Commodore PET startup sound (again)

I’d posted the commodore pet startup chime before without comment, but last weekend’s goings-on at World of Commodore 2025 have made me revisit it.

Josh Bensadon had brought some very silly little devices he’d made that played the startup chime when you pressed a button. They contained a small PIC chip programmed with the PWM sequence, a tiny speaker, a battery and the switch. For certain people, it brought so much joy. Retrocomputing folks can be easily amused, it seems.

I don’t have a PET, but I do have fond memories of the one we borrowed from school during the holidays around 1980. The PET doesn’t have great sound capabilities, as Dave at Tynemouth Software notes. But looking at the captured waveform dumped from VICE’s audio output, it looks suspiciously close to four repeats of these 7 steps:

1. 80 cycles of 4329.0 Hz;
2. 40 cycles of 2164.5 Hz;
3. 20 cycles of 1082.3 Hz;
4. 10 cycles of 541.1 Hz;
5. 20 cycles of 1082.3 Hz;
6. 40 cycles of 2164.5 Hz;
7. 80 cycles of 4329.0 Hz.

Conveniently, all of these steps have the same duration (0.01848 s). The odd series of frequencies seem to be coming from a clock divider: 4329.0 Hz is 1,000,000 ÷ 231, 2164.5 Hz is 1,000,000 ÷ (2 × 231), and so on.

The end result, cobbled together by many calls to sox, sounds like this:

It sounds not too bad. It doesn’t have the fade-in effect caused by the PET’s power supply coming on, but it has the right character.

Its spectrogram is particularly special:

If you want the sound for a phone notification:

• pet_chime.ogg is for Android;
• pet_chime.m4r is for iOS.
  (I had to zip it because WordPress)