Elektron Musical Instruments

> what does the term "phase accumulated" mean? 

Well, phase accumulated generally refers to the fact that an
index into a wavetable is used to play back a sample.  For a 
given frequency, there will be a delta which is added at some
regular time interval (e.g. 1/44100 s) to the current index into
the wavetable, and that sample is played through a D/A conv.

In most typical wavetable players (e.g. mod players, many non- analog
emulation synths and more), some extra computation/dsp work is performed
on the sample data.  This is to account for many things which are
just barely on the edge of my understanding.  But, basically, it has
to do with playing back wavetables at too slow of a rate.  For example, a 
waveform could have overtones in it that go above the Nyquist
rate when hte sample is played at a sufficient pitch.  Therefore, you've
go to filter them out, or suffer aliasing.

However, people often describe the sid as "phase-accumulated" instead of
wavetable in order to refer to the SID's superior synthesis method.  What
is so special about it?  Nothing, except two things: 1) the waveforms
are not stored in memory somewhere.  They are generated based on the
index (I believe it's a 12-bit accumulator?) and sent to D/A converters.
That alone is not such a big deal, it just means the SID doesn't require
external waveform memory, however, when you combine it with the other
superiority, it's important.  

2) The waveform generators in the SID are
generated (I believe) at 1MHz.  (Someone please correct me if I goof
on the exact value here )  Therefore, even the highest frequency sounds
that you're going to generate will not generate neigh imperceptible
amounts of aliasinhg distortion.

e.g. a 20KHz Sawtooth wave, which has a frequency spectrum roughly
equal to 1/h, which h is the harmonic number.  The Nyquist rate for a 
1MHz plyaback rate is 500KHz.  500KHz/20Khz=25.  The magnitude of that 
frequency, and those beyond it, is between 3 and 4 % of the fundamental's
amplitude.

Hope I got that right,
-N


> i have seen it in reference to the Sidstation's oscillators but
> have no idea what it refers to. anyone? 
> cheers,
> wendell.
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