Turns out that it's actually 2-bit and not 1-bit. It uses a combination of the EAR and MIC jacks, and when you output through the MIC jack, it slightly boosts the EAR jack. That gives you 4 possible levels, but they are not evenly balanced.
I gotta try this on real hardware, reading the original article it seems it's barely audible without an amplifier, unlike on an emulator.
Will post an update after I get back to my parent's home, where I have these beauties that launched me into computer science: https://imgur.com/a/GKrA0vX
I created a program like that back in the day. In my case, I recorded my voice through the "MIC" input and played it back through the TV. A few seconds of sampled audio required the entire available memory. While I fully understood what was going on, I thought it was magical that the Spectrum could "speak with my voice".
On ZX-Spectrum? There was no "MIC" input on ZX-Spectrum, not that I know of. I was in high school at the time with only a rough understanding of the hardware but it was kinda obvious to me that whatever I plugged into the casette input port was only read as "0" or "1" by the IN <port> (forgot which) instruction. So not really possible to record voice at the time (didn't even know of PCM anyway), but I tried to decode Morse transmissions from radio. Never got anywhere with the software but my ear got good enough at some point that I could decode them myself by hearing :)
With proper volume control, even one bit could probably sound voice-like enough to a person in the 1980s.
It may be feasible to retrieve a pcm for a very short clip by playing the same recording at different levels which then could be converted on the same machine to a sequence of opcodes playing a more realistic voice.
There was Z80 code to record from the MIC socket in one of the small books (Melbourne House maybe?) that came out in the early 1980s. I remember borrowing it from the library and it worked! Heavily quantized of course by the snapping to 2 or 0.
Also some games had sound samples, Activision’s Ghostbusters had a couple.
Nope. I did the same thing myself with my Spectrum. I guess I was 14. I didn't have a microphone so I tried it with my dad's stereo headphones. I got the idea to try it after looking at the Z80 machine code of cassette saving and loading routines. Maybe it was also after ETX came out, first game I remember with speech samples. (Terrible game). I forget the details now but it's all about the timing between turning the mic/speaker on or off.
