[Mp4-tech] Regarding LTP reference decoder

[juha.ojanpera nokia.com]

  Dear Vinay,
I took a closer look at the LTP reference decoder implementation and you are right. There is a possibility that spectral bands beyond 'max_sfb' might get added to the reconstructed spectra. This is something that will be fixed in the future releases of the reference software as the current behaviour of the implementation may lead to different LTP outputs depending on the platform. If 'sfb_prediction_used' gets initialized to zero when resources are allocated to LTP structure and 'max_sfb' does not vary from frame to frame, then everything should be ok, otherwise different LTP output results can be expected.
The predicted spectra should never get added to the reconstructed spectra (i.e., current frame) beyond min('max_sfb', 40) value, unless those values are set to zero.
Thanks for pointing out this issue in the reference software.
Br,
 Juha
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Juha Ojanpera                          email: juha.ojanpera nokia.com
Nokia Research Center                  tel: +358 7180 35325
Audio-Visual Systems Laboratory        mob: +358 5048 35325
P.O. Box 100 (Visiokatu 1)             fax: +358 7180 35899
FIN-33721 Tampere, Finland; 
-----Original Message-----
From: ext Vinay M K [mailto:vinaymk emuzed.com]
Sent: 21 August, 2003 07:40
To: MP4-Tech lists.mpegif.org
Subject: [Mp4-tech] Regarding LTP reference decoder
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