Product Description
(This module is included in the System B)
The spectral filter is a powerful concept that gives you the complete freedom to edit both the frequency and phase response across the audio spectrum. With morphing, stepping, CV modulation and skew features.
The full spectrum from 30Hz to 20kHz is split to several bands (shown as bars) that are manually editable. There are two modes for editing, Amp and Phase changeable through a switch. In Amp mode, bars higher than middle amplify and bars lower than middle attenuate. In Phase mode, bars are bipolar. Highest will cause a phase shift of +180° and lowest will cause a phase shift of -180°, in between is in between this range. Middle is no phase shift*.
Tip: Phase shifts have no immediate effect on the filtered sound. Correctly so, specially at higher Quality settings. You’ll have to mix in something post the filter or do some further processing or modulation to hear the difference. The reason is usually understood by advanced users and is out of scope of this description.
Freq knob/CV controls the frequencies of all bands (bars) together as if you were controlling the cutoff of a regular filter. Res knob/CV controls the resonance. This is done by amplifying the bands that are higher than middle. Dis knob/CV controls the dissonance of the filter. This is done by attenuating the bands that are less than middle. Res and Dis knobs have different settings for each of the two modes (Amp and Phase).
Tip: Connect both Freq CV and the tuning CV of your oscillator to the same CV source. You can get interesting results as the tuning changes the character of the filter. (ie. filter tracking).
You can choose the number of Bands (bars) and you can choose the Size which is a multiplier that specifies the total number of bands, including the ones that are above and bellow audible spectrum. The higher the size, the more bands there are when you sweep the Freq. You have to experiment with that to understand how it works.
There is a total of 100 filters that you can manually edit, morph between using Knob/CV and you can step through them with an external trigger into Trig In. And Rst In is also available to reset and pause like all sequencers in SoloRack. Note that Rst In will only stop/pause sequencing, it will not stop morphing or CV modulation.
All sequencing and morphing will range from filter 0 to the last filter. if passed last filter, it will loop/morph back to filter 0. The red LED will flash when you are at the last filter. You can change the last filter by pressing the 
button. The last filter will also change automatically if you edit a filter that is higher than the current last filter (for ease of use).
There are Copy and Paste buttons to ease duplication between different filters. But notice that the Copy button is also an initializer!! If you press Copy the first time, it will do a normal copy. The second press will initialize the bands to zero. The third press will initialize to middle. The fourth press will randomize, if you keep pressing the Copy button further, it will keep randomizing. To return back to copying, tweak any other control or press Paste.
Smooth will smooth out filter transitions caused by a modulation, automation, sequencing etc. At higher values, it has a nice verby effect, at lower values, the filter is more responsive to changes. Optimize will equalize adjacent closely valued bands (bars). This is solely meant for better CPU usage. The module is able to optimize modulation performance better when more bands (bars) have equal values. Quality knob dictates how well the filter follows your drawn bars. More quality will increase the sharpness of the slopes in the spectrum but will also consume more CPU. Less quality will smooth and round out the slopes but will consume less CPU. Mod Speed dictates the internal poling speed for CV modulation, morphing and automation speed. More mod speed makes it smoother and more accurate but consumes more CPU, less speed is the opposite.
It is important to understand that Quality here doesn’t necessarily refer to better or worse sound. It only refers to how accurate the filter follows your edited bands (bars). It is also important to note that quality in bass differs from quality in mid range, that in-turn differs from quality in treble (high frequencies). The lower the frequency is, the harder it is to achieve high quality. However, you can change this behavior when you apply Skew or VariQ. But more on that bellow.
The Extreme Quality button will multiply the Quality by a large factor. Once pressed, the spectrum slopes will become extremely sharp and even sharper as you increase Quality knob. We added this feature in v2.21 because we realized it does make a significant difference in the sound, specially in lower frequencies. Unfortunately it is very CPU hungry. It is worth noting that Extreme Quality doesn’t do anything special. It simply forces more calculations to achieve sharper slopes.
High Quality values will also start introducing latency in the filter. This can be very audible when Extreme Quality is pressed. This is where the VariQ knob omes in to rescue. VariQ has 4 modes of operation:
1. VariQ at minimum: This will give whats known as a “linear phase” filter response. In layman terms, it means that when all amp and phase bars are at middle, the output waveform will be almost exactly the same as the input waveform without any phase distortion. The side effect of that is that as Quality increases, there will be more latency (delay) between input and output. Use this mode if you don’t care too much about latency and you want to preserve the phases of the input signal as much as possible.
2. VariQ at maximum: This will give whats known as a “minimum phase” filter response. Practically this means that when all amp and phase bars are at middle, the output waveform will still incur some phase changes inorder to minimize latency between the input and the output. Use this mode if you want almost no latency between input and output.
3. VariQ is less than 12 o’clock. Will give less quality to higher frequencies than lower frequencies inorder to defeat some of the latency at high frequencies.
4. VariQ is more than 12 o’clock. Exactly the same as mode 3 but uses a different method to achieve the same result.
Inorder to hear the difference between what mode 3 and 4 do. Introduce a harmonically rich input to the filter (like a saw tooth). Set all bands (bars) to middle, then repeatedly tweak VariQ and watch how it effects the spectrum. Notice, that VariQ near 12 o’clock is the least quality.
Skew is an interesting effect that gels really well with this type of precision filtering. There are 4 skew modes that sound different (off will disable skew). Changing Skew knob may sound like your changing Freq at first. But it’s actually not. Skew directs the quality factor towards lower frequencies as you decrease it. Although the Skew knob is uni-polar, Skew CV is bipolar. The Skew Color changes the character of skew. Notice that Skew Color is sensitive to the bar level. The higher the bars, the higher Color is prominent. Color will be reset automatically when changing skew modes. This is a bit annoying, but it’s necessary to avoid unexpected high gains.
Tip: In skew modes 2 and 4. Changing the phase bars will change the character of the sound ever so slightly without changing the filtering levels.
A DC blocker is also provided at the input because any DC offset will cause unwanted artifacts with such filtering. However, note that the DC blocker will introduce some phase shift around 0Hz. This is practically inaudible. But if you are sure that your input has no DC offset at all, there is really no need for the DC blocker in that case, you can simply set the DC block knob to minimum which will disable it.
Technical details
Higher Quality will always demand higher CPU usage. But higher bands count and/or higher Mod Speed will only demand higher CPU during modulation or automation. Higher Size does NOT cause higher CPU in any case. Using Skew (Not off) requires much more CPU in general.
* Note that these phase shifts are relative, not absolute. This is because the filtering it self does apply intrinsic phase shifts to do it’s job. So the relative phase shifts that you edit are applied on top on the already happening intrinsic phase shifts.
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