Home
Search
Links
Manuals
Rare Analog
Synth DIY

Site Intro
 

Overview
VCO's
Filters, TGs
Microprocessor
Console
Keyboard
Sequencer

 Email me

Created by
 Western Graphics
Last modified
 08 February, 2006
 

Copyright Kevin Lightner, used with permission.   Manuals * Audity Brochure 1980        188 KB * Audity Voice      Architecture 1.2 MB

Audity Zero Project - Filters

Circuit Description  An initial review of the Voice Card schematics reveals a number of interesting issues:

• The LFO is based around a SSM2030, a very simple design with no temperature compensation.

• The LPF and HPF are stock SSM2040 designs, the same as on the SSM Voice Card - therefore east to replicate.

• The Resonant Filter uses a SSM2020 like the Modular version, but it has voltage controlled Q. Easy to clone.

• There are 5 software controlled Lag Processors that delay some parameters by 4ms to 4 seconds. These were used to cross fade between voices. As there are no hardware controls, and only one voice, they will be omitted.

• There are hardare delay controls on the filter TG's, but no hardware in the voice card to delay the SSM 2055 TG's, so they must have been software features, especially as each TG has its own trigger and gate. I will not implement them.

• There are 16 8-bit DAC's per voice (gulp!), 15 are used as Digital Controlled Attenuators for the control of module audio signal paths (sometimes exponential). The 16th drives a set of 16 sample and holds for voltage control of the SSM chips. Whilst expensive, I will keep with this design.

• The noise generator uses the MM5837 digital noise chip - tricky to locate and a poor choice that has an audible repetition! I will use the E-mu SYstems digital noise source. The Pink Noise is created using a very nice -3dB filter design - nice one Dave!

• The next step is to put the schematics into PCB CAD software and design a number of boards, unlike the massive single voice card of the original which is not practical.

Detailed documentation on the CPU and console sections of the Audity are not available, however the advert from 1980 provides enough information on the console layout. So the electronics are easily replicated.

• A decision on using a single powerful PIC programmer has been made - and I will try this idea out on the AM3372 module (Programmable Filter based on the Matrix 12 CEM3372).