Spectre

spectre is a resonant tone shaper, which adds both filtering and feedback delays to the five oscillators. It features three analog bandpass filters which are sent through two BBD delays in a dynamic feedback loop. The filter settings are captured in a sequence, allowing them to be stored and recalled rhythmically with the lens sequencer, with three levels of morphing.

spectre knob

spectre is both the name of the processor and one of its parameters.

The spectre knob is a macro control, controlling the balance of dry, filtered, and delayed sounds, all while driving the feedback intensity. Think of it as a dynamic scan through the discrete facets of the circuit. This control can feel very different depending on both the source material (the five voices) and the other settings within this region. This macro traverses three zones, cleanly moving unique parameter scalings through an expressive range. Starting fully counter-clockwise and moving clockwise:

Moving spectre from counter-clockwise to 10:00 crossfades from the dry signal to the filters' output without any delay. Reminiscent of the sculpting found in Three Sisters.
As you increase spectre clockwise to around 2:00, the output of the filters is progressively fed into the stereo delay, adding feedback as you go. The delayed signal is heard only through this feedback path, leading to ever-growing resonant echoes.
Continuing to increase spectre to full clockwise crossfades the filtered signal into the dry signal with direct delay output, while reducing feedback. Maximum feedback at the start reduces to ~50% at full clockwise.

Each section of spectre has its own quality and is useful as a static tone shaper for the whole instrument. Both edges of the range will prioritize the dry signal, leading to the brightest full-spectrum tones. Meanwhile the centre of the range is decidedly coloured and allows the spectre processor to shift your sound more dramatically. It can be really fun to modulate this control, but feel confident to leave it in a static position & define a resonant shape for your patch!

time

The time knob controls the delay time for spectre's feedback path.

The delayed signal is only heard when the spectre knob is above 10:00.

As is common in Atrium, higher settings (clockwise) are faster:

Turn time counter-clockwise to set the delay time to 800 milliseconds.
Turn time fully clockwise to speed the delay up to around 1.5 milliseconds.

While this might sound straightforward, there are a lot of interesting things that happen across this range, interacting strongly with the filter settings. At the shortest times you'll approach some clangorous tones, applying a comb-filter effect — this can be fantastic for imparting a strong resonant quality to your patch.

Conversely the slowest speeds are notably lo-fi, owing to the nature of bucket-brigade delay technology. At these long times, the sampling rate is greatly reduced and high-frequencies will be lost. When using long delay times, focus on filter frequency settings in the lower half of the range.

When modulating time, if the mapping source is polyphonic, voices 1 & 5 will be used to control the left & right channels independently. Try mapping (just a little) lfo to time, then spread out the phase to hear the echoes bounce against each other. Perhaps even modulate phase for a through-zero chorus effect.

spectrum touchplate

The arch around spectre is a touchplate that controls the spectrum of frequencies to be filtered. Think of it as an equalizer that passes only three selectable bands. The range from left to right is around 100Hz to 12kHz allowing for dramatic shaping of your voices.

Swipe with one finger to adjust the cutoff of all three filters, adjusting each frequency band by an equal amount. If you continue to push to either extreme, the filters will coalesce at a single frequency — they will retain this alignment even if you then move in the opposite direction. To bring all three filters' frequencies to the centre of the arch, hold clear and touch the spectrum touchplate.

To spread the filters, changing their relationship to one another, employ a two-finger touch instead. You can either:

Hold one finger down to 'pin' one side and move another finger on the opposite side to manipulate that filter's cutoff value without moving the other two.
Spread both fingers away from the arch's centre to pull the left- and right-panned filters further away from one another, which keeps the centre steady.
Push the bands together with two fingers to bring them both closer to centre.

While the factory patches typically place the left filter to the left of centre (and vice versa), this can lead to an imbalanced stereo field where the left channel has more bass. You can however use the third two-finger gesture to cross over the centre channel, flipping the spectral panning. When using two-fingers, the left-most touch on spectrum will always correlate to the left output channel — use this to determine the organization of frequency bands.

lens & morph

As you explore spectre, you'll find that the placement of the filters strongly affects the final shape of the signal. Atrium offers a sequencer to spectre, named lens, which stores up to 11 snapshots of different cutoff frequency landscapes.

Press lens to step forward through the sequence.
Cutoff adjustments (with spectrum touch) directly modify the current sequencer step.
To change the sequence bounds, see sequences.

The abrupt changes of lens steps can be softened with three levels of slew. Press the morph key to cycle through slew times. When morph is active, touches on spectrum will also be smoothed out. This can be lots of fun with the longest morph setting and a spectre setting with lots of feedback — listen as the feedback latches onto different harmonics in the sound while it slides about.

morph moves the frequencies linearly, at a fixed pace. As such, morphing frequencies that are far apart will take much longer than those that are close together.

Stereo Field

Put simply, spectre is a stereo effect, but part of its magic comes from entangling the left and right channels. While these are implementation details, perhaps they will give a curious few of you new ideas on how to use spectre.

How the signals flow:

Atrium's five voices are mixed down to a stereo pair. Each voice is spread evenly across the stereo field, with voices 1 & 5 at hard-left and hard-right respectively.
This stereo mix of the voices is sent to each filter: Left->Left, Right->Right, and Left+Right->Centre. The central filter thus operates on a "mono" mix of the signal.
Filter outputs are also mixed (again) to stereo: Left->Left, Right->Right, Centre->Left+Right. This is the signal you hear as the "filtered" sound, but is also used for feedback...
Stereo feedback is sent into the delays, up-mixed the same way as in step 2, and recirculated into the three filters.

The result is feedback that cross-pollinates across the stereo field via the centre frequency band. You'll find that spectre can often have a narrowing effect on the sound which is often useful to make Atrium sit better in a mix; taking up less space. By the same token, you can exaggerate the stereo effect with dramatic frequency settings, lens sequences that alternate left/right panning, and polyphonic modulation of time which pulls the stereo width out considerably.