which ground to use for the SID ?

[Michael Buchstaller]

Hello Gurus,

after completion of my monophonic SID-Synth (Monovoice), i am working on a
polyphonic version.

I want to have a controller board and 7 Voicecards.
The bus will be made from a standard 50-pin flat cable (like an internal
SCSI cable).

The Voicecards will run from an unregulated 20 VDC power supply
and incorporate 2 78xx voltage regulators to get the +5V for the SID
and bus glue logic as well as the +9 or +12V for the SID output
amplifier.
I had some trouble in the Monovoice with noiose from the digital circuitry
appearing on the SID´s audio output (8 MHz and 1 MHz clocks showing up
at a level of about 10 mV -  of course i could filter it out, but i want to prevent
this by design)
I suspect that having separate ground connection for clean (Audio) and dirty
(digital) will help. Also excessive bypassing and the separate regulators for
every voice card might help.

But, the SID chip does have only one pin to connect it to GND. Shall i connect
that to the clean or dirty GND ?
Personally, i think i should use the dirty GND, because the sound generation and
logics registers are all digital and only the output amplifier and filter are analog.
Or shall i connect it to the clean ground and risk polluting that one with the digital
noise from the 1 MHz oscillator and the switching transients from the MCU controlling
the SID ?

Has anybody experience with that ?

Or should i try to isolate the SID´s ground somehow else ? 

-Michael Buchstaller