> looks like an MDAC
It is; but unlike most MDACs it is *dB* input (yes, parallel).
> That limiter circuit is typical of the time
The circuit details are so 1970, but the overall conception seems very novel to me.
Moog/ARP were doing dB-scale VCAs but analog control.
1dB steps could be brutal for a live real-time controller. 1/2 and 1/4 dB steps could be added without more gain-stages, same as TR12 and TR13 do small-steps in one node. This also gets you to 8 bits, which suits many microcomputer sizes. If you must use a 4-pin PIC, then add a shift-register to offload your 8 bits through one pin.
As a real-time limiter, you let the gain-reduce work about like the original, selecting the UJT buzz-rate to give a decent attack rate. That will only reduce, never release. For some work I have done, this could be acceptable. Generally you will want to feed slow-gain-up pulses so gain sneaks back up. Then you also want a max gain-up (unless you want your noise-level to rise, as in speech-only AGC). And then you think about faster release for isolated transients and slower release if the signal has slammed a lot recently. Also a gain-hold function for when the signal drops into the noise (between musical movements, or pauses in a movie's sparse soundtrack). All we do with voltage processing can be done just as well with pulses. Perhaps better, because analog voltage is hard to store for many seconds, while the flops will just sit there.
> copying tracks off LPs onto cassettes. Hey, Dolby B ruled back then, right?
You must be a child of the 1970s.... you do not remember them.
Philips Cassette existed in 1970 but had not found its niches and was rarely seen. Dolby was still costly studio gear, Dolby B came even later.
I am thinking "language lab". The tapes wear out. You make working copies and keep the masters in the cupboard. The masters arrive with all different recorded levels. The lab users whine about having to use their volume control knobs (some cheap labs didn't give the users knobs). Given labor (student workers), the usual approach was to re-record all tapes at MAX clean level. Given lazy or slovenly labor, a peak-finder is a great help.
I did similar work but with music-appreciation tapes. These were usually fairly decent levels, the playbacks did have knobs, and it was one more add-on job-duty for me so I didn't really give a drat.
