Protracker CIA (Complex Interface Adapter) Timer Tempo Calculations:
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Fcolor                        = 4.43361825 MHz (PAL color carrier frequency)
CPU Clock   = Fcolor * 1.6    = 7.0937892  MHz
CIA Clock   = Cpu Clock / 10  = 709.37892  kHz
50 Hz Timer = CIA Clock / 50  = 14187.5784
Tempo num.  = 50 Hz Timer*125 = 1773447

For NTSC: CPU Clock = 7.1590905 MHz --> Tempo num. = 1789773

 To calculate tempo we use the formula: TimerValue = 1773447 / Tempo
 The timer is only a word, so the available tempo range is 28-255 (++).
 Tempo 125 will give a normal 50 Hz timer (VBlank).

 A normal Protracker VBlank song tempo can be calculated as follows:
 We want to know the tempo in BPM (Beats Per Minute), or rather quarter-
 notes per minute. Four notes makes up a quarternote.
 First find interrupts per minute: 60 seconds * 50 per second = 3000
 Divide by interrupts per quarter note = 4 notes * speed
 This gives: Tempo = 3000/(4*speed)
 simplified: Tempo = 750/speed
 For a normal song in speed 6 this formula gives: 750/6 = 125 BPM

 Lars "ZAP" Hamre/Amiga Freelancers 1990