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The attached patch implements sinc based resampling in rockbox.
I have only tested it stand-alone so far (i.e. the code outside of rockbox – I am waiting for a replacement for my broken android phone to test it there).
There is (of course) quite some room for improvement.
Things which could be improved:
- buffer handling
- calculate the correct sinc spanning the whole table for downsampling. Then the downsampling and upsampling code would be the same (no need for sinc_increment, saving some additions). Also the result would be more accurate resampling.
- Save some memory by always calculating the correct sinc and putting it into resample_data. Would need to use fp_sincos() for that.
- Save some more memory by taking advantage of sinc's symmetry, only storing half of it in the table.

I know that I'm breaking the coding-style by using more than 80 chars in some lines. I will fix that when I have time. I wanted to get this code finally out to get some feedback after it sitting for two weeks (basically unchanged) on my HD.

This patch changes some of the settings of the AS3515 to highest audio quality, similar to  FS#11304  but for AMSv1.

AUDIOSET2_IBR_DAC_50 → AUDIOSET2_IBR_DAC_0

0% reduction (instead of 50%) of DAC bias. This will probably increase power consumption.

Not set AUDIOSET3_IBR_HPH.

Increases HP amp bias. Recommended by the data sheet for 16Ohm loads (and will probably reduce THD).

Would be great if these settings could be changed during audio playback to listen for any actual changes in quality :)