User guide

25
Technical Details
The digital poron of the SHAPESHIFTER module is implemented in an Altera Cyclone IV Field Programmable
Gate Array (FPGA). The use of Gate Array technology rather than a general purpose programmable microcon-
troller or DSP chip allows highly ecient implementaon of fast parallel operaons.
The primary logic blocks are clocked at a rate of 25MHz. The phase accumulators of the 9 oscillators are up-
dated at this rate, as well as the nonlinear combinaon operaons. This results in very low aliasing noise for
the nonlinear operaons and for the frequency and phase modulaons. The phase accumulators are 55-bits in
width, allowing for very low frequency operaon and high quality frequency and phase modulaon.
The contents of the relavely slow wavetable EPROM are copied to fast SDRAM memory when the module is
rst powered up. The SDRAM is then clocked at a rate of 100MHz. This results in a fast read rate of the wavet-
ables - when CHORD mode is o the wavetables for each oscilllator are read at a sampling rate of 3MHz. When
chord mode is on the wavetable read rates drop to 700KHz per oscillator, which is sll high enough to minimize
aliasing noise. Linear interpolaon is performed between successive samples, providing smooth waveforms
even at low frequencies. Each wavetable sample is digitally represented with 16 bits, and each waveform in the
wavetable consists of 512 samples. There are 1024 dierent waveforms stored in the wavetable.
Most of the signal processing in the module is done at a 25MHz sampling rate. The signals are then ltered and
down-sampled to 98KHz (25Mhz/256) before being output by the Digital to Analog Converters. Some of the
signal processing (the delay line and the vocoder lter banks) is done at the lower output sampling rate. The
PULSE output is sent at the primary 25MHz sample rate, although it is passed through an analog buer which
rolls o the high frequencies.
The input signals MOD A and FM1 are converted to digital form by 24-bit Analog-to-Digital Converters running
at a sampling rate of 98KHz. The input signals PITCH1, PITCH2, INT. FM, RATIO, SHAPE1, SHAPE2 and MOD B
are converted to digital form with 12-bit Analog-to-DIgital Converters running at 25KHz sampling rates. The
SYNC input signal is sampled at 25MHz and converted directly to a 1-bit digital representaon. The FOLD and