Tiptop Audio are old hands in the modular scene, but since the release of their Z2040 low-pass filter in 2009 they’ve been missing a multimode option in their ever growing stable of modules. In a blaze of sci-fi laser zaps (plus the odd other sound), the Forbidden Planet is here to fix that.
Forbidden Planet is of course a nod to the 1965 sci-fi classic (starring none other than Leslie Nielsen before he discovered his comic flair). Awash with drifting oscillators, spring reverbs and tape delays, the soundtrack is accepted as the first ever 100 percent electronic film score. Interesting fact: the American Musician’s Union refused to classify the soundtrack as music, instead crediting it as ‘electronic tonalities’.
The Forbidden Planet filter embodies just a little bit of its quintessentially analogue namesake thanks to a 2-pole, 12dB/oct filter inspired by the 1975 Steiner-Parker Synthacon. Clearly aimed at the cost-conscious end of the market, there aren’t a lot of bells and whistles. You get three modes: high-pass, low-pass and band-pass, as well as frequency and resonance controls, two CV inputs (both controlling frequency), a volume control, and a single output.
A single output? Really? On a multimode filter? Well, yes. The filter has a separate input for each mode and just one output which carries the combined signals of each filter type. This is the reverse of most modular multimode filters, which offer one (or sometimes two) inputs and a separate output for each filter type. This seems perplexing at first but it doesn’t take long to see the potential.
You can put two or three completely different sources through each filter type simultaneously and have them neatly mixed at the output, something not possible with the more common single input, multiple output design. Strap two different signals to the low-pass and high-pass inputs and the frequency control becomes a crossfader that operates frequency instead of amplitude. Other combinations of inputs lead to interesting gluing of source material where two or more sounds mingle in fascinating ways.
Because the mixing of filter types is done inside the module, you do lose the possibility of processing different filter types elsewhere in your modular. And because there are no input attenuators, you will have to make sure that different signals are at the right levels before they reach the filter. Nonetheless, I enjoyed having this unique configuration as an option amongst the more traditional multimode filters in my system.
The Synthacon filter is well known for its resonance, which can be used to create all manner of unstable, grainy, harmonically rich, self-resonant tones that lend themselves easily to sci-fi sound design — amongst other things of course. Think of it as a resonance control that goes to 11. It also has an agreeable characteristic whereby an increase in resonance does not cause the output to decrease in amplitude, as is common in some other well known filter designs. I can attest to this being true of the Forbidden Planet too. It makes it perfect for all manner of tweakathons where you want the signal passing through the filter to remain solid even with gratuitous changes in resonance. It is a shame therefore that there is no CV input for resonance as this would seem like the ideal filter for it.
One old trick that I found the Forbidden Planet struggled with (actually it failed completely) is in making self-resonant kick drums. Leave the inputs unconnected, crank the resonance up to self-oscillation, and apply a short snappy envelope to the frequency input. From this simple patch one can usually coax all sorts of kick drums, ’70s disco toms and assorted electronic percussion (plus a few laser zaps worthy of a Forbidden Planet foley library) . However, I struggled to get any low end out of the self-oscillation. Kick drums were elusive. Closer inspection of the filter’s output on a spectrum analyser revealed that the sine wave caused by the self-oscillation starts to decrease in volume at around 200Hz. By 150Hz it has disappeared completely. My Analogue Systems multimode filter, by comparison, will go right down to 23Hz when self-resonating. It’s a shame. Perhaps one shouldn’t expect all filters to be equal but be warned, this one simply won’t make self-resonant kick drums, no matter how hard you try.
Like the mysterious unearthly beast in Forbidden Planet that rips unsuspecting extras limb from limb (off camera of course), the sound of a filter can be a difficult thing to pin down. Because of the aforementioned lack of volume dip when the resonance is increased, the filter has a solid sound. The resonance is pliable, going from smooth and creamy at low levels, through to vocal-like at medium levels. ‘Wow’ sounds aplenty. At high levels the resonance starts to crumble and break up, but still with a strong sine wave at the cutoff frequency. This area is really where the Forbidden Planet sci-fi influence is most evident.
On the whole I like the Forbidden Planet a lot. It’s a great sounding filter, it doesn’t use a lot of HP, and despite a few foibles, its unusual approach to inputs and outputs gives it a character of its own. It covers the basic filter types you would want from a multimode, and can be coaxed into sounding anywhere from subtle and sublime to gnarly and capricious. The price is excellent and I suspect it will find a lot of happy homes amongst racks of all sizes, making both ‘electronic tonalities’ and, let’s hope, music.