diff --git a/doc/7-systems/zxbeep.md b/doc/7-systems/zxbeep.md
index 6381814a8..e724e0943 100644
--- a/doc/7-systems/zxbeep.md
+++ b/doc/7-systems/zxbeep.md
@@ -2,7 +2,10 @@
 
 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 only a Follin/SFX-like engine with 6 channels of narrow pulse wave and click drums.
+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