To: "'\[AH\] Analogue Heaven'" analogue@hyperreal.org,
From: "Caloroso, Michael E" CalorosoME@corning.com,
cc:
Subject: [AH] Moog ladder filter on a chip (was: SSM)
Date: Mon, 11 Oct 1999 19:35:11 -0400

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The only bona fide Moog transister ladder filter on a chip that was ever fabricated is...

The National Instruments DM7670, better known as the 71 microchips under each key in the Polymoog. Yes, each IC contained a waveshaper, a mixer, an envelope generator, two VCAs, and a *transistor ladder filter*.

Don't believe me? It's buried in the text on the first page of the Modulator Card section of the service manual.

...don't get too excited though:

This was the "brightness" filter whose cutoff was controlled by the presets; a front panel control was available on the prototypes and eliminated from the production models :( There is no feedback provision for resonance control :(

The VCA EG was hardwired to the filter, so if the cutoff was low enough the filter would open in synchrony with the EG. The EG level was also fixed.

These filters are separate from the VCF section of the Polymoog; while there was a brightness filter per key, the sole VCF with variable resonance encompassed the entire sound chain.

Now if only the brightness filter wasn't so crippled and had a separate EG with variable resonance; the Polymoog probably would've been more respectable today. So why didn't Moog go the distance? Back in 1974-5 during the development of the Polymoog, custom microchips were very expensive and Moog was limited to a fourteen pin package within their budget. After you assign two or three pins to the power rails, that doesn't leave a lot for a fully polyphonic synthesizer.

MC

"And now for something completely different... a man with a Moog IIIc up his nose"

Opinions (and mistakes) expressed herein are my own and not those of my employer.