The Organ Bass
The M1 organ bass under half of house is a Hammond in software: drawbars that hold, a percussion harmonic that comes in 17 dB above them and decays in 150 ms, and a click of 31 dB of high frequency in the first five milliseconds. All three read off the sound, the percussion to within 10 ms of its setting. It fires only on a note played alone, so a legato phrase gets it once, and that's the feel.
What it is
The organ bass: the Korg M1 preset under half the house records ever made, which is a Hammond drawn in software, and the two things a Hammond does at the front of a note that make it an organ rather than a stack of tuned sines.
The drawbars are sines at fixed levels, five of them here from the sixteen foot to the two foot, and they hold for as long as the key does. The percussion is one extra harmonic, the third, that comes in loud and decays away however long the key is held. The click is a few milliseconds of noise from the key contacts. Here the keys play it, the percussion and click switch on and off, the percussion decay has two settings, and the demo plays a house organ line at 125 with two legato pairs in it, because the percussion has a rule about those.
How it works
Each of the three parts is a number off the sound.
The drawbars are read as the level of each harmonic against the eight foot once the note has settled: 0.80, 1.00, 0.50, 0.25 and 0.20, which is what they were set to, to two decimals.
The percussion is read off the third harmonic’s level over time. It starts 17.4 dB above where the drawbar alone would put that harmonic and falls back to it, and the decay is fitted to the stretch between 1/e and 1/e squared of the way down, which is the part that’s neither attack nor floor. Set to 150 ms it reads 160. Set to 400 it reads 390. Switched off, the third harmonic is flat to within a dB.
The click is the high frequency energy in the first five milliseconds against the same band a tenth of a second later: 31 dB, and 1.4 dB with the click switched off, which is the noise floor of the measurement.
The Web Audio graph is one periodic wave for the drawbars, whose fundamental is the sixteen foot so that every bar is a whole harmonic of it, a separate sine for the percussion with its own decaying gain, and a burst of highpassed noise for the click. Rendered offline it gives 160 ms, 17.4 dB, 31.3 dB and the same five drawbar levels, against the core’s 160, 17.3, 31.4 and the same five.
And the rule. On a Hammond the percussion is single trigger: it fires when a key is pressed with no other key down, and not otherwise, so a phrase played legato gets it once, on the first note. The click follows the same rule. This page does the same, and the demo pattern has two legato pairs in it so you can hear the second note of each arrive without the flare. The spectrogram shows it plainly: the third harmonic lights up on every detached note and stays dark on the tied ones.
What surprised me
The organ is in the first 150 milliseconds. With the percussion and the click switched off, the same five drawbars at the same levels are a perfectly pleasant set of tuned sines, and nobody would call them an organ. Switch the percussion back on and it’s a Hammond. One harmonic, 17 dB up, for a sixth of a second. That’s the whole identity of the instrument, and it’s measurable.
The single trigger rule turns out to be why the M1 bass feels the way it does. Play the line with every note detached and it clatters, every note flaring. Tie the notes into phrases and the flare marks where each phrase starts, which is a rhythmic accent you didn’t play and the instrument added for you. House organ basslines are phrased around it, whether the people playing them knew that or not.
The instrument that went wrong was the floor. I fitted the percussion’s decay by subtracting the drawbar’s own third harmonic, read at the end of the render, and read 290 ms for a 400 ms decay and 13.7 dB of lift for 17.4. A one second render still had 8% of a 400 ms percussion in its last frame, so the floor was too high and the excess above it too small. Three seconds and it read 390 and 17.6. A floor has to be read where the transient has actually gone, which for an exponential is several time constants out and not wherever the render happened to stop.
What I would do next
A real Hammond’s percussion is not an extra sine, it borrows the third harmonic’s drawbar and takes it away from the sustained tone while it plays, so switching percussion on quietens the held sound. Measuring that trade on a recording would say how much of the organ’s punch is addition and how much is subtraction.
Then the click, which on the real thing is nine contacts closing at nine slightly different moments, and the spread of those moments is a number with a sound attached.