Reconstructing AAC CPE

Part of Understanding AAC

This page describes the process of reconstructing PCM data based on the decoded CPE parameters in an AAC bitstream.

Reconstruction Process

 +-specrec.c:reconstruct_channel_pair()
   +-specrec.c:allocate_channel_pair()
   +-specrec.c:quant_to_spec()
   +-pns.c:pns_decode()
   +-ms.c:ms_decode()
   +-is.c:is_decode()
   +-ic_predict.c:ic_prediction() (main profile)
   +-ic_predict.c:pns_reset_pred_state() (main profile)
   +-(LTP & LD stuff)
   +-tns.c:tns_decode_frame()
   +-drc.c:drc_decode()
   +-filtback.c:ifilter_bank()
   +-ssr.c:ssr_decode()
   +-lt_predict.c:lt_update_state()
   +-sbr_dec.c:sbrDecodeInit()
   +-sbr_dec.c:sbrDecodeCoupleFrame()

reconstruct_channel_pair(2 ic_streams, 2 spec_data arrays(16 bit ints))

 declare 2 1024-element float arrays for spectral coefficients (spec_coeff1 and spec_coeff2)
 quant_to_spec(ics1, spec_data1, ics1)
 quant_to_spec(ics2, spec_data2, ics2)
 if ics1.ms_mask_present
   pns_decode(ics1, ics2, spec_coef1, spec_coeff2)
 else
   pns_decode(ics1, spec_coef1)
   pns_decode(ics2, spec_coef2)
 ms_decode(ics1, ics2, spec_coef1, spec_coef2)
 is_decode(ics1, ics2, spec_coef1, spec_coef2)
 // main profile decoding
 // LTP decoding
 tns_decode(ics1, spec_coeff1)
 tns_decode(ics2, spec_coeff2)
 // DRC stuff
 ifilter_bank(ics1, spec_coeff1)
 ifilter_bank(ics2, spec_coeff2)
 save window shape for next frame (I thought frames were independent)
 // LTP stuff
 // SBR stuff