Subharmonic Frequency Enhancement

Does anybody has info (or patches) about the processes involved in subharmonic frequency enhancers like the Aphex big bottom, DBX 120XP, SPL Vitalizer, Waves Maxbass etc.?

Subharmonic generation is literally something like applying magic, the results of magic only exists in our imagination, not in the physical world. So most of the times subharmonic generation is trying to trick the mind into hearing things that might very well not be there physically. Very similar to politics... J

There are several ways to go about. The simplest is to boost the low band a couple of dBs with a lowpass filter or EQ set somewhere to 60 to 100 Hz. This simply boosts subharmonics that might already be weakly present in the signal. But this can also make the sound muddy, it works best if there is a single bassline present that does not conflict with a bassdrum in the same area. In general, if the bass is very low the bassdrum is pitched higher or vice versa if the bassdrum is pitched lower the bass is played with 'tighter' strings. So results may differ much when using only a filter. Cheaper devices like the Behringer Enhancer seem to use this filter method.

Another way to go about is to make use of interference frequencies, eg if there is a pitch of 220 Hz and a pitch of 330 Hz present then there is a potential 110 Hz difference frequency present which is the subharmonic of the 220 Hz pitch (330-220=110). The difference frequency can be generated by applying a slight nonlinear distortion to this sound. So using a lowpass filter set to a cutoff of 300 Hz followed by a little overdrive type distortion followed by a 100 Hz lowpass filter can generate subharmonic frequencies. This means that audio with very simple chords might produce nice subharmonics but complex chords might give a very muddy sound. A good example of this interference thing is overdrive on a guitar, when playing chords on the guitar with lots of overdrive one can clearly hear the sub frequencies appearing, making the sound very full.

The only way to generate a pure subharmonic from a single pitch is to use a frequency divider that triggers on a positive or negative zero crossing of the waveform cycle. This generates a pulsewave half the frequency of the original wave. This pulse is used to open or close a gate where the original signal goes through. The output of this gate is added to the original signal. The effect is that every other cycle is somewhat louder, which introduces the subharmonic wave effect. But this can only be done reliably on a single pitched sound, if there is a complex chord the result might be very disharmonic. Lowpass filtering helps again.

Another thing is that if a pitch lacks its fundamental the mind tricks itself in still hearing it. An example is this "little transistor radios from the early seventies" thing, whose speakers didn't reproduce frequencies below perhaps 200 Hz at all. Still people would hear the basslines in the songs and have the correct pitch sensations. So the mind 'fills in' lacking fundamentals. There has been some studies at the Philips research lab about this phenomenon, which Wout Blommers knows more about. Mathematically its easy to produce higher harmonics, Chebyshev functions can do that, but generating a subharmonic from a single pitched sound is impossible without a 'binary' frequency divider. However, if a fifth chord is present the subharmonic of the lowest pitch in this chord is potentially present and can be enhanced by any nonlinear function. All the rest is magic, literally making people believe a subharmonic or fundamental is present while it actually isn't.

Personally I prefer the method with the frequency divider. If the source material is right it can produce a real subharmonic that will shake your audience, provided your speakers can actually reproduce that low end. Two patches attached, one showing the general divider principle with a single oscillator, the other one a subharmonic generator for audio, knob 4 turns the effect on and off, knob3 adjusts level of the subharmonics and knobs 1 and 2 can be used for some fine tweaking on eg a drum or sample loop.