ZX-3 (MSG) - ZX Spectrum beeper music engine
Original Z80 code by Jan Deak, published circa 1991
1tracker version by Shiru, 01'2018


Likely the first of the Jan Deak's series of engines. Features three 
channel of tone with detune support. The first channel has slightly 
weaker sound. There is no drums. 

1tracker version has been reworked a lot, totally different song format 
and data processing code, but sound generation loop remained intact. 


The columns are:

T1 d T2 d T3 d
.... .... ....
|  | |  | |  |
+ch1 +ch2 +ch3

T1,T2,T3 : Note, G-2..B-7
       d : Detune, 1..9
