[sdiy] Enveloppe follower

[rsdio at audiobanshee.com]

Great question!

I’ll take a stab at an answer, but you’ll surely need to refer to other sources to get the whole picture.

Really understanding how an envelope follower works means that you need to understand how sound works, what amplitude is, at least a little about human perception, and finally, of course, how electronic circuits work.

My pet peeve are those books that spend three chapters reviewing the same basic concepts that every other book on the subject covers. I sometimes wonder how much smaller and lighter my bookshelf would be without all of those repeated pages. Here I am, though, about to do the thing I hate. The good news is that it’ll be reasonably short, and you don’t have to pay for a book or find space to store it.

How Sound Works:
We all live in air that is approximately one atmosphere of pressure. We cannot hear that constant pressure, though. It’s equivalent to a DC signal. What we hear are pressure variations that cycle through slightly above normal pressure to slightly below normal pressure. Of course, regular variations are pitch, and irregular variations are noise. Sound can be recorded by turning pressure variations into variations of some other value, such as analogue voltage or digital numbers. Sound can also be created from scratch by creating variations in a value that eventually end up converted to air pressure variations by one or more transducers. You’ll note that the air pressure never goes negative, at least not in absolute terms. What we consider the “negative” part of a waveform is just the part that drops below normal air pressure (of about one atmosphere). Even a pure vacuum is not negative pressure, but merely the lack of any pressure - a zero.

What is Amplitude:
Without jumping directly to human perception, amplitude is the intensity of a sound or its loudness. It roughly corresponds to the difference between the lowest air pressure and the highest air pressure. If the difference is small, the amplitude is quiet. If the difference is large, the amplitude is loud. For completely different reasons than how there are no negative air pressures, there happen to also be no negative amplitudes. The smallest amplitude is zero. Silence. The largest amplitude is going to be dangerous. In the digital domain, 24-bit samples have enough resolution to handle everything from the quietest sound ever heard by a human to the pressure wave a short distance from a nuclear blast. Not any analog circuit can handle all of that without noise (at the quiet end) and clipping (at the loud end) reducing the dynamic range.

Human Perception:
Here I’ll really try to be brief. Human hearing only spans frequencies from 20 Hertz to 20 kiloHertz. There is also a minimum amplitude under which we hear silence even though an expensive microphone might pick up something. All of this is a blessing in disguise, because if we could hear that low, we would be constantly annoyed by changes in the weather that affect air pressure over time. When it comes to amplitude, humans have twenty frequency bands that are analyzed by our brain. Within each band, added frequencies do not contribute as much to our perception of increased loudness. In contrast, added frequencies that fall into separate bands are interpreted fully as increased loudness. For this and other reasons, sometimes a frequency in one band can mask a frequency in another band. There are even some very strange details of psychoacoustics due to the fact that our brains interpret sounds slowly enough that sometimes a sound occurring later at one frequency can actually mask an earlier sound at another frequency. It is as if our brains have not finished figuring out the early sound, and when the later sound arrives our brain just completely ignores the partial analysis and throws out the earlier sound. Anyway, most of this is not important for an Envelope Follower except for one thing: What you think you hear may not always be the same as what the Envelope Follower says, because our brains are way more complicated than any circuit we can build simply.

How Electronic Circuits Work:
Our friend is the capacitor. It stores electrons as charge, building up a voltage, and holds on to that charge as well as it can according to its capacitance. If you build a circuit that can control the charging and discharging of a capacitor, then you can use the voltage in the capacitor as a CV for an Envelope Follower. Our next friend is the diode. It allows charge to flow in only one direction. This allows better control over the speed of charging or discharging the same capacitor, since each can be treated separately, perhaps with a difference resistance to slow the rate of change of charge. Op-amps are another friend, because they can buffer voltages without drawing current from their inputs, and because they can greatly amplify currents in a precise manner.

Jacob has already described some ways to use these in a circuit to build an Envelope Follower. There are many variations, but hopefully the concepts summarized above will help.

Brian Willoughby


On Dec 4, 2019, at 7:44 AM, Jean Bender <lofideadbeat at gmail.com> wrote:
> Well, i know i could just pick up any schematic i can find on the net
> for building myself an enveloppe follower as many exist actually, but
> i would rather really understand how it works so..
> 
> I would be happy to grab any advice, text, info, history about the way
> they are built.
> Also if you have schematics you find useful, or because they offer
> specific way of building a such module..
>