LFOs
LFOs are a common modulation source. They are found on synthesizers and effects processors, and are used to produce rhythmic, periodic effects such as tremolo, vibrato or auto-wah. “LFO” is short for “Low Frequency Oscillator” and that is precisely what it is: an oscillator that runs (in most cases) at frequencies below the audible range.
Each knob-based, combo-box based or LED-based parameter in a given module can be modulated by its own LFO. This allows for some very complex sonic manipulation. When you select a parameter, the LFO settings in the main plug-in window will change to reflect the values of the LFO attached to that specific parameter. The LFO is activated with the blue LED button just to the right of the LFO label — this button is only visible when a parameter is selected.
Period
Section titled “Period”This parameter determines the speed (or frequency) of the LFO. There are two ways to adjust an LFO’s period, the “free” mode and the “synchronized” mode. You will most often use the latter.
In “free” mode simply drag the slider to choose your desired value. In practice however, you’ll usually want your LFOs to follow the rhythm of your track and this is where the “synchronized” mode kicks in. In synchronized mode SpectrumWorx uses time, tempo and time signature information from your host DAW to turn the period slider into a snapable control that will allow you to walk through and select only the periods that are meaningful for the current tempo and time signature (i.e. whole number ratios of the beat and bar durations). The range of the period slider is also automatically adjusted according to the active time signature. The minimum duration is set to 1/8th of the current time signature’s beat/base note duration and the maximum duration is always 16 bars. For the typical 4/4 measure this gives you values from 1/32 to 16/1.
Instead of always using power-of-two ratios as is the usual (and wrong!) practice, the values that fall between the durations of one beat and one bar are also carefully calculated based on the current time signature. Example: for the 4/4 time signature you get to set the value of 1/2 bar — which is meaningless for let’s say a 9/8 time signature, where you get to set 1/3 of a bar, or in a third case 1/2, 1/3, 1/4 and 1/6 for 12/8 time signature. Time signatures with prime number numerators have no whole number bar duration dividers so, for example if you have 7/8 time signature, you’ll jump straight from 1/1 to 1/7, then to 1/14, 1/28 and 1/56 (which is the duration of the 8th part of a quaver expressed as a part of the duration of a bar in the 7/8 time signature). Might look complicated, but it really isn’t!
As you have probably noticed, SpectrumWorx synchronizes its LFOs to the bar duration, as opposed to the beat duration. This might require a bit of getting used to but we hold this to be more musically correct and it plays better with creating presets that are supposed to work across different time signatures. So if you, for example, create a preset with an LFO synced to the duration of a bar (1/1) under a 4/4 time signature, when you load it under a project with a 3/4 time signature it will still be synchronized to a bar (as opposed to 4/3 of a bar).
The story, of course, does not stop here. The jump from “free” to “synchronized” is not a simple on/off switch. To turn on host tempo synchronization SpectrumWorx provides the usual three modes, independently selectable using the N, T and D text buttons. The N represents a normal mode that enables straightforward host tempo synchronization described above. For more complex rhythms, the T and D buttons also enable snapping to triplet and dotted notes, respectively. When all three sync modes are deactivated, the LFO switches to “free” mode.
As usual, for this magic to work correctly, SpectrumWorx relies on the host to correctly implement and provide the timing information functionality. There are some hosts that do not provide timing information at all; for such hosts SpectrumWorx will assume a 4/4 measure with a 120 BPM tempo and set its LFOs to free mode and disable the sync buttons. If you load a SpectrumWorx preset with a synced LFO in such a host, SpectrumWorx will load the preset, adjust any synced LFOs for the mentioned “default tempo and measure” but leave the sync buttons disabled. This is done so (as opposed to automatically switching all LFOs to free mode) to prevent the destruction of the preset’s LFO synchronization setup by subsequent saving (accidental or automatic) of the preset while in such a host.
Finally, it is important to note that SpectrumWorx, as a frequency domain effect, inherently processes data in blocks (or frames) and, as such, cannot update its LFOs (i.e. their phase) for each sample but for every block. What this means in practice is that, unlike for time domain effects, the SpectrumWorx LFOs do not change perfectly smoothly, and that not all period values make sense given a particular setup — values comparable to or smaller than the current block size will cause the LFO to exhibit aliasing effects. Whether or not the effects of “quantized” LFO updating will be audibly noticeable depends on the period you set and on your host and SpectrumWorx engine setup. Currently, LFOs are updated every frame or every (host) data buffer block, whichever is larger. You only need to make sure that your LFO periods do not go near or below this value — unless of course you want the “undefined behavior”!
Waveform
Section titled “Waveform”Each LFO can output one of several different waveforms. This waveform has a lot to do with how the effect will sound. For instance, the regular and smooth Sine wave will result in a steady, even rise and fall of the destination parameter, while the Square wave will switch abruptly between the high and low parameter values as determined by the Range setting.
In addition to Sine and Square you get Triangle, Sawtooth, reverse Sawtooth (htootwaS), Exponent, three random waveforms (Hrandom — random and hold, Grandom — random and glide, and Whacho — sort of a totally-random waveform), and two impulse waveforms (Dirac up and Dirac down).
You can make a pretty good guess at how each will sound by looking at the little graphic that goes along with each of them.
Phase control enables the waveform to start from some shifted, non-zero value. It can go from -50% to +50%.
This allows you to tell SpectrumWorx exactly how much of the LFO you would like to apply to a given parameter’s value. It takes the form of a slider with dual handles. The left-most handle controls the lowest range of the LFO, while the right determines the maximum effect to be applied.
For instance, say you want to apply an LFO to the Start frequency of the Slew Limiter module from the first screen grab. You want the LFO to drive it all the way to the fullest range, but never dip below the halfway point. You’d set the left slider handle about halfway across, and the right one all the way to the right. The minimum and maximum range values will of course differ depending on the selected parameter.