If you were alive in 2000, you probably remember the sound of monophone ringtones. If you took a bus to middle school in 2000 you definitely remember that sound. Would there be some soundfont around that mimicks this sound?
There is Nokia Composer by zserge, and while the website has one of those unmistakable GSM fonts, it doesn’t sound like a mobile phone. You may recognise the sound from something else — it is similar to music from the good ol’ PC speaker from the 1980’s, but it does not sound like a phone.
First question is, what type of waveform is it?
r/audioengineering on Reddit had the answer: It is a simple sawtooth wave…
Saw wave. This sounds not at all like a GSM.
…but with strong filtering: the character of that sound is created by the speaker itself: it has a strong resonance around a couple of thousands of hertz, so that the tiny speaker in these phones can create enough volume to hear the ringtone.
Saw wave after applying the filters
Most of the sound here is the resonance of the speaker: You can see it decaying between the drops of the original sawtooth wave. A tiny speaker like that can’t produce low frequencies, so most of what you get are overtones.
If you make a soundfont with this sort of sound, you’ll need a sample for every note. This is because shifting a sample up or down will also shift up this resonance peak, which will very quicky make it sound wrong.
(This is also why voices on synthesizer keyboards are so unmistakably synthesizer. The thing you subconsciously notice is the formants being shifted up and down, something that does not happen if a singer sings another note.)
This contrasts with instruments like bells. Carillon bells are tuned to have very specific overtones relative to the pitch, so you could make a decent sounding soundfont with a single sample.
So since we’re into old school sounds, what about that PC speaker?
These use 50% duty cycle square waves. Even this simple waveform already sounds like an old PC. But as with our phone ringtone, it is also shaped by the small speakers inside the typical 1980’s and 1990’s PC cases. So there isn’t actually one PC speaker sound. Originally, a small dynamic speaker was used, a small version of the ones you still find in speakers today. Later, when Sound Blasters and other sound cards became ubitiquous, nothing really used the PC speaker anymore and you got those tiny moving-iron speakers. It is easy to tell apart those two, with the latter producing a very tinny and resonant sound.
So there’s two variants of the PC speaker. Both start with a square wave but one has a much more narrow band filter to produce the ‘tiny speaker’ sound.
Square wave, after filtering. The filtering is quite similar to that for the GSM tone.
Even though that filtering, it very clearly is not a GSM. A 50% square wave only produces odd overtones (1, 3, 5…) which gives it a characteristic and easy to recognise hollow sound.