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MPEG-1 Layer 3 Question: Pause at beginning of files after decoding (not framing)

Dear list,

When I encode a WAV-file into MPEG-1 Layer 3 (MP3), no matter if I use the
"original" patent pending Fraunhofer algorithm or the LAME algorithm I
always get a few thousand sample values digital Nulls:
When I decode those MP3-files again to WAV again, the signal starts
3000-4000 samples later than in the original WAV.
I know about framing in MP3 - but AFAIK this should just happen to the end
of the file, not to the beginning of it.

There is no signal cut, the signal is just being moved forward.

Does anyone know why this happens and if this is an artifact or purpose
(e.g. because the decoder needs time to start)?

I'm doing the whole thing for looking at simultaneous masking on one of the
patent pending Fraunhofer MP3-features(not the mp3-pro).
I always have to move manually the mp3-decoded file sample-accurate to the
original Linear PCM-file to get the difference-signal via phase inversion of
one of them.

Thank you in advance,
Hermann Stemberger