Calling all Analog Technical Guros!

[Palaver]

Hey guys,

 

I was wondering what actually makes the waveform in an analog synthesizer? Someone asked me the other day about "how does an analog synth actually create the waveform?"

 

I thought about it, and couldn't answer it. I figure it is some sort of chip... but how does it generate a 'sawtooth' or 'sin' wave for example? How does an oscilator work? I know in VAs, these waveforms are sampled, but how are they REALLY made?

 

Cheers,

PB

[ElectricPuppy]

The gritty details vary from synth to synth, but in most cases, the oscillator is based around the generation of a sawtooth. The sawtooth waveform is produced by controlling the charge rate on a capacitor. When the capacitor reaches a threshold, it's quickly discharged and the cycle repeats, resulting in a sawtooth. By controlling the rate of charge, you can control the frequency of the sawtooth.

 

A pulse wave is derived from the saw by comparing sawtooth level against a threshold level. When the sawtooth wave is above the threshold, the pulse is high, and when the sawtooth is below the threshold, the pulse is low. By varying the threshold, you vary the pulse width.

 

Triangle is usually made from saw by rectification. At the halfway point on the sawtooth ramp, the output polarity is reversed, or "mirrored", which gives a triangle. There's often a glitch at the discharge point, though, but this is usually filtered out.

 

Sine can be approximated from triangle via some selective non-linear distortion. I've seen this done with diodes, usually.

 

Not all synth oscillators work exactly the way I've described, though, this is just one way.

[Palaver]

 

The gritty details vary from synth to synth, but in most cases, the oscillator is based around the generation of a sawtooth. The sawtooth waveform is produced by controlling the charge rate on a capacitor. When the capacitor reaches a threshold, it's quickly discharged and the cycle repeats, resulting in a sawtooth. By controlling the rate of charge, you can control the frequency of the sawtooth. A pulse wave is derived from the saw by comparing sawtooth level against a threshold level. When the sawtooth wave is above the threshold, the pulse is high, and when the sawtooth is below the threshold, the pulse is low. By varying the threshold, you vary the pulse width. Triangle is usually made from saw by rectification. At the halfway point on the sawtooth ramp, the output polarity is reversed, or "mirrored", which gives a triangle. There's often a glitch at the discharge point, though, but this is usually filtered out. Sine can be approximated from triangle via some selective non-linear distortion. I've seen this done with diodes, usually. Not all synth oscillators work exactly the way I've described, though, this is just one way.

 

 

Awesome. I really appreciate the response.

 

Thanks,

PB

[ClavAnother]

Nice job, Professor Teh Puppy.

 

Next weeks lesson: Ladder Filters

[dereksljuka]

 

I know in VAs, these waveforms are sampled

 

 

This is wrong, a true VA synth uses no samples. The waveforms are generated in real time through mathematical calculations running on DSP chips.

 

When the Nord Lead (first true VA) came out in 1995, one of its selling points was, "No Samples!"

[AfroRouge]

Tasty analog gyros!