[sdiy] Niche ACM-1 how's this thing work?

brianw brianw at audiobanshee.com
Tue Mar 21 09:07:07 CET 2023

I had one of these Niche rack units back before my studio had much in it, and never got around to using it. The original was stolen from storage, so I bought another. I can't ever remember getting either one to work! It's supposed to do MIDI learn when you hold the button, but that never seemed to work.

I also tried to take a look inside, and found the same big chip with the part number scratched out. That's such an old-school tactic for IP protection, although they surely didn't make the chip themselves so I don't quite understand how they could consider it Intellectual Property. I think Sweetwater were unloading them for $80/ea. Somewhere in my notes, I have the CPU and maybe even ROM contents.

They must be built around an 8-channel DPOT, like you're guessing. I don't see why it would be an MDAC, as others are saying, because there's no DAC. The signal stays in the analog domain.

By the way, there would be distortion when changing volume, due to the stepped levels of the digital attenuation. If I could get mine working, I'd be curious to see whether they implemented 14-bit MIDI CC resolution, just 7-bit, or somewhere between. The manual says "16-bit" so I wonder how they get that out of 14-bit CC messages. Interpolation is possible, but MIDI doesn't have a sample rate, so interpolation would be a guess, at best.

The manual says 95 dB S/N and "virtually flat" from 30 Hz to 30 kHz. It lists the maximum input signal level as +20 dBu, so I'd better not run my 8-channel audio interface into it at full blast. Then again, my audio interface has DPOTs on every channel, so it'd be rather redundant. Lots of professional mixers can output well over +20 dBu, but I suppose you could just take it easy on the levels.

If I can find mine, I should dust it off and implement a SysEx program so I can skip past the MIDI Learn obstacle that made me give up last time.

Brian Willoughby

On Mar 20, 2023, at 6:55 AM, mark verbos via Synth-diy <synth-diy at synth-diy.org> wrote:
> Those of us as old as I am might remember this thing from the 90s. It is patched into the inserts on a mixer and then adds MIDI controlled automation. All the chatter around is about how it is passive, so it has “infinite” headroom, 0% distortion, 0% gain. I got one recently for a low price. I looked inside, eager to know the trick…. Vactrols? Motorized pots? something I don’t even know about? Inside the is a big chip 28pins or something with the label scratched off. Does anyone know what is actually going on in this thing? Is that an 8 channel digitally controlled pot? Just curious.
> Mark

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