fix documentation regarding ZX beeper samples

papers/doc/6-sample/README.md

@@ -48,7 +48,7 @@ due to limitations in some of those sound chips, some restrictions exist:
 - Neo Geo (ADPCM-B): no loop position supported (only entire sample), and the maximum frequency is ~55KHz.
 - YM2608: the maximum frequency is ~55KHz.
 - MSM6258/MSM6295: no arbitrary frequency.
-- ZX Spectrum Beeper: your sample can't be longer than 2048.
+- ZX Spectrum Beeper: your sample can't be longer than 2048, and it always plays at ~55KHz.
 - Seta/Allumer X1-010: frequency resolution is terrible in the lower end. your sample can't be longer than 131072.
 
 furthermore, many of these chips have a limited amount of sample memory. check memory usage in window > statistics.

papers/doc/7-systems/zxbeep.md

@@ -2,9 +2,12 @@
 
 Rather than having a dedicated sound synthesizer, early ZX Spectrum models had one piezo beeper, controlled by Z80 CPU and ULA chip. It's 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, as of 0.6pre1 Furnace supports only one - Follin-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 0.6pre1 Furnace supports only one - Follin-like engine with 6 channels of narrow pulse wave and click drums.
 
 # effects
 
-- `12xx`: set pulse width
-- `17xx`: trigger overlay drums.
+- `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!