fredag 21. juli 2017

XM8 filters

I want the XM8 filters to be as flexible as possible. I've studied the various IR3109 based roland filter implementations and the CEM3320 ones found at Electric Druid, and found designs for state variable filters based on both.

I am thinking about having exchangable filter cassettes, possibly with two identical filters per cassette. I tried to find a common control scheme, but right now I think that just using a shift register to serially control the filters will be the most flexible - that way variations can be software controlled instead of implemented in hardware.

I also want to have two filters per voice, configured in either series or parallel - one state variable and one permanent LP.

IR3109
I've found six different implementations using the IR3109 chip or similar. The JP4/6/8, the Juno 6/60, the JX-3P and the MKS-80 all use the IR3209. The MKS-7, MKS-30 and Juno 106 use the 80017a filter chip which is basically a combination of an IR3109, two BA662 and some additional components. I have not studied the use of these in detail, just compared the resonance and control circuitry.

SH101 and MC202 also use the IR3109, I have yet to look at those.

There seems to be a progression in the circuit through the years. Exact component values varies but all except two of the implementations use the same circuit for the filter part. The resonance feedback control and the cutoff frequency control circuits are quite different between the synths.

The JP-6 and the MKS-80 stand out however. The JP-6 has a state variable filter that allows both HP, LP and BP filters - the two first 24dB and the last 12dB. It would be easy to add 12dB HP and LP too, but this has not been done. The MKS-80, weirdly enough, has the same configuration as the JP-6, but hardwired to a LP.

The JP8 and the Juno 6/60 uses the exact same filters/component values, except for a few minor differences in the resonance feedback and cutoff frequency control. The JP8 can be tapped at 12dB or 24dB whereas the Juno is 24dB only. An interesting thing to note is that the Juno 6 datasheet indicates the CV for cutoff: 0.48V/oct.

All implementations have a posistor in the control circuit to compensate for temperature variations. The JP6 and MKS-80 have a posistor in the resonance circuit too.

State variable filter
I currently consider these implementations:
- The JP-6, with the added option of 12dB HP/LP
- The Elka Synthex CEM3320 based filter
- The CEM3320 multiple identity filter
- The Wasp filter.

Permanent LP filter
- The moog ladder filter
- possibly the Juno filter, if it actually sounds different than the JP-6 filter
- Possibly a SSM2044 (SSI2144) based filter - Korg PolySix or MonoPoly
- Korg MS20
- ARP4270 (ARP 2600)
- EMS Diode ladder filter

I will start out with the JP-6 and moog ladder filter and go from there.

Interface
SVF
digital: serial input, clock, load
analog: resonance CV, cutoff CV

LP
digital, 12dB/24dB switching
analog: resonance CV, cutoff CV

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