Hi all - Dave here.
It's been about 40 years since I was at Texar as Senior Design Engineer. In my old age, I am reliving memories of my favorite days spent designing audio processors and twisting on the knobs of the best stations in the world. I thought I would try to replicate the unique signature sound of the Audio Prism in Stereo Tool using what I know from back in the day about the implementation constants and laws that governed the dynamics. I have a few physical prisms on the bench which I used to compare and contrast against this file, and I *think* I have it pretty close - or close enough for some feedback from those who remember and appreciate that box from years ago.
A lot of the original law is implemented, including the Safety Buffer dynamics (you'll see that as a separate AGC), band crossover frequencies and slopes, most of the gate dynamics, and the windowing constants used in the M101 cards. Its pretty amazing how close the Stereo Tool Adaptive Compression algorithm can be adjusted to mimic the unique sound of the Vactrol. A combination of the Windowing feature under Behavior along with the advanced gate parameters come pretty close to the CMOS digital logic Glen implemented.
The only elements in the attached Settings file are the "ACG" (Safety Buffer) and "Compressor 1" (The 4 Band Compressor). There is no limiting and phase correction, no I/O or repair. You will need the license for the Adaptive Compressor feature as that is used here.
Drive the input so the AGC barely winks at you under nominal peak levels. We never wanted the safety buffer to influence the 4 band, it was only there for protection and it wasn't great to begin with, but it got the job done. There was no real "sweet spot" for the 4 band section on the Audio Prism, but the box worked the best when driven to 15db avg GR in the mid bands - same with this file.
The gating and core timing is set for a pop/rock format, and the defaults I used are for a tighter but open sound and punchy sound, not for loudness, although it does densify the audio envelope as you would want on-the-air. I am using this on a stream with some repairs enabled and a touch of limiting and it sounds pretty good to me. (You can hate it, it won't hurt my feelings
More or less density? Play with the Windowing parameter first, wider or tighter windows on the bands will mimic what the CMOS logic did when you adjusted the Density Control with a Greenie. Recall that was Glen's brilliant but simple idea - you can do one more thing besides compress and expand - you can do nothing- and that was what the windowing did. You can adjust the tonal mix by going in to the Levels section of the compressor and adjusting the Mix to taste. Then if that isn't enough for you, you can unleash you inner-self on to the tiny knobs scattered all over Compressor 1 which I diddled with for many weeks. Do us all a favor - do a better job on this than I did.
Play around, let me know your thoughts. (Feed it Roundabout by Yes and turn it up)
Kudos to the kids who make Stereo Tool!
Dave Van Allen
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