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Navigation
Demystifying the PPG wave 2.2 & 2.3
Thanks to MiK for his knowledge and his FLUKE PM3394B 200 MHz
Combiscope.
PPG wave 2.2 = 8 bit and wave 2.3 = 12 bit?
The PPG wave 2.2 with the original wave 2.2 analog boards (named "OF
4"") have AD558 DACs per voice. These are 8 bit DACs. The PPG wave
2.3 uses the analog boards named "OF 4 12 bit" with the 12 bit AD7545
DACs per voice. The wavetables are not stored in a higher resolution
(bit depth) on the PPG wave 2.3. So the wavetable sounds are played
back by using the upper 8 bit of the 12 bit DACs. So the playback is
still 8 bit for wavetables. Therefor the digital part is PPG wave 2.2
alike.
The full 12 bits are only used to playback samples imported via the
PPG waveterm B or the PPG PRK-FD keyboard (PPG PRK with built in 5 1/
4" disk drive). The waveterm A sends sample data to the PPG wave 2.2
in just 8 bit depth.
There might be differences on the code for the envelopes and
different VCF calibration when comparing the PPG wave 2.2 against the
later wave 2.3.
PPG waves 2.2 & 2.3 oscillators of one voice are never mixed!
PPG wave 2.3 signal of a voice DAC PPG wave 2.3 signal of a
voice DAC zoomed
Two OSCs playing triangle wave with same as left, but zoomed
only a little detune
Notice the rapid switching from one to another oscillator sample word
Yes, the main and the so called sub oscillator of a PPG wave 2.2/3
voice are never mixed or added! The digital data of each oscillator
of a voice is sent to the DAC one after another in an interleaved
way. So there is one sample word of OSC 1 and then one of the OSC 2.
And every oscillators got a sample rate of about 183.8 kHz (= 25 MHz
quartz /8 /17 ticks of PAL cycle). But the DAC gets data twice that
often (one value for OSC 1 and then one value for OSC 2) at about
367.6 kHz. And due to the nature of our ear and the very, very high
sample rate (CD = 44.1 kHz vs. PPG 367.6 kHz), we hear the two
oscillators like being mixed and not interlaced. We kind of hear just
the smoothed integral of the DAC output. But we get plenty of high
pitched aliasing beyond our hearing range using this strange method.
The individual eight voices are mixed to the output in the analog
domain.
PPG wave 2.3 PAL20x10_Output with PPG wave 2.3 PAL20x10_Output with
SW:3 (one OSC) SW:0 (two OSC)
OSC 2 for all 8 voices set to off OSC 2 for all 8 voices set to on
(SW:3 in Digital page) (SW:0 in Digital page)
Notice the double width pulse in the upper channel of the scope
Some signals from the PAL 20x10 (Programmable Array Logic) of the
Proz-Board (top to bottom):
* voice select A (bit 0)
* voice select B (bit 1)
* DAC enable (one pulse for each oscillator of all voices)
* global DAC enable (active for 16 ticks during the whole update
cycle, pause of one tick)
Voice select C left away due to only 4 scope channels available, it's
the bit 2 of the binary counting sequence seen for A and B already.
Goes low-high-low-high, so one full update is for OSC1 of all 8
voices, followed by OSC2 of all voices, followed by the pause of one
tick.
Due to a slight pause between the switching of the PAL between
selecting OSC 2 to OSC 1, the second oscillator sample word is hold 1
/8th longer as the sample word of the first oscillator. The PAL 20x10
triggers the 8 first oscillators of a voice (8 "ticks"), then the the
8 second oscillators (another 8 "ticks") and then "stops" for one
"tick". So there are 17 "clicks" together in one cycle of updating
the audio of all eight voices. This cycle is repeated 183.8 kHz,
hence the sample rate per oscillator (and twice per DAC). The values
for the second oscillators are hold at the DAC output for one "tick"
longer as the main oscillators value. What this does cause
mathematically to the output in our hearing range is beyond my
knowledge.
The measurements were taken on a PPG wave 2.3 running in the wave 2.2
mode.
PPG waves 2.3 voice update rate
PPG wave 2.3 CV multiplexer switch
The multiplexer for the CVs of the VCF, VCF resonance and VCA are
updated every 38,4 ms (about 26 Hz). This is rather lame compared to
todays synths. But never less, the PPG does not sound bad. It got
character!
Disclamer
PPG wave 2.3 with 4 scope test probes
No PPG wave was harmed during these measurements. No shortcut was
produced. No pin bent. I asure the reader, this PPG wave 2.3 was
alive and playing well before and after. And only CO[2] free
generated power was used.
Don't try this at home, unless you know what you are doing!
Link for additional informations
* www.tauntek.com/PPGWav23.htm by R. Grieb
* www.virtual-music.at/en/ppg-zone/#4 by Virtual Music e.U. - Vintage
Synthesizer Service in Vienna