different type than handled by this element.
Additional debug info:
/var/tmp/portage/media-libs/gst-plugins-base-1.8.2/work/gst-plugins-base-1.8.2/gst-libs/gst/riff/riff-read.c(262):
gst_riff_parse_file_header (): /GstPipeline:pipeline0/GstAviDemux:avidemux0:
Stream is no RIFF stream: 0x62643030
ERROR: pipeline doesn't want to preroll.
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