[Mp4-tech] Regarding LTP reference decoder

[Vinay M K]

Hi,
This query is with respect to Mpeg-4 AAC LTP decoder, reference software
(refsoft) downloaded from:
http://www.iso.ch/iso/en/ittf/PubliclyAvailableStandards/ISO_IEC_14496-5_200
1_Software_Reference/audio.zip
In the file nok_lt_prediction.c line #277,
'mdct spectral' lines of the predicted signal are set to zero depending upon
the flag
"sfb_prediction_used"  till the maximum coded sfb given by
"info->sfb_per_bk".
But this value can be less than the maximum permissible sfbs for a given
sampling frequency, which means those spectral lines which are beyond
'info->sfb_per_bk' are not set to zero.
In the file nok_lt_prediction.c line #285,
All the 1024 (or 960) spectral lines are added to the current frame upon
which Imdct
is performed. Hence for those spectral lines which are beyond
'info->sfb_per_bk'
sfb predicted ltp signal gets added, which may not be the intended action.
As a result output decoded signal will have signal energy even beyond
maximum coded sfb.
Is this a bug in reference software? Or am I missing something.. Can any one
clarify?
thanx in advance
regards
Vinay
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