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--- atrac3p:final_band_combination_imdct_fir_filtering [2009/11/14 22:25] – created megadiscman
+++ atrac3p:final_band_combination_imdct_fir_filtering [2009/11/14 23:03] (current) – megadiscman
@@ Line 1: / Line 1: @@
 ===== Final band combination and FIR filtering =====
-This step is completely analogous to MPEG audio: Samples from the 16 bands are transformed via a standard type-IV-DCT, and the resulting 16 samples are sent through an FIR filter. While the MPEG FIR filter (also called the polyphase filterbank) is in fact 32 FIR filters (MPEG has 32 bands) with 16 taps each, ATRAC3+ uses 16 FIR filters with 12 taps each. The filter coefficients for the ATRAC filter bank are given in [[polyphase coefficents]].
+Samples from the 16 bands are transformed via a standard type-IV-DCT, and the resulting 16 samples are grouped into 2 8-sample blocks.
+The FIR filter operates on the current and 11 most recent sets of results for the first block and on the 12 most recent sets of results for the second block.
+  * The first eight output samples are obtained from the sum of sending both blocks through an FIR filter bank, with the first block reversed in order, so the 1st output sample depends on the last sample of the first block and the first sample of the second block.
+  * The second eight output samples are obtained from the sum of sending both blocks through another FIR filter bank, this time reversing the second block.
+The coefficients for the filter bank are given on [[polyphase coefficents]]