How to build chords from a single note?
I'm working on a simple theory to apply both in real life and in a computer program I'm writing.
Basically, I realized that we can create (or add) any chord above a melody (vocal track, e.g.), as long as the chord contains the note of the current melody.
Let's take as example the famous song "Happy birthday to You".
Basically we can play it using 3 simple chords: C, F and G (considering we're in C key). Then, to improve its chord progression or just to replace it with another chords, see it:
Chords: C G ...
Words : Ha-ppy Bir-thday to You...
Notes : G - G A - G C B ...
Now, we can replace and/or add chords for something else which contains the melody notes, such as:
Chords: Bb7+ Am7 G#°...
Words : Ha-ppy Bir-thday to You...
Notes : G - G A - G C B ...
To find these new chords I just built anything which contains the notes randomically and that was the result.
I know these new chords have some relation with the key or melody but I'm don't know theory at this level. Wondering this, can someone tell me if there a better/consistent way to build these "new chords" instead doing it randomically?
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I see you ask about a better/consistent way to build these "new chords" instead doing it randomically? As stated earlier, it's a very broad question.
However, your project looks like a lot of fun ahead, and to get you started i'd recommend looking into these cornerstones and go from there:
How the major scale is built on it's note intervals.
How the major scale can serve as a foundation for building chords.
How the chords of a scale can be described as having different functions.
How the chord functions relate to each other in the circle of fifths.
Good luck/have fun!
This won't work in all cases but a simple method of harmonization is:
Determine the key or keys of the melody. In this case, it's the key of C
Construct the harmonized scale of the melody. A harmonized scale uses the notes of the key to construct chords with each note of the scale being the root of the chord. In this case the notes of the scale are C D E F G A B so the harmonized scale is C Dm Em F G Am Bdim.
Add chords to the accented beats of the melody using the chords of the harmonized scale matched to the notes of the melody. For 4/4 time this is usually beat 1 and 3.
When adding chords prefer the first, fourth and fifth chords of the harmonized scale for the melody. For the last chord prefer the first chord of harmonized scale.
You've demonstrated one pitfall already in your 'Happy Birthday' example. Yes, the 'right' chord on the first strong beat, 'Hap-py BIRTH-day to you' is C major. But the melody note on 'BIRTH' is A. And A isn't in the C major chord!
The music notation program Sibelius has a plug-in called 'Add simple harmony'. It makes a fair job of 'Happy Birthday', though the chords it chooses are not, perhaps, the expected ones!
The guy who wrote the plugin is called Bob Zawalich. He is active on the various Sibelius support forums, and MIGHT be prepared to take the time to discuss his approach with you if you take the trouble to search him out. He donates a lot of effort to the Sibelius community.
I'm sure it is possible to write a program that does what you want. The reharmonizing or harmonizing thing is a part of arranging and/or composition. It's a wide subject. I think you have to be able to do it manually, before you can write a program that automates the process.
The things you need to learn are functional harmony (ii - V - I, what tonic and dominant and secondary dominant mean, borrowed chords, etc.), movement and voice leading. You have a harmonical starting context and a goal context, and you create intermediate transitional contexts between the start and the goal - or actually, for any melody longer than just a couple of bars, you have several given intermediate goals. But between each goal, it's a stepwise transformation process from something to something, and the melody sets constraints for the process. You want to create tension and release, play with the expected and the unexpected.
Saying that the melody note has to be one of the chord tones is one possible constraint ... but not necessarily every note of the melody, as you can probably figure out. Do you want to have a melody note as a chord tone or a passing tone? On the other hand, you can use more chords than the melody has notes. With a slightly slower tempo, you could use two, three or even four different chords for every melody note of Happy Birthday. Which melody notes are more harmonically significant? It has to do with the perceived rhythm and meter of the song and the arrangement - and your harmony changes are a part of the perception.
One method is to decide which chords to set as fixed non-movable goal posts. Then take one voice of the harmony and start moving it step by step to some direction, or up/down, anything. For example the bass note is an easy pick to start moving. Take that note and the forced melody note (if you decide that the melody note is a significant must-have chord tone at that point, and not a short nice-to-have passing tone) and for every step, pick a chord that has those two. And - if you want - has the rest of the notes so that they all fit the key. But you should also consider if this makes sense in a simple I - IV - V sense. Will it sound corny? Will it sound like it was made by some teenager just fresh out of a jazz lesson? Does it support the melody well, or just barely?
Good luck!
Harmony isn't a formulaic thing.
When you harmonize a melody, you're first thinking about where it sounds like a chord change would work, and then you're considering ALL of the notes that will be played against that chord. They don't all have to be chord tones, but in general most of them will be.
Look at the harmony for Happy Birthday: you have the C chord against an A note in the melody, which isn't a chord tone. But the C chord is held during the G and C that follow, so the C chord works over the phrase as a whole.
In general, the non-harmonic tones will be:
passing tones (non-harmonic tones that fall between two adjacent chord tones)
neighbor tones (a chord tone that moves briefly to a non-harmonic tone and then back - as Happy Birthday's melody goes) or
appogiatura (a leap to a non-harmonic tone that resolves by step to a chord tone in the opposite direction of the leap)
Those aren't the only possibilities, but they're the most common.
You CAN use more (or fewer) chords when you re-harmonize a tune, but you want to be aware of how the melody is moving against the chord, and if you're not handling the non-harmonic tones it's a signal that there might be a better choice.
As far as the selection of chords goes there's more to it than just containing most of the melody notes. Good chord progressions progress by moving smoothly from one chord to the next. In general, chord roots should move by steps, fourths, or fifths. And in general, chord changes that change fewer notes will sound much smoother.
For example, let's say you start the C chord on the first note of Happy Birthday, and you want a chord change at the A, because it doesn't belong to a C chord. Bb7+ has the notes Bb-D-F#-Ab, so it has NONE of the same notes of the C that came before it - it's going to sound kind of jarring.
If you want to establish a solid progression of chord roots you might decide a I-IV-V sounds good, and use an F chord over the A-G-C; that gives you two of the three melody notes as part of the chord, follows a common chord root progression, and has a common tone with the preceding chord.
Or you might decide that smooth voice leading is more important and use an Am (A-C-E) which shares two tones with the C.
There are a LOT of ways to re-harmonize a tune. Random chords that contain the melody notes is certainly one way, but musically it won't usually be the best option.
I bet if you start generating many random chords combinations this way most of them will sound bad but occasionally it will pull out something really interesting. Then when you leave the cozy harmonic context of "happy birthday" thighs might get even more hairy.
IMO the code generating the chords should:
look at the whole melody and try to infer things like the key of the melody,
or identify the key centers in the melody,
find a sections with altered notes that could suggest a dominant section,
attempt to match some well know progressions to different sections
fill out the gaps by looking from the progressions that would require minimal movement of tones between chord A and chord B.
Studying theory of harmony might be of great help here. Rules of voice leading will be very important.
In general I think that mixing strategies and analyzing material up front might give a more natural sounding results.
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