[sdiy] Buchla 295 10-band comb filter topology

[Mike Bryant]

What I did was put a 4*4 multiplier in a 256x9 ROM with a carry bit and addressed it through some 4/8/12 bit shifters multiple times.  Saves a lot of single bit shifting.

Whatever happened to x9 memories - used to be commonplace back then.
________________________________
From: Donald Tillman <don at till.com>
Sent: 25 November 2022 21:36
To: brianw <brianw at audiobanshee.com>
Cc: Mike Bryant <mbryant at futurehorizons.com>; synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Buchla 295 10-band comb filter topology

The AMD2901 was a general purpose ALU slice with a set of simple operations.  It couldn't do multiplication by itself.  You had to use a lot of shift and add microcoded cycles for a multiply operation; which ends up taking a very long time.

What you need is a circuit that can do a 16x16 multiply and accumulate in a single fast cycle.

The TRW MPY-16 chip came out around 1978 and could do that in about 200nS for $300.00 and a significant amount of support circuitry.  (From memory, don't quote me.)

Before that you would have had to use an awful lot of gates and MSI chips.

  -- Don
--
Donald Tillman, Palo Alto, California
https://www.till.com

On Nov 25, 2022, at 11:53 AM, brianw <brianw at audiobanshee.com<mailto:brianw at audiobanshee.com>> wrote:

Wow! A quick search for AMD2901A mentions floating point processing. I'm impressed for '78.

I couldn't find a data sheet. Most of the search hits seemed to land on some other part.

The TMS320 DSP came out in 1983. Fixed-point DSP can work quite well, if you don't mind coefficient quantization moving your filter knee at bit away from ideal. Some older "DSP" gear just has discrete math chips in TTL or equivalent, with an A/D and D/A around it and a slow CPU to change controls signals without actually processing the audio itself.


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