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