[sdiy] Prophet VS a phase accumulator design?

[tu at alphalink.com.au]

On Fri, Feb 4th, 2011 at 9:20 AM, Scott Nordlund <gsn10 at hotmail.com> wrote:

> > My guess would be they're using a phase accumulator, just the like PPG
> > does to play back samples (remember the wave 2.2 and 2.3 could
> > playback samples recorded using the Waveterm).
> 
> I dunno, I just vaguely remember them advertising that 
> the Emax could transpose over 8 octaves without aliasing, 
> or something like that, and I've lately been somewhat 
> obsessed with the "best of both worlds" combination of 
> zero order hold harmonic images on the low end and no
> aliasing on the high end.

I believe you are thinking of the Emax II, which used Emu's G-chip to transpose over a 10 octave 
range (+/-5 octaves). The G chip had a fixed output sampling rate and allowed 32 channels of 
sample playback. It was also used in the Emu proteus line and EIIIX but I believe later samplers 
had an improved version. Assuming the G-chip matched with the Emu patent on its technology, it 
used 32 times oversampling with a polyphase FIR and then linear interpolation on the oversampled 
signal to fill the gaps. Upward transposition was not as alias and distortion free as the advertising 
claimed but still an improvement over many other samplers of the day. 

The earlier Emax used the E-chip to perform add/drop sample transposition at a fixed rate around 
1MHz, but bespite this had only very limited upward transposition. At the 41.6k sampling rate it 
could only shift up a few semitones but could transpose down a long way. Because of the output 
rate exceeded the sampling rate by 20 times or more, the result was more similar to using a 
variable sampling rate but with considerable sideband distortion at non integer intervals. Given that 
the samples were compressed down to 8 bit it probably was not so noticeable anyway. 

/Tristan