Magic Chord-O-Matic Machine!

August 31, 2009 1:04 PM

I was playing around with a keyboard at a Guitar Center that had an auto-harmonize feature. Below a certain note pressing a key would play a chord instead of a single tone. What was interesting was that playing chromatically (white and black keys) it played chords that retained a firm sense of key (C major) - if you jumped around randomly or did chromatic runs it didn't sound like there were a bunch of modulations taking place. What formula do you think it used to do this?
posted by phrontist to technology (8 comments total)

Probably the easiest way to design it would just be to have a lookup table that gives a chord to play based on the note you're pressing and the last chord that played. For example, if you hit Bb after a C major chord, then the next chord should be C7. But if you hit Bb after a F major or Am, then maybe you'd want Bb major instead. You could probably cover a whole lot of common progressions that way, and it wouldn't take very long to have someone fill in the table manually.

Do you remember the brand name?
posted by equalpants at 1:33 PM on August 31

equalpants: Unfortunately, no, I don't.
posted by phrontist at 1:38 PM on August 31

Did it always leave you feeling like you were in C major?

Or was there a way to explicitly specify a different key?

Or do you feel like it was actually deducing a key from what you played, and then sticking to that key as you kept playing?

The first two strike me as much easier to implement than the third.
posted by nebulawindphone at 2:08 PM on August 31

I don't think it was trying to recognize key. In any case, what I'm interest in is how you could come up with a chord for every pitch class that implied major tonality in a given key.
posted by phrontist at 2:28 PM on August 31

It may have just used chord substitutions that were the most relevant to the first key pressed.

For instance, if you start on C, then move to a B, the chord would play an E minor (the relative 3rd of C), not a B major (straight root).

This is pretty easy to implement. There are only 12 notes, and they can all be resolved to some inversion of I, IV, V, IIm, IIIm or VIm.
posted by Aquaman at 2:33 PM on August 31

It's tough to answer without knowing what you heard as "C major" while traversing chromaticism.
Cdominant7 gets you a Bb, which gives the feeling of "C major", and give some easy passing tones for other notes.
If a C# is used as the root for a half diminished chord, it sounds "like a C major scale chord" if you progress to a D-, or resolve down to a C.
Or an F# can be the first inversion of a secondary dominant, which sounds really "C major-ish" as a turnaround (| D/F# | G7 | C |), even though a D major chord on its own sounds not very C major-y.

If I were building a toy to do this, I'd have a few different mappings for the chromatic scale.
posted by lothar at 2:43 PM on August 31

A slight aside, but an interesting point - you can usually play out-of-key chords by using more than one key - press C and get C major. Press C and Eb together, get C minor. Press A, get A minor. Press A and F# together, get A major. Etcetera, etcetera. This way you are not limited to only playing in one key (so long as you're willing to use more than one key at a time).
posted by Brother Dysk at 5:37 PM on August 31

In any case, what I'm interest in is how you could come up with a chord for every pitch class that implied major tonality in a given key.

Well that's a different question. lothar's got the right idea. Here's a progression using all 12 pitches as bass notes that will sound like C major:

C A7/C# Dm B7/D# Em F D7/F# G E7/G# Am Bb G7/B C.

However my guess is that your hearing the key of C had more to do with you than anything fancy the keyboard was doing. If you've got C major in your ear, any chromatic run you play is going to sound like it wants to go back to C, because that's the key you're thinking. It's a matter of perception.
posted by ludwig_van at 6:23 PM on August 31