Squeeker - ZX Spectrum beeper music engine
Original Z80 code by Zilogat0r, circa 07'2000
Modified and improved by utz, 2015


Four channel engine based on a very specific idea that much later found 
its use in may more developments. The engine was first developed by 
Zilogat0r in 2000, which kinda makes it beginning of the 1-bit music 
renaissance in the new millenium. As it had no editor, and interest to 
the beeper music among composers remained low until 2010s, when more 
accessible editors appeared, it only got attention much later. In 2012 
the code was size optimized, then a new data format and loader were
added by utz 2015, which made it easier to adopt in a tracker. 

There is four channels of tone with detune, no drums, per-row speed.

All tone channels has uniform capabilities and sound, but there is a
global  duty cycle setting, which can be changed to 1..128 from the
Instrument editor tab (F4), that affects to the tone of all four
channels at once. 


The columns are:

Sp T1 d T2 d T3 d T4 d
.. .... .... .... ....
   |  | |  | |  | |  |
   +ch1 +ch2 +ch3 +ch4
   
Sp : Speed, 1..99
Tn : Note, C-0..B-7
 d : detune, 1..9
