Hi fans !
As some of you might know, your "Uncle Bernie" has been on a crusade for a year (or so) to root out the problems with the notorious Apple Cassette Interface (aka "ACI") once and for all. Religious people may see this as a holy mission to drive out the demons lurking in that piece, but this is tongue-in-cheek as no demon can possess a dead piece of hardware. Scripture teaches that only living beings - humans or animals - can be possessed by demons (so many of the Stephen King horror novels got it wrong). But I mention this demon topic for an allegoric reason: despite being a seasoned analog / mixed signal designer who believes in Laws Of Physics, so there must be an explanation for the "evils" in the ACI based on science and engineering, but it turned out to be a hard nut to crack for anyone who really wants to understand what really is wrong with it. It would be easier to say it's due to "demons" and then pursue some other, more rewarding goals in life. There certainly are no laurels to be earned by fixing a 47 year old cassette interface design. Besides the fact that real cassette recorders are almost extinct. But being retired, and having no real purpose in life anymore, I need to pick some windmills to fight like Don Quixote. The notorious ACI is one of these windmills.
PREFACE (the historical background, you can skip to my upcoming next post in this thread unless really interested)
So in the past, some progress was made. I wrote an additional firmware page (256 bytes) which has additional functions (such as the Apple II style checksum for recordings) and I fixed the volume indicator LED circuit. The "extended format" PROMs have been in my IC kits since a year now, but to get the benefits, Apple-1 builders had to cut some traces and add some wires, which is despised by some, so I also made a PCB revision which includes these mods, and I gave away the leftover PCBs as a bonus with my recent kits (now they are gone, none left). Here are some links on how all this looks and works:
See Post #9 and #16 of this thread:
... and here is an example what you can do with these mods:
But alas, all this work, despite bringing improvements, did not really get to the bottom of the ACI rabbit hole. I had hopes that the modified PCB would allow me to reduce the hysteresis to the level seen in the Apple II cassette interface. Alas, this was not possible despite I had added a high performance, modern, SMD, power supply bypass capacitor right at the LM311 comparator. This measure brought some improvement but not enough: it allowed to reduce the hysteresis by half, but not to a third of what would be needed. So I got a big "F" - FAIL - like in school. (BTW, the most traumatizing experience in my whole life --- if I had children I would never, ever send them to school - home schooling is the way !)
You might ask why I did not see the problem with the hysteresis reduction, before I made the PCB mod. Well, reduction of the hysteresis was not the target objective of that "Gen 1" ACI PCB mod, so I never tried hysteresis reduction before I sent the PCB layout for production. Also, manually adding power supply bypass capacitors to any circuit in an attempt to improve its robustness is a hit-and-miss endavour. The leads of these capacitors are inductors in the nH range and these will impair the effectiveness of the mod. It happens to work with the "reliability mods" I have published to fix the DRAM reliability problem in the Apple-1, see here:
... but this success can't be generalized. I have seen cases in my professional realm that adding bypass capacitors actually made matters worse (and I know why). The whole topic is extremely complex and the industry has spent lots of money (10's of millions of US$ at least, if not billions) to get a handle on this issue.
The problem boils down to this: any power supply grid on any PCB is a resonant LCR network. The digital ICs sitting on this network produce current spikes when switching / being clocked. These current spikes may incite the whole LCR network to resonate. If you happen to hit a resonance frequency, the power supplies / ground rails may oscillate enough to crash the whole system. Bad !
This phenomenon even extends into the microscopic world of ICs --- such as PC CPUs. Less experienced IC designers think that just generously adding enough power supply bypass MIM capacitors in the IC will make it work. A fallacy. The whole power supply grid needs to be properly modelled and simulated. This is a multi million US$ CAD boondoggle and lots of money is still flowing into that dark hole.
A typical remedy is to add "spoiler tank circuits" which comprise integrated inductors, resistors, and capacitors. They are tailored to ruin resonance effects in the larger network. A mechanical analogy is the dual mass pulleys you can find on some car engines. These are made out of two parts bonded together by a rubber insert. The rubber element provides the damping (a resistor). And some springyness - an inductor. The outer rim of that pulley is a mass - a capacitor - completing the spring / mass system. The electrical equivalent is a LCR spoiler tank circuit.
