Analog Delta-Sigma interpolation DAC

[kokoriantz]

20 samples didn't make anything better, the image is at -28db. It looks the 30 samples is the best.
20samples.JPG

[Hans Polak]

With 30 point plus sophisticated post interpolation you have found a very nice optimum.
You can’t escape physics, when you improve on one aspect, you wil lose somewhere else.

Hans

[kokoriantz]

I will try to interpolate the midpoints of the derivatives, by this the sinc ringingings get subjected to delta sigma filter. After all I am interpolating the cubic spline of the derivatives and no more that of the data.
I will also use the heterodyne to add instead of filtering the 68k-88khz images.
There is still lot to do.

[Hans Polak]

I tried a 36point Blackman Harris window while not using the outer 4 points because they were practically zero, so filter length stayed still the same as before.
Below, at the left Blackman Harris in Red t and the Hamming with 0,45% gain at the right in Blue.

FR is now a bit smoother around 20Khz, but stopband is much deeper that the Hamming window with 0,45% gain.
Suppression at 24.1Khz is a bit less, but to be realistic, at this frequency the spectral content of music is quite a bit lower, see second image.

Hans
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