[Mrs Beanbag]

Originally Posted by pandy71

ABCDEF > A0B0C0D0E0F0 > (lowpass) > AaBbCcDdEeFf but AABBCCDDEEFF is also valid (albeit spectrum after upsampling will be slightly different http://www.dsprelated.com/showarticle/761.php).

But amplitude will be reduced by the zeroes. Can also do a low pass filter on the second example, that will not reduce the amplitude but should otherwise be the same.

Problem with sinc is that it is suboptimal from human perception perspective as it introduce ringing.

True, you can actually see it sometimes on images rescaled with such a technique, it is worse with a large window, and much better on smooth functions, not so great on square waves but fine for interpolating sine waves.

I have used a convolution kernel [0, -27, 0, 155, 256, 155, 0, -27, 0] to good effect for upsampling by a factor of 2.