Now here is how this trick works: the whole dual mass pulley is bolted to the crankshaft. The crankshaft may have a critical resonance where the engine would blow up. The dual mass pulley dynamic subsystem is tuned such that near the critical resonance frequency of the crank shaft it will start to resonate itself and drain resonant energy from the crankshaft. So the crankshaft oscillation gets dampened. I once had such a car and when driving it at 135 mph in top gear and flat down there was a woiiiig-woiiig-woiiing sound from the engine and that is how the dual mass pulley dampend the crankshaft oscillations down to a harmless level. Otherwise I would probably not be here to tell the story because the engine would have exploded !
I told you this to show you that engineering is a very interesting profession. Once you grasp the basic concepts, you can see them being applied everywhere. Heck, even in tall buildings the architects put in "resonance spoilers" such as large swinging pendulum masses somewhere in the top levels. Without them the right wind speed could make the skyscraper oscillate at its resonant frequency and the building would collapse. As far as I am concerned I would not want to be near any such building. Because I know the math of such a dynamic system. Under certain circumstances, its response may become chaotic, and then all bets are off. Hopefully Mr. Architect has checked the math thoroughly and it was not done by H1B slaves hired from third world countries. Where you can buy your M.S. or Ph.D. for a pittance. Maybe I'll buy myself a Ph.D. from there, too, if it's cheap enough (I'm a cheapskate). To be able to brag. Like Saul Goodman (of "Breaking Bad") proudly displayed his law certificate from some dubious university in American Samoa, an American Territory. Oh, I got my divorce cheaply in another American Territory, Guam. Seriously. What an awesome deal ! The airfare was more than the lawyer fees ! The parasitic lawyer here in Colorado Springs wanted a higher retainer (!) alone than the whole Guam divorce did cost me, including airfare, hotel, lawyer, etc. And I got a real divorce certificate from the Superior Court of Guam ! No American can brag to have gotten a divorce certificate from any "Superior Court" here in the continental U.S. ! No "sworn financial statements" needed in Guam, either ! You don't want your financials being part of a court record, ever ! So if you want to get rid of your trophy wife, go Guam !
But enough of that. The point I wanted to make is that once power supply noise / resonance effects get their fingers in the pie, things get complicated. And not easy to analyze. So I avoided to waste my RQLT on these effects in the ACI until I had the final PCB in hand. Which, alas, turned out not the the final one. Because it did not allow the reduction of the hysteresis down to the level I wanted.
You might ask why I want to get the hysteresis down to Apple II levels. The reason is - compatibility.
I want the same characteristics in the Apple-1 ACI card as seen in the Apple II cassette recorder interface. Which was good enough to satisfy the Apple II users for a full year until the famous Disk II system came out. If the Apple II cassette interface had been as bad as the notorious Apple-1 ACI, there would be no Apple (nowadays one of the most valuable corporations in the world). Angry customers would have torn them down and maybe set their headquarters on fire, who knows. But as we all know this did not happen. Because the Apple II cassette interface was certainly "good enough".
This is where I want to get with the improved Apple-1 ACI. I think I can reach this goal based on my Gen 2 ACI cards, which currently are in lab evaluations. But, man, the journey was a strange one. It all begins with analyzing the Apple II cassette interface, which is similar, but distinct from the ACI of the Apple-1. Other than the TAPE IN circuit and the checksum lacking in the Apple-1 recordings they are much the same. The encoding / modulation method is the same. You can actually read any Apple-1 tape recording on the Apple II and vice versa - except when reading on the Apple II, you have to ignore the missing checksum stalling the read or the checksum error which might appear or not. My "extended format" PROMs for the Apple-1 ACI cure the checksum problem, though.
The tape storage systems on both machines are so similar in terms of hardware and software that it is mind boggling to see how the Apple II system succeeds to satisfy the user while the Apple-1 ACI drives the user nuts and makes him/her want to smash everything with a sledge hammer. In my following posts in this thread I will show you why.
Stay tuned !
(And please don't comment until you saw my next post !)