furnace/doc/7-systems/zxbeep.md

ZX Spectrum beeper

rather than having a dedicated sound synthesizer, early ZX Spectrum models had one piezo beeper, controlled by Z80 CPU and ULA chip. its capabilities should be on par with an IBM PC speaker... right?

not really - very soon talented programmers found out ways to output much more than one square wave channel. a lot of ZX beeper routines do exist, but as of 0.6 Furnace supports two engines:

• a Follin/SFX-like engine with 6 channels of narrow pulse wave and click drums.
• QuadTone: PWM-driven engine with 4ch of pulse wave with freely variable duty cycles and 1-bit PCM drums.

effects

• 12xx: set pulse width.
• 17xx: trigger overlay drum.
  • xx is the sample number.
  • overlay drums are 1-bit and always play at 55930Hz (NTSC) or 55420Hz (PAL).
  • the maximum length is 2048!