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Topic: Hybrid filtering in PS AAC (Read 2023 times) previous topic - next topic
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Hybrid filtering in PS AAC

Hi all,

I have some trouble understanding Hybrid filtering as used in PS AAC codec.

Output of the hybrid filterbank in case of 20 bands isn't as nice as in 34 bands case! After filtering, some recombination needs to be done.

Hybrid analysis as shown in the Figure 8.20, page 51 (ISO14496-3 amd 2) is somewhat unclear to me.

Let's look at the first filter. Hybrid filtering of the first qmf band gives 8 new bands, which are then recombined to form 6 output bands (labeled as 's' in Figure 8.20). How is the recombination done?

Is this ok:

recombined[0] = hybrid[6]
recombined[1] = hybrid[7]
recombined[2] = hybrid[0]
recombined[3] = hybrid[1]
recombined[4] = hybrid[2] + hybrid[5]
recombined[5] = hybrid[3] + hybrid[4],

or is this ok:

recombined[0] = hybrid[0]
recombined[1] = hybrid[1]
recombined[2] = hybrid[2] + hybrid[5]
recombined[3] = hybrid[3] + hybrid[4]
recombined[4] = hybrid[6]
recombined[5] = hybrid[7]

?

Thanks,
Daniel