I would like to recreate this sound:.
It is a string sound from the Dave Smith OB6 (which I do not have).
Can anyone tell me which oscillators are used, and how to get the smooth and silky high frequencies?
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One important aspect of synth strings that is part of the OB-6 is the filter, which is a two-pole filter that has a 12 dB per octave slope. The very popular Moog "ladder" filter and copycats of that are four pole filters with 24 dB per octave slopes. It's much harder to get a convincing strings sound with a Moog filter.
There are two popular ways to synthesize strings with an analog synth. One is to use saw waves and the other is using square/pulse waves with pulse width modulation (PWM). If you have PWM on your synth, this is usually considered the best approach. If you don't have PWM, then you'll probably have to make do with saw waves or a mix of saw and square/pulse waves.
If you only have one oscillator per voice and you don't have PWM, it's going to be very hard. If you can add a chorus effect to this, then you might be able to make it work.
Basic two-oscillator options for strings:
You'll want to play with the LFO settings used to modulate the pitches of the oscillators and/or pulse widths. Look for a rich, slightly buzzy, chorused sounding tone. It normally will be too bright - which is where the filter comes in.
Take a wide open low pass filter with zero resonance and slowly bring it down to eliminate the buzz and fizz on top of the sound you have. Getting this right will make the difference between a great strings sound and a bad brass sound. As I mention above, you want a 6 dB per octave or 12 dB per octave slope max. It's possible to fake strings with a 24 dB per octave filter if that's all you have, but I've never been satisfied with any of my attempts in that area. Do not modulate the filter or apply any envelope to the filter. You can play with a slightly slow attack, but too much envelope sends you into brass territory again, as will any amount of filter resonance.
For the amp envelope, here you have some room to play with. Usually lush string pads will have a slow attack and a slow release. I would leave the sustain all the way up and decay at zero. If you have velocity control over your amp EG, you can try making it so when you play harder the attack is shortened.
Finally, I like to have the ability to volume swell. If you have a pedal for that, excellent. Otherwise, some aftertouch linked to the amp EG sustain or overall volume can give you the ability to make string swells.
There's no greater source I've found for understanding synthesis than the Sound On Sound ("SOS") Secrets of Synthesis series. Here are the two articles on strings: