Guitar tutor - chord chasing

Following on from our guitar tutor shift-register-LED project, after some discussion (and the usual "musical differences" that accompany any discussion involving two or more musicians) we've been adding some extra functionality to our firmware.

So far, the light-up fretboard shows a scale, pattern or chord in a specific key. And while simply playing the same old scale over an entire progression won't ever sound too offensive, it can quickly become a bit "boring" for the listener. True musical expression comes from playing with the underlying progression. i.e. you actually need to take note of which chord you're playing over, and adjust your melodic phrasing accordingly.

So for "chord-chasing" sequences (where you play from a scale that matches the dominant chord being played at any one time) we've put some "shortcut" buttons on our microcontroller, to select the relative second, third, fourth, fifth and sixth chords from the dominant key (which is dialled in at the start of the song). For anyone not familiar with music theory, let's assume that we're playing in the key of A (major).

• This means that our root chord or primary chord is A major.
• The second chord (or, in music notation, the II chord) is B minor (two frets up from the root).
• The third chord (or the III chord) is C# minor (two frets up from the II chord).
• The four chord (IV) is D major (this is probably familiar to anyone who plays classic blues sequences, or pop songs that follow the "three-chord-trick"
• The five chord (V) is E major (pop songs and most blues progressions follow the I-IV-V rule)
• The six chord (VI) is F# minor (two frets up from the V chord)
• The seven chord (VII) is G# diminished (although this is rarely used; Peter Green's "Need Your Love So Bad" is one of a few blues songs to actually make use of the diminished sound).

On our guitar body we will eventually have seven buttons. This allows the player to "dial in" the primary key that they are playing in (let's say A major for now). When they hit the first button, the corresponding scale (A major) can be displayed (although for guitar solo playing, perhaps the A major penatonic scale might be preferable).

As the song continues, maybe the IV chord comes along. Instead of the player having to work out which chord this is, stop playing and dial in the new chord, they can simply hit the fourth button. The firmware will work out the relative "fourth" chord for the key and display the appropriate scale/pattern (in this case, D major).

This allows the player to quickly see all the relevant notes not just for the key that they are playing in (after all, we can all blast an A minor pentatonic scale over any song in the key of A) but also to match the tonal qualities of the underlying chord sequence - a way of producing harmonious music that actually fits with the song being played, and not just a load of widdly-widdly scale-shredding!