The DW-6000, DW-8000 and EX-8000 were Korg’s first synthesizers to use completely digital oscillators. The DWGS system generates 16 phase accumulator oscillators with oversampled waveform data to obtain reasonably good sound quality from simple hardware. Contrary to what’s often reported, the DVP-1, DSS-1 and DSM-1 have nothing to do with DWGS.

The DWGS oscillators are based around two simple custom gate arrays, MB62H133 and MB64H129.

DWGS

The master clock is a 6.4 MHz VCO. This is modulated for pitch bend, vibrato and tuning. The variable clock means that pitch modulation doesn’t introduce aliasing, but it can only modulate all oscillators globally. The oscillator tuning parameters and the pitch envelope in the DW-8000 work instead by changing the oscillator’s frequency register. There’s a single time multiplexed 8 bit DAC (DAC08) that operates at 800 kHz (8 clocks per oscillator). The sample rate is 50 kHz (128 clocks per sample) and the interpolation is truncation.

MB62H133: This implements a 20 bit phase accumulator. Each oscillator has its own 20 bit frequency and phase registers, stored externally in 2 banks of 5 74S189 16 x 4 bit SRAM ICs (640 bits total). The MB62H133 IC contains the CPU interface that allows the oscillator frequencies to be written to the frequency registers, a 20 bit adder, and circuits to generate various timing signals like the control signals for the waveform DAC demultiplexer. It has 64 pins and contains about 800 gates.

MB64H129: This has 40 pins and about 400 gates. The input is the upper 12 bits of the oscillator phase data from the MB62H133, plus octave data to select the correct waveform multisample. The waveform may be up to 4096 samples per period, with no fractional bits for interpolation. The multisampled waveforms are different lengths for each octave, so the bits of the phase data are masked in order to play the correct address range. The output of this IC addresses the waveform ROM, and the waveform ROM output goes directly to the waveform DAC (DAC08). The MB64H129 can address a waveform ROM up to 64k x 8. The DW-8000 uses 128k x 8, so some external hardware is added to handle that.

Waveform multisample lengths:

  • octave 1: 2048 points, 320 harmonics
  • octave 2: 2048 points, 160 harmonics
  • octave 3: 1024 points, 80 harmonics
  • octave 4: 1024 points, 40 harmonics
  • octave 5: 512 points, 20 harmonics
  • octave 6: 512 points, 10 harmonics
  • octave 7: 512 points, 5 harmonics
  • octave 8: 512 points, 2 harmonics

Depending on the octave, the waveform data is oversampled by a factor of at least 3.2. Compared to critically sampled waveform data, this means that the high frequency images that result from truncation are considerably reduced and cause less audible aliasing, but this comes at the expense of a relatively large waveform ROM. The complete set of multisamples for each waveform is 8192 x 8, which is much larger than most similar synthesizers. The DWGS models are also somewhat dull sounding. At the low end of the range for each multisample, the harmonics only extend to about 10.5 kHz.

The waveform ROM stores the full waveform period. The waveforms are all odd functions (summations of sines). The ROM size could have been reduced by half with minimal modifications to the MB64H129 by resynthesizing the waveforms as even functions (summations of cosines) and inverting the phase accumulator data for the second half of the waveform period (this would add only a small number of gates to the MB64H129). It could also be done with odd functions by using a sign-magnitude DAC that additionally inverts the DAC’s output for the second half of the waveform period. If a sign-magnitude DAC isn’t used, a two’s complement negation before the DAC of the second half of the waveform period is also possible, but this would require more ICs.

As far as the digital oscillator portion goes, the DW-6000 and DW-8000 are very similar. The hardware supports 16 oscillators, and the DW-6000’s oscillator design carries over mostly unchanged to the DW-8000. However there are many other differences, including different PCBs, CPUs, features, waveforms, etc. The DW-6000 also has the ability to use external waveform ROMs, although this section of the PCB (marked “I/O Block”) is left unpopulated. It’s omitted from the DW-8000.

Both the DW-6000 and DW-8000 use software to generate all control voltages and modulation, but there are significant differences. The CV DAC in the DW-6000 is 8 bit (BA9201). The DW-8000 also uses an 8 bit DAC (HA17008), but adds a 4 bit resistor array DAC that feeds into the main DAC’s reference voltage pin. The 8 bit DAC’s output is multiplied by the 4 bit DAC’s output. This is not equivalent to a 12 bit linear DAC. I suspect that only one bit of the 4 bit DAC is high at a time. In this case, it’s a floating point DAC with an 8 bit mantissa and 4 step exponent, 11 bits total dynamic range. To make up for its CV DAC’s low resolution, DW-6000 also uses 3 BX7114 custom thick film hybrid ICs in the control voltage demultiplexing circuit. These are made by Rohm and are apparently marked BX7200 on the package. They act as sample and holds, and also smooth the VCF and VCA envelopes. The other control voltages, for the master clock frequency, oscillator levels, noise level and resonance, use a conventional sample and hold with simple first order smoothing. The BX7114 isn’t described at all in the service manual, but it seems to implement a more sophisticated kind of smoothing that eliminates audible stepping while still preserving fast transients. Various companies like Sequential Circuits, PPG, Casio and Wersi sometimes used RC smoothing circuits with added diodes so that fast transients can pass through unchanged. It’s possible that this works in a similar way. The BX7114 ICs have 10 pins and process 4 control voltages each. The DW-8000 uses conventional sample and holds for all control voltages, with simple first order smoothing on the filter cutoff and VCA control voltages only. The envelope shapes are also different, piecewise linear in the DW-6000 and exponential in the DW-8000.