[sdiy] Prophet VS a phase accumulator design

[karl dalen]

tu at alphalink.com.au <tu at alphalink.com.au>:

> I would be interested to know what sort of 
> transposition was used by the Akai samplers, such as the
> S1000 and later. The Akai sound was 
> clean and bright while still being able to shift upward by
> large intervals without audible aliasing.

Saw somewhere the S1000 was based on MOT 56xxx DSP or
was there also custom ASIC for the sample replay?
 
> The K2000 has an excellent interpolator for transposing
> samples downward, or upward to the 48k 
> system sampling rate, but there is no decimator! The
> maximum playback rate supported is 96k, so 
> you can shift a 48k sample up an octave, but with that you
> get considerable aliasing distortion. I 
> tested this once using a 20kHz sinewave sampled at 48k and
> a scope. As I went up the scale the 
> sinewave went to nyquist at 24k and then the frequency
> started descending again. The K2500 
> extended the upward transposition to two octaves at 48k
> using the "skip sampling" mode. I believe 
> this mode did some crude filtering to reduce the aliasing
> from skipping samples but was 
> acknowledged in the manual as having reduced fidelity.

The ordinary saw/sin oscs seams to be simple counters, 
they aliais terribly just as one enters a couple of Khz.

The hard sync mode in one of the algos are more less
useless due to the aliasing.