Wednesday, September 10, 2008

Chip Tuning the Sega Mega Drive / Genesis

PAL tuning has been added to the Mega Drive / Genesis interface. The NTSC tuning has also been updated. Below you can see a table of the pitch identities and their tuning in relation to an 'internal' C.

This internal C has a frequency of approximately 261.5 Hz for a middle C. The middle C of equal temperament has a frequency of 261.626 Hz. As the Mega Drive / Genesis uses FM synthesis, the pitch identities and the spaces in between them are the same across all octaves. As a result, the tuning differences shown here for one octave applies to all eight octaves across the range of the sound chip.

For the sake of completion, the entire pitch space within an octave block of the sound chip is shown. However, it should be noted that the lower fifth and the upper fifth are there to demonstrate the pitch bend range when using the interface, and do not represent discrete pitches or notes as such. This is because the maximum pitch bend range is plus or minus seven semitones. The core identities for one octave - from C to B - are the integral pitches.

PAL Tuning (in relation to an internal middle C at 261.5 Hz):
F: -1.952 cents
F#: -0.773 cents
G: -1.609 cents
G#: -1.069 cents
A: 0.316 cents
A#: -0.906 cents
B: -1.785 cents

C: 0 cents
C#: -1.343 cents
D: -0.847 cents
D#: 0.883 cents
E: -0.911 cents
F: 0.046 cents
F#: -0.778 cents
G: 0.17 cents
G#: 0.61 cents
A: 0.311 cents
A#: 0.589 cents
B: -0.374 cents

C: -0.005 cents
C#: -0.086 cents
D: 0.339 cents
D#: 0.879 cents
E: 0.145 cents
F: 0.041 cents
F#: 0.162 cents
G: 0.166 cents

NTSC Tuning (in relation to an internal C):
F: -0.594 cents
F#: -1.64 cents
G#: -0.763 cents
G: 1.414 cents
A: 1.129 cents
A#: -1.533 cents
B: -0.714 cents

C: 0 cents
C#: 0.313 cents
D: -0.021 cents
D#: 1.084 cents
E: 0.926 cents
F: 1.437 cents
F#: 0.278 cents
G: 1.046 cents
G#: 1.41 cents
A: 1.124 cents
A#: -0.011 cents
B: 0.721 cents

C: 1.354 cents
C#: 0.308 cents
D: 1.185 cents
D#: 1.08 cents
E: 0.922 cents
F: 1.432 cents
F#: 1.234 cents
G: 1.042 cents

Pitch Bends
The interface now has a much-improved pitch bend function. It only responds to the most significant byte of a pitch bend message, giving a possibilities of 128 discrete steps of micro tonal control over pitch.

The user can set the desired pitch bend amount using a MIDI control message. This sets the pitch bend range to between plus and minus one and seven semitones. At a setting of plus and minus one semitone, the resolution of the pitch bend is 1.5625 cents. At a setting of plus and minus seven semitones, the resolution of the pitch bend is 10.9375 cents.


Elfreda said...

Thank you for the article, very worthwhile material.