The VS-1 compared to old and new analogs. Technical details

I just watched an amazing video from Abstract Instruments, the makers of the VS-1: https://www.youtube.com/watch?v=4WwXlRYw_S0&feature=youtu.be.

It compares implementation details of the OB-X, JP8, Prophet 5, Rhodes Chroma and OB-6 and gives tons of useful information. I've tried to summarise it here:

Autotune

Autotune can only do so much for high frequencies, so a good initial trim is essential (for all synths)

JP-4:
Has no autotune, relies solely on the trimmers

OB-X:

• Measures C5 (around 1kHz), does not adjust scaling, only initial tune offset.
• Measures each oscillator twice: Once to measure frequency, second to
• confirm the adjustment. Measures up to six times, if that fails it disables the voice.
• Autotune takes < 2 sec

DAC:

• 0 to 5.333V
• 5.333V / 64 notes = 83.3mV step over 5 octaves, 1V/octave
• 10 bit but only needs 6 bits to represent 5 octaves
• In addition: Common VCO frequency CV, 4 octave and 1 octave from left hand control panel for a total of 10 octaves
• Oscillator bias CV runs through 10M resistor (on a 100k for 1V/oct style summer)

P5 DAC:

• 7bits, 10.666V reference = 10 octaves, 83.3mV, 1V/octave
• 7bits fine tune for a total resolution of 14 bits

Rhodes Chroma

• 12 bit main DAC for CVs, 3 cents resolution (reasonable for the time)
• 8 bit that sets reference voltage for main DAC, skews tracking, corrects scaling errors in 0.1% intervals
• Measures the periods at 6 octave intervals for each oscillator [error, should be 6 something else??]
• Calculates scaling bias (done in 8 bit dac). Also calculates initial tune offset added to main DAC

Prophet 5

• Rev 1 & 2:
• Four checks: C3, C4, C5, C6
• Bias CV resolution is about 1 cent with 128 possible values, about 1 semitone range
• 7 bit bias CV dac mixed through a 10M resistor (on a 100k for 1V/oct style summer)
• 40 autotune measurements (4 points x 10 CVs). Takes a long time. ( > 10 secs)
• Bias CV 1 is used for bottom up to 1/2 oct above C3
• Bias CV 2 C4 +/- 0.5 octave
• Bias CV 3 C5 +/- 0.5 octave
• Bias CV 4 C6 - 0.5 octave and up

Rev 3:

• 7 checks per osc: C3-C9
• 14 bits bias CV added as parts of Key CV, resolution is 650uV or 0.8 cents
• Bias for C0 to C2 is calculated from the others as it takes too long to measure ( > 60 secs)

Jupiter 8

• 10V output, 3cents/step for 12 bit, < 0.8 cents for 14 bit
• Measures C3 and C8, uses a formula to calculate a bias CV for each key

Sequential OB-6

• samples several points and deriving a high resolution correction curve
• temp sensor, saves temp profiles
• can recall profiles, does not do any real time calibration per se
• has "slop" settings to introduce variation

Analog voices

OB-6:

• No trimmers, everything is done through CV
• Sub osc on VCO1
• Can mix waveforms using 2164 VCA
• Taps all poles of HP/BP/LP filter, mixable using 2164
• 2164 for resonance control
• 2164 for panning
• Splits after voice mix to dry and fx which is mixed afterwards

VS-1:

• 2164 panners
• Bi-timbral
• 4 input analog chorus
• Analog polyphonic glide

Digital control

• OB-X: 2.5MHz MCU
• OB-6: 32bit PIC at 200MHz
• VS-1: 32bit ARM at 600MHz

Reading/updating CV:

Loop times:

• OB-X: 14-19ms
• P5: 9-11ms
• JP8: 3-6ms

OB-X

• Pots are scanned using a DAC and a comparator (successive approximation)
• 10bit CVs, 10 iterations per pot
• 1-6ms to scan pots (19 on OB-X)
• Scans pots then updates CVs, thus updating CVs less frequently when loop time is long
• More complex sample and hold circuit for pitch CV (check, possibly just shifting voltages?). Low leakage polystyrene caps for pitch CV assures stability between updates.

