CEFT: Difference between revisions

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== Constrained-Energy Fourier Transforms ==


=== Problems with current implementation and possible solutions ===
==== Overlapped FFT not critically sampled ====
In the current implementation, we encode 4/3 times more samples than necessary because we use 256-point FFTs with 64 samples overlap.
Ideas:
* Use an MDCT instead of the FFT
* Do extrapolation on the input and use a wider FFT. Then optimise the search only for the "real" samples
==== Non-harmonic signals (i.e. music) ====
CEFT only works on speech because most of its coding efficiency is provided by the pitch predictor.
Ideas:
* Sinusoidal prediction
* Use two (or more) pitch periods and choose one for each bin/band/whatever
* Use two (or more) pitch periods at the same time and use energy conservation to keep everything stable.
==== Sparse spectrum ====
CEFT tends to have musical noise, especially at high frequency when there are very few bits/bin.
Ideas:
* Use a "rotation matrix"
* Prediction from lower frequencies
[[Category:Opus]]

Revision as of 03:28, 26 August 2015