I found this recording of the complete "harmony smurf" tape on youtube:
It uses a single audio frequency, so it reminds morse code more than FSK when you hear it...
Interesting. Conversion to a digital signal may be fairly easy.
My first guess would be that the longer intervals are the beginning of a frame or the beginning of a message.
Somebody who's good at disassembly can probably spot the detection of similar patterns being looked for by the code.
Somebody with a Kid Vid, a digital storage Oscilloscope and a tape deck could lay up the digital signal out from the Kid Vid next to the audio track to see if there's a direct one-to-one correlation between "tone present" and a digital 1 or 0 from the Kid Vid digital output..
I wonder if the decoding could be as simple as amplifying and rectifying the signal with a filter capacitor to smooth the output. The output doesn't have to be ripple free and super clean. It just has to stay above and below the digital logic thresholds for the duration of a bit. Seems like a scheme like that would be appropriate to the time period.
An op-amp comparator set up with an appropriately low reference voltage could probably crank out a 90% duty cycle square wave from a sine wave. Again, a little filtering to keep the signal from dipping too low in the "off" times might work.
Hearing the "sound" of the KV digital output could tell how sophisticated the demodulation is.
Just throwing out some thoughts that anyone is free to run with if they so choose.
As ridiculous as it might be, I wouldn't put it past somebody around here to eventually do a homebrew Kid Vid compatible game.
A quick glance at the comments seems to confirm a typical serial data stream.
That means, in my estimation, that the control track is translated to a pure digital signal outside of the console.
Tracing the execution in Stella and counting cycles, one should be able to figure out the expected bit rate.
Comparing that theoretical bit rate to the waveforms on the control track, it could be determined if the control data represents a one to one bit per signal change relationship. That would tell whether I'm right in guessing that it uses a simple FSK encoding of the data.
I remember playing with a tool that could translate Supercharger audio to binary. (wav2bin?) Something like that with a means of tuning the bit rate could decipher the audio from the tapes if captured to wav files.
Has anyone disassembled the ROM from the cartridge? That'd probably be informative.
My initial thought was that the tape would only play one tone on that channel and that any data (such as "which game") would be interpreted from a serial data stream, not that a specific tone would represent a specific game. I don't think the Atari could sample fast enough to discern very many different frequencies.
At most, I was thinking the control channel would play two different frequencies each explicitly representing a 1 bit or a 0 bit (FSK). The internal works of the Kid Vid would have a demodulator like a moDEM to translate the tones to digital. In that case, playing the tape in a normal deck, you'd hear "modem sounds" (providing you're old enough to know what that sounds like an young enough to still be able to hear).
At the simplest, I thought tone or absence of tone might be used to represent 1/0.
But, I'm just making wild guesses. For all I really know, there might be DTMF tones recorded on the control channel.
Has the audio from both tracks been ripped and put on the internet somewhere? I didn't find it with a quick search.
It's kind of an interesting subject, now that I've started thinking about it. Now I wish I hadn't flipped the Kid Vid that I just knew I'd never have any interest in playing with...
I suppose it would depend on the jitter being observed. I can see how ripple that's not in sync with the sampling frequency could cause the paddle input to vary with no actual change in resistance, but I wouldn't expect it to amount to much.
I haven't noticed that happening on the 2600. If you're seeing that on the 2600, I don't see much harm on trying it. I guess if there's substantial resistance between the power supply and the chip somewhere this could cause the TIA to power up too slowly, but that's a stretch.
But, jitter on the 2600 is widely known to be caused by dirty or otherwise malfunctioning pots. I haven't seen anybody claim that addressing the pots didn't fix the jitter problem.
Before the hack and before I had a Harmony cart, I built an adapter to externally remap the Up direction to work as the booster grip button did. It took a little bit of active electronics but was a fun first controller hack.
Hi, thank you so much for your input. Could you please describe a bit more the 90 degree out phase comment?. I think I get it, but I want to make sure I fully understand it. Currently, the slots are parallel to each other, equidistant. So, 90 degrees means, when one is in open, the other must be in close relative to the wheel? Thanks a lot! Al
What you're describing would be 180 degrees out of phase. You're saying that they would both change states at the same time (and would always be in opposite states). With 90 degrees out of phase, a channel changes states as the other channel is half way through it's current state.
In your description the output of the two channels would be 01, 10.
In a 90 degree phase difference, the output of the two channels would cycle through these 4 (hence "quad") values 00, 01, 11, 10.
(That bit pattern is known as "Gray code")
Taking the perspective that one of the channels "leads" the other in phase, by knowing the state of one bit you can tell which direction the quadrature encoder wheel is turning. Here's a diagram I put together when I was trying to wrap my head around how to decode quadrature signals in a microcontroller program I was working on.
Those crazy patterns are typical of bad connections between the cartridge and the slot.
The cartridge and/or the slot contacts possibly just need to be cleaned. If the problem is dirty contact surfaces, you may be able to get it to work temporarily by repeatedly removing and inserting the cartridge to scrape away oxidation on the contact surfaces.
I have a partially functional Japanese Space Invaders knockoff called "Super Space Stranger" sitting in my garage waiting for me to fix it. I was never aware of them at the time, but there must have been a bunch of these Space Invaders "inspired" machines back in the day.