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→3.7 Base frequency
===3.7 Base frequency===
In the most trackers on the MSX computer music timing was done by the VDP, this has a few disadvantages:
- The music timing is always in some way releated to the interrupt frequency of the VDP ( 50 or 60 Hz). The only way the created different speeds is to skip an interrupt or more. (This is done by the tempo command of Moonblaster). This does not give a satisfying tempo setting.
- The music can not be played at the same speed on 50 or 60 Hz. This was solved by frequency speed equalizer, but this caused timing errors.
The solution for these problems is: Using the timer of the OPL4. This timer has an accuracy of 80.8 micro second and is in Moonlbaster selectable from 1 until 133 , resp. 48Hz until 100Hz. In Moonblaster this value is called: Base frequency.
To calcate the interrupt frequency use the following equatation.
s = ( 256 - Base_freq.) * 80.8^10-6
Interrupt frequency = 1/s
Example: Base frequency = 80
s = ( 256 - 80 ) * 80.8^10-6
Interrupt frequency = 1/0.0142208
Interrupt frequency = 70.32Hz
The Tempo determines the devision of the interrupt frequency.
Example: Tempo = 20
This means that every 5th interrupt a step is played:
(22 = every 3th int,21 every 4th int,etc)
So if you have a interrupt frequency of 70.32hz and a tempo of 20
the step/min will be:
70.32
------- * 60 = 848 step/min
5
You can select bewteen 2926 different tempos.
The maximum speed is 2012 Steps/min (tempo 22 , base freq. 133)
The minimum speed is 121 Steps/min (tempo 1 , base freq. 1)
====3.7.1 Base frequnecy setting====