xtone - ZX Spectrum beeper music engine
by utz 09'2015
1tracker version by Shiru 12'2017


xtone is basically like Tritone, but with 6 channels. Inevitably, the 
sound is quite a bit grittier though, and duty cycle control is less 
accurate. All channels has the same volume.


- 6 channels of square wave sound
- limited duty cycle control
- 16-bit frequency resolution
- 3 interrupting click drum sounds
- per-row speed control
- 16.2 KHz mixing


The columns are:

Sp T1 dW T2 dW T3 dW T4 dW T5 dW T6 dW D
.. ..... ..... ..... ..... ..... ..... .
   |   | |   | |   | |   | |   | |   |
   +ch1+ +ch2+ +ch3+ +ch4+ +ch5+ +ch6+
   
Sp : Speed, 1..99

T1,T2,T3,T4,T5,T6 : Tone channels
   Tn : Note, C-0..B-7
    d : Detune, 1..9
	W : Duty cycle, 1..4 (50% square wave to 6.25%)
	
 D : Drum channel, 1..3