OB-6

• 12bit ADC over SPI to scan pots and external CV input, pitch and mod wheels. Scanned 256 times per second.
• Updates CVs at 24kHz - must be fast to update software LFOs and envelopes fast enough
• 24kHz gives sub 1-ms attack times for 0 to 5v envelopes (looks like 2.5v to me in oscilloscope pics)
• Smooth LFO rates up to 500Hz
• Looking at the oscilloscope screenshots, a full attack takes around 0.4ms-0.5ms (each grid line is 0.2ms), at approx 5 samples per 0.2ms (= 24kHz), giving 10-13 samples for attack.
• Uses pair of 8ch 16 bit dacs for 120 CVs
• Uses independent CVs per voice instead of common CVs, to be able to add offset biases on a per voices basis, it has NO trimmers!
• 4051 multiplexers for sample & hold
• S&H caps on mainboard, S&H opamps on voice cards
• Separate microcontroller for CV updating, tables in memory for calibration
• Core logic runs at 1.2V
• Samples VCO and filter waveforms using 24bit ADCs, AKM 24bit stereo codecs

VS-1

• CVs updated at 48kHz
• Single 8 ch DAC
• 85 CVs
• Single ADC to scan the endless pots, scan rate is 1kHz

Cross modulation on JP8, JP6 and JX-8P

I was trying to find info about how the JP8 crossmod works last night, but ended up answering a post about it instead. Here is what I wrote:

TL;DR:

JP8 and JP6 crossmod is exponential FM of DCO 1 by DCO 2, JX-8P is amplitude modulation (ring modulation) of DCO 2 by DCO 1 (plus sync if wanted).

Details:

All of this should be fairly easy to figure out from the service manuals for the various synths.

Right now I'm looking at the JP8 service manual. Here, the output from VCO 2 - tapped after the waveform selector - is fed back to the exponential (v/oct) input of VCO 1. There is nothing going into the reference current input of VCO 1 (the circuit around op amp 1A on page 12 of the service manual), so the JP 8 has no linear FM.

The signal from VCO 2 goes through IC18A, which is a BA662 OTA, this acts as a voltage controlled amplifier/attenuator, and this is what is controlled by the panel potentiometer.

The same is the case for the JP-6, although here the signal is fed through a CEM3360 VCA. It is still sent back to the exponential input of VCO 2. Nothing is fed into pin 13 of the CEM3340 VCO so the JP-6 has no linear FM either.

The JX-8P is completely different. It doesn't have VCOs, it has DCOs, meaning the frequency is digitally controlled. It does NOT let the output of DCO 2 control the frequency of DCO 1, neither exponentially nor linearly. Instead, when crossmod is enabled (pos 1 or 2 on the PG-800 switch), the output of DCO 1 controls the amplitude (volume) of DCO 2, which I guess means that the output of DCO 2 is now DCO 1 times DCO 2, or exactly what is described as ring modulation in figure 2 here (given that it follows the gain requirements stated): https://www.keyboardmag.com/gear/on-synthesizers-amplitude-and-ring-modulation

PS: This means that, unlike what OP says about crossmod on the JX-8P, DCO 1 does NOT modulate the FREQUENCY of DCO 2, it modulates the AMPLITUDE.

In the circuit diagram the control for this part of the X-mod is labeled 'Metal', so it is very possible that this has the same effect as 'Metal' on the JX-3p, which is also described as a ring modulator type of sound. Interestingly enough, I cannot find any similar circuit on the JX-3p.

The JX-8P (and JX-3P) does also have a sync input on DCO 1 going to Q18, which is the transistor that resets the sawtooth of DCO 1 (i.e. sets the frequency). Sync comes from the same input that resets DCO 2, so whenever DCO 2 resets, so does DCO 1 (which means this is just normal hard sync I guess).

The post I answered can be read here: https://music.stackexchange.com/questions/57210/what-exactly-is-the-cross-modulation-on-roland-jupiter-and-jx-analog-synthesiz

Coming soon: the Xonik Waveshaper

On my breadboard today: The Xonik Waveshaper - insert a non-centered 0-10V saw wave from the Xonik DCO and you'll get the following (centered) waves:

- Saw
- Inverted saw
- Triangle
- Sine
- Pulse/square with VC-PWM and VC amplitude (no VCA needed)
- Sub oscillator with square -1oct, square -2 oct, saw -1 oct and saw -2 oct.

The current triangle/sine circuit is based on the Jupiter 8 and Yusynth modular, the sub oscillator is a simplified version of the Xonik Sub oscillator. The pulse circuit amplitude control idea (but not circuit) is lifted from the Juno.

Jupiter 8 X-mod

The Jupiter 8 has two oscillator, and the output of oscillator 2 can frequency modulate oscillator 1.

Oscillator 1 has four waveforms:
- Saw
- Triangle
- Pulse
- Square

I am not sure why both pulse and sqare are present as only one can be connected to the output at the time.

Oscillator 2 has these outputs:
- Sine - which replaces the saw form osc 1
- Triangle
- Pulse
- Noise

The x-mod input is tapped after the oscillator 2 waveform selector, to any of the osc 2 waveforms may modulate osc 1. X-mod has its own VCA The X-mod output (a current, as it is the output of an OTA) is mixed with the rest of the linear CV input, so x-mod is linear.