[sdiy] Unrolling the OBX voice card

[Steve Lenham]

From: "Antonio Tuzzi" <antoniotuzzi at gmail.com>:

> I decided this time to create a monosynth from my obx card...
> Yep I know is not an easy task, many control should be created from
> scratch (lfo, pots voltages.)
> But with (our)your help the task would be possible (! )
>
> to start this, here are the connection with codes present on the card.

OK, I'll bite (having recently worked on somebody else's OBX). BTW, have you 
got the OBX service manual? I think it's on the web somewhere.

> a3 FT1A ? frequency on VCO1
> a1 FT1B ? frequency on VCO2

These are the autotune correction voltage inputs. They are effectively a 
second CV in with a restricted range.

> b2 CMOD cross modulation

This is a digital enable signal - on (+5V) or off (0V).

> c3 GATEIN gate in ?5v

Yes, a +5V logic signal.

> c1 CVIN control voltage in ?0 -> 5v

This is the CV from the key assigner. Range is 0 to +5V.

> d10 VCO1F ? Vco 1 frequency
> d9 VCO2F ? Vco 2 frequency

These are the modulation CVs for each oscillator, used for transposing, 
vibrato, pich bend, etc. Sensitivity is 1V/oct and these can swing up to 1V 
negative in the OBX for downwards pitch bend.

> d6 VCO2L ?

This is a +5V logic signal which, when high, attenuates the OSC2 signal. It 
is used in the OBX for the HALF OSC2 option.

> d5 VCFP ? VCF frequency

This is the external VCF footpedal input. Not sure of span but looks like it 
should be 0 to +5V.

> d1 FSUS VCF ADSR sustain ?0 -> +5v
>
> e10 FRLS VCF ADSR release ?0 -> -5v
> e9 FDCY VCF ADSR decay ?0 -> -5v
> e8 FATK VCF ADSR attack ?0 -> -5v

Yes, the attack, decay and release voltages are 0 to -5V (MINUS 5V) and the 
sustain voltages are 0 to +5V (PLUS 5V).

> e6 VCFM VCF frequency modulation

This sets the amount of VCF envelope modulation. Range is 0 to +5V.

> e5 VCFF ? VCF frequency fine

This is the main VCF fequency input. On the OBX it already includes any LFO 
modulation, etc. Can go slightly negative.

> f4 -19V -19 volts (a +15 -15 linear psu can be directly used)

The cards have onboard +/-15V regulators, so > +/-17V is required unless you 
link them out.

Hope this helps - best of luck!

Steve L.