quattropic - ZX Spectrum beeper music engine
by utz 08'2015
1tracker version by Shiru 12'2017


quattropic is the big brother of ntropic. It features twice as many 
channels, and you can even vary the sound of the noise. This comes at a 
small price, however: There is no per-row speed control in this engine. 
There is no dedicated drum channel, but one of the channels can be used 
play noise of varying length, pitch, and timbre, and fast pitch slide on 
one of channels is available to produce percussive sounds. 


- 4 channels of square wave sound
- Full duty cycle control
- One of the channels can play noise or fast pitch slides instead of regular tone
- Noise/slide channel also supports note cut
- 16-bit frequency resolution
- 15.6 KHz mixing


The columns are:

T1 dD T2 dD T3 dDS T4 dDN
..... ..... ...... ......
|   | |   | |    | |    |
+ch1+ +ch2+ +ch3-+ +ch4-+

T1,T2,T3,T4 : Tone channels
  Tn : Note, C-0..B-7
   d : Detune, 1..9
   D : Duty cycle, 1..4 (square 50% to 6.25%)
   S : Slide down mode flag, ch3 only
   N : Noise mode flag, ch4 only. Different notes will produce
       various metallic and noisy sounds
