QChan - ZX Spectrum beeper music engine
by Shiru 03'2011, released into Public Domain
1tracker version by Shiru 04'2018


The engine features four channels of PFM tone with global pseudo volume, 
volume decay, interrupting clicky drums, per-row speed. All tone 
channels has uniform capabilities. Even though it has a decent note 
range, the lower end is lacking due to the design. 

Compared to the original release, engine code was modified a bit to be 
supported in 1tracker. Don't use decay and speed changes too often to 
minimize song data size. 

There is also a 3 channel version called 3chan. It is exactly the same,
but with three channels instead of four, which reduces the data size,
and technically increases the sound quality a tiny bit, even though it
is really difficult if possible to tell the difference.


The columns are:

Sp T1 s T2 s T3 s T4 s D
.. .... .... .... .... .
   |  | |  | |  | |  |
   +ch1 +ch2 +ch3 +ch4
   
Sp : Speed, 1..99

T1,T2,T3,T4: Tone channels
  Tn : Note, C-1..B-5
   s : Decay speed, 1..8 (fastest to slowest)
   
 D : Drum sound, 1..9
