Filters, Filters... It was a filter what led me down this road of complex pleasures... and I never even stopped to thank it...
1972: I am trying to design a 5-pole Chebyshev high pass filter that will carefully attenuate excessive bass below 100Hz, like on "Leaving on a Jet Plane" by Peter Paul and Mary, or "Magic Carpet Ride" by Steppenwolf. I am driving an audio amplifier putting out 33,250 watts RMS to modulate a 66,500 watt AM transmitter that puts out 50Kw. We have extensive AGC (Automagic Gain Control) to push maximum modulation without invoking the ire of the FCC by exceeding 100%, and the bass in those songs pumps the rest of the spectrum up and down horribly. Also those cheap transistor radios sound like crap with the 2 inch speaker flapping back and forth at frequencies they could only handle in their dreams.
So, attenuate the Low lows. BUT the FCC insists that we be within 3 db at 100Hz. BUT we only have to MEASURE at 100Hz, so, hey,if there was a little ripple and the curve just happened to go thru 100Hz at 0db, WhatTheHeck.
Try doing that with a sliderule.
So I used a desk calculator. I could do a design in about, oh 1.5 hours. Then I could build it, test it, and miss big.
So the local SUNY branch had just got a COMPUTER. A DEC PDP11 running BASIC. Whatever THAT was. Ran 5 teletypes.
I spent a few nights sneaking into the computer room. They had a 1-page paper about BASIC taped on the wall. The Guru had the Book locked in his office. Eventually I got the same answers for all the resistor values as a manual design! . The next week I got a LOOP that made a TABLE of values. Holy Crap.. Maybe there was something about these computer things. Maybe I ought to learn more about them.
I built some Opamps with matched dual transistors from Fairchild in the first stage, and 3 other modern SILICON transistors, running on + and - 24 volts to have plenty of headroom for line level audio. Built the filter, came in at 1AM Sunday with the whole station to myself, and ran it. The frequency response curve was within 1 db at 100HZ and within 3 db elsewhere. Legal! And it went down like a stone below 100HZ. Listened to some pop songs, and the troublesome PPM and Steppenwolf. Did AB. On our test set of 5 different radios including a car radio there was NO apparent loss of bass, just cleaner bass, and the damn volume pumping was gone! Later demoed for music director, DJs, Management. It was declared a Trade Secret and I was not supposed to talk about it or leave the diagrams anywhere.
Of course today every podunk radio station has a digital audio processor that does all this and more.
...Don't blame Crossroads for setting me off on stuff like this :roll_eyes: