This little monosynth patch was christianed Kitty as it snores and purrs nicely away if fed a nice sequence. Just connect your sequencer, select a pattern, press play and start tweaking. Contraty to my habit of sending patches without knob and CC# assignments this one has a full set of assigned knobs and CC#s.

It features:

So I thought it interesting enough to share.

The filter is made with two parallel SVF filters. The signal of the second filter is inverted and the mix is two times the first LP output _minus_ one time the second LF output, the minus is done by inverting the signal before it enters the second filter. The resonance for both filters must always be exactly the same. The effect is that if settings for both filters are the same the filter is a standard 12 dB LP filter (2* filter A minus 1* filter B = 1* filter A). However, if the resonance is raised to a fairly high value and the cutoff of the second filter is slightly raised the peak of the second filter is subtracted from the frequencies just above the cutoff of the first filter increasing the slope of the filter. This makes it sound more like a 24 dB. This technique is called elliptic filtering as the total of four poles are now in a elliptical configuration. The Offset knob controls how much the second filter is detuned relative to the first filter. Increasing the Offset value creates squelchy filter effects. The Offset can also be negative which produces a slightly 'talkative' formant effect. The modulation envelope can be applied separately to both the Cutoff and the Offset to create interesting filter sweeps. There is a button 'Legato' that controls whether the modulation envelope is or is not restarted if notes overlap. Glide is also controlled by note overlap, so if there is legato note overlap there is also a slight glide to the second note.

I had a look at the schematics of the 303 and as far as I can see the 4-pole Moog-type ladder filter does not track the oscillators pitch. Instead they use a trick where the cutoff frequency is lowered if more envelope modulation is applied. I put that in the patch as well, you can hear it if you set the modulation envelope to a very short pulse and vary the amount of envelope modulation. It is not on the Offset envelope modulation. The LFO rate is controlled by the Midi clock. I used the simple pulse integration method where the midi clockpulse is set to a small fixed value of 1 msec. Two smooth modules integrate the pulse to a control value that increases exponentially if the midi clockrate is raised. A control mixer is used to adjust the value to more or less make a LFO ratio of 1:1 the same to a 16th beat. Although this makes the LFO track the midiclock quite well it is not exactly accurate over a longer period. So a clcok divider is used to to regularly reset the LFO, a division rate of 16 makes the LFO reset at the start of ecery 16th bar, a division rate of 32 on every second bar.

The LFO is used to modulate either the cutoff directly or to modulate the modulation level of the modulation envelope. The direct LFO modulation effect is inverted compared to the modulation depth modulation. So if both are applied they try to counteract each other instead of fortify each other. This to keep the timbric modulation in the same part of the spectrum, for the same reason as why the cutoff is lowered if the amount of envelope modulation is increased.

Well, just play with knobs 5,8 and 6,9 and you get the feeling.

The LFO modulates a panner as well.

To add some slight distortion the BP outputs of the filters are used and distorted. This signal is simply added to the LP outputs. In most cases I don't like severe distortion too much, I more like it if distortion is just that little pinch of pepper and salt. But that depends a lot on what more is in the mix. Well, of course you can mess up this patch any way you like. Although the synth can be played by keyboard it is specifically crafted to be used with 80 to 160 BPM sequences. This as the AD modulation is most expressive in the first 200 msec of the notes. Also the LFO only works if a midi clock is present. So consider this patch to be made for the regular '303' stuff.

If the VelAccSwitch knob (17) is set to eg 50 all notes with a velocity value between 100 and 127 are accented. The depth of accent effect is set by knob 18 and can be gradually increased in the positive or negative direction, the last effectively reverses the accents.

The Midi CC# assignments are chosen in a way that the knobs on my Yammie DX200 control the same parameters. Perhaps they need reassignment for other sequencers. Just press F9 (on PC) to see the CC# assignments. Using the 'Send Controller Snapshot' fucntion from the Synth menu can be used to record different knobsettings on any place in your track when using eg Cubase or Logic. And tweaks on all 18 knobs can be recorded as well as all are assigned to a CC#.

This info on your filter sounds similar what I was trying to do, only I used a notch filter in series after the first lowpass.

Is this configuration effectively a model of what happens in the "4-pole Moog-type ladder filter " used in the actual 303?

No, the TB-303 filter is a traditional ladder-type with four poles in series and one feedback path over all poles. To me there seems no special thing in the design to account for the 303 sound, only reason for a 'different' sound I see might be component quality or something. Probably cheapo components with lots of tolerance as the TB-303 was a cheap gadget when in the shops. Which proves that doesn't necessarily have to be a bad thing.

My dual filter is based on subtracting the difference between two filters. This can create 'unusual' peaking characteristics but might sound totally different to standard filters. To my knowledge there are no prepatched synthesizers that used the difference between two filters to create the filtering action. This has always been the domain of modular synthesizers and especially 12dB SVF filters. Basically _subtracting_ an equal amount from the LP output on one filter from the LP output of another filter creates a bandfilter with two corner peaks. Other amounts and offsets create different effects.

Here is a little filter based on the same principle, nice on drumloops and samples. Extreme settings can produce extreme results J

Yet another tb303 patch, sounds really good, me thinks with e.g. maq16/3 sequencing - accent at velocity 127 - slide controlled by modulation - if the modwheel is over 64 the envelope is opened - figure out the knobs...