[pjc50]

Clipping in the digital domain like this introduces an extremely hard corner, effectively corresponding to a set of signals near the sampling frequency and its harmonics that are required to perform the shaping. That's where the "interesting distortion" comes from.

Real diodes are not quite as sharp, because there's an exponential region between forward voltages of 0V and the forward voltage of the junction (~0.7V for P-N diodes, ~0.3 for Schottky).

And vaccum tubes are softer still, with a considerable rolloff region.

[alexjplant]

Cliff Chase, the guy who started Fractal Audio [1] to build the Axe-Fx digital guitar processor, often writes about these types of disparities [2] since his business is modeling analog components in the digital domain. I'm not a trained EE and have a very surface-level understanding of the subject matter but I find these types of talks fascinating nonetheless.

[1] https://www.fractalaudio.com/

[2] https://forum.fractalaudio.com/forums/tech-notes.77/

[Turing_Machine]

Some still insist on old-school germanium diodes and transistors for things like clones of the classic guitar effects pedals. Those are also ~0.3 V, but their proponents argue that the shape of the "knee" and other parameters are different to silicon, thus leading to a perceptible difference in sound quality (in this application, the devices are often intentionally operated in non-linear regions) I've also heard that the specs for old germanium devices varied a lot more than modern silicon ones, thus leading to stories that (e.g.) Jimi Hendrix would order a dozen pedals and pick the one he thought sounded best.

[squarefoot]

> Some still insist on old-school germanium diodes and transistors for things like clones of the classic guitar effects pedals. Those are also ~0.3 V, but their proponents argue that the shape of the "knee" and other parameters are different to silicon

This is true. The important feature of germanium diodes, wrt this application, isn't their lower forward voltage as Schottky diodes are even better at that , but rather their characteristic V/I curve which is softer than silicon (0) and produces less odd harmonics which would otherwise contribute to a harsher sound.

Regarding germanium transistors, they're mostly used to replicate old school fuzz guitar pedals. In that application, aside the above characteristic, the particular configuration in which they're employed contributes to the sound: old fuzz pedal produce a very pleasant asymmetric distortion in which one half wave is much less distorted than the other, so that the output can contain a blend of clean and distorted sound. R.G. Keen at Geofex (.com) wrote a thoroughly informative article about how that circuit works which anyone interested in building guitar pedals will find very informative (1).

(0) https://commons.wikimedia.org/wiki/File:V-a_characteristic_d...

(1) http://www.geofex.com/article_folders/fuzzface/fffram.htm

[ilyt]

That reminds me about podcast with some mic guru that was talking about zener diodes. They used ones to stabilize high voltage used for a capacitive microphone and found out that while initially they didn't care much as long as voltage matches, some models had harder or softer knee and the softer ones reduced the noise level coz they provided more noise-free power.

[atoav]

If you want that sharp corner in the analog domain you can use an active clipper circuit (e.g. diode inside an opamps feedback path).

[someguydave]

As long as your ADC and DAC are not clipping, I don’t see why could could not model any arbitrary distortion process with DSP

[duped]

I said this in another comment, but the fundamental difference is that the frequency spectrum in discrete time is periodic and therefore finite in the range that the DAC can recreate, while distortion creates infinite spectra beyond the confines of that period. This is a fundamental limitation of DSP.

The way around it is to make sure the upper harmonics generated by distortion beyond the limit is below the audible threshold, either by making the period larger (over sampling) or modifying the distortion process. It's not lossless and a difficult design problem when factoring in computational limits. It's often cheaper to use discrete analog components with all their flaws than a good-enough processing chain and computer that can evaluate it.

Even offline whitebox modeling tools like SPICE, and realtime black box tools like Volterra series models are limited by this.

[someguydave]

you can find very high dynamic range ADCs and DACs that cover the audio spectrum plus a few megahertz. Distortion due to spectral aliases above the Nyquist frequency is simply not an issue that one needs to worry about with decent hardware.

As for the cost efficiency of DSP emulation vs using genuine parts - sure maybe it makes sense to use real analog hardware. It depends on the specific requirements of your design.