[Mp4-tech] [Audio] SBR decoding

[Devilal]

Hi,
I am doing SBR decoding with some streams downloaded from ISO website. Some
of the streams are: al_sbr_cm_48_1.mp4, al_sbr_cm_48_2.mp4,
al_sbr_e_44_2.mp4, al_sbr_i_44_1.mp4, al_sbr_ps_02.mp4(this steams has
Parametric Stereo also), al_sbr_sig_48_2_sig1.mp4 and
al_sbr_sr_24_2_fsaac12.mp4.
I have a couple of questions related to SBR decoding:
*1.* In one frame the following syntactic elements are coming -
ID_CPE/ID_SCE i.e Channel Pair Element(for Stereo) or Single Channel
Element(for Mono)
ID_FIL - Fill Element
ID_FIL - Fill Element
ID_END - to end the syntactic elements
My question is why do u need fill elemets two times?
*2.* The fillExtType being decoded is 13 for fist call of fill element and
is 0 for second time.
I understand that 13 is corresponding to EXT_SBR_DATA and 14 for
EXT_SBR_DATA_CRC i.e. SBR with CRC which has 10 extra bits in the SBR
Extension data element. I assume that we have streams without CRC check, so,
the number 13 is corresponding to EXT_SBR_DATA and is correct.
But what is the significance of second fill elements in the syntactic
elements? Moreover, it does not do anything in the decoding as it always
decodes 0 as fillExtType.
*3.* In case of fillExtType == 13 (the first time decoding of fill element)
it does decode SBR *header_flag* bit and *it is always 0 *. After knowing
that header_flag is 0 (i.e. header_flag NOT SET), the decoder does not
decode SBR header and it just does up-sampling and not SBR decoding(i.e. HF
Generation, HF Adjustments and other steps).
*This is happening with all the streams I mentioned above. The reference
software from ISO i.e. mp4mcDec is also doing the same thing. Is this
correct decoding for these streams or something wrong with my decoding? Are
there other streams for SBR decoding which follow the missing decoding in
the current case?*
-- 
Regards,
Devilal
--------------------
Mobile: +91-9986423985
email: devilal gmail.com
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