Hysteresis

[tkingen]

Here's what I came up with while trying to figure out what hysteresis has to do with recording. The concept is clear - but what does it have to do with tape emulation plugins and how can you hear it?


What's Hysteresis?

Webster's Seventh New Collegiate Dictionary tells us:
hys-ter-e-sis:
n [NL, fr. Gk hysteresis shortcoming, fr. hysterein to be late, fall short, fr. hysteros later]
a retardation of the effect when the forces acting upon a body are changed (as if from viscosity or internal friction); esp: a lagging in the values of resulting magnetization in a magnetic material (as iron) due to a changing magnetizing force. -hys-ter-et-ic adj

There seems to be no etymological link between hysteresis and either hysterical (fr. L hystericus of the womb) or history (fr. Gk, inquiry, history, fr. histor, istor knowing, learned). This is too bad, as there are scientific connections to both words. (There is no link, scientific or etymological, to histolysis, the breakdown of bodily tissues, or to blood.)
Hysteresis represent the history dependence of physical systems. If you push on something, it will yield: when you release, does it spring back completely? If it doesn't, it is exhibiting hysteresis, in some broad sense. The term is most commonly applied, as Webster implies, to magnetic materials: as the external field with the signal from the microphone is turned off, the little magnetic domains in the tape don't return to their original configuration (by design, otherwise your record of the music would disappear!) Hysteresis happens in lots of other systems: if you place a large force on your fork while cutting a tough piece of meat, it doesn't always return to its original shape: the shape of the fork depends on its history.



Hysteresis loops happen when you repeatedly wiggle the system back and forth (cycle the field up and down). The magnetization of a tape will ``lag behind'' as the field sweeps up and as it sweeps down. The memory in the tape is the magnetization remaining as the field is released to zero from a large value. In magnetic tapes, this lag is repeatable: the shape of the loop after the first cycle is roughly the same as it is after many cycles. (This is convenient for doing multiple recordings on the same tape.) This is not true of many other systems: forks, for example, after being bent back and forth many times, will actually become stiffer (``work hardening'') and then break. There is a class of metals (called shape memory alloys) that can be bent or stretched plastically large distances back and forth many times without work hardening: this superelastic behavior is only one property of these interesting materials.

Many hysteretic systems make screeching noises as they respond to their external load (hence, the natural connection with hysteria).

 

[Ford Van]

Think of it as the Release knob on a compressor and you will get the concept.

 

[SouthSIDE Glen]

Here's what I came up with while trying to figure out what hysteresis has to do with recording. The concept is clear - but what does it have to do with tape emulation plugins and how can you hear it?

Ed has the right general idea in that it represents an action that is not instantaneous, but rather occurs in a slope over time, much like compression release.

In this case, it's a matter of the change of the mageentic level of the tape. The oxide particles take time to both absorb and release their magnetism, it's not necessarily an instantaneous transfer. For example, if you have a spike transient in your audio that the record head hits with a magnetic strength of, say, 200nW, the full 200nW of magnetic strength may not be absorbed by the tape because the transient goes by too fast. This will result in a playback from tape where the peak is not quite as strong as the original and the shape/slope of the peak will be skewed because of the slow magnetic absorbtion and release characteristics of the tape.

This is a big part of what many people like about the sound of analog tape, the "warmth" that results from the "smoothing" effect of tape hysteresis. A hysteresis control on a tape emulator lets one artifically synthesize and dial in the hysteretic response of the tape being emulated.

At least that's been my understanding of it. I'm sure beck or one of the other analog guys will come along and correct my somewhere

G.

 

[tkingen]

Thanks Ed and Glen - it's getting clearer now.

Glen, from your explanation (tape not fully absorbing the initial transient) it sounds like it's a combination of both attack and release.

 

[chazba]

Getting hysterical about it!!!!

Mag Tape recorders apply an AC signal to the tape which is several times higher than the audio freq's that are to be recorded. The function of this "bias" current is to keep the magnetic domains of the ferric oxide stirred up (overcoming hysteresis) in order to make it easier for the audio signal to be applied to the tape by modulating the bias signal. Over simplified you say???? Probably but I did it w/o any math. Anybody wanna buy a R2R machine???


lol


chazba

 

[SouthSIDE Glen]

Glen, from your explanation (tape not fully absorbing the initial transient) it sounds like it's a combination of both attack and release.

Well, yes, because the tape is slow to change in both directions. But in this case the "release" is what makes magentic tape work as a stroage medium at all; the extremely slow release is what holds the magnetism to the tape. It's called "recording" .

A good (but not perfect) analogy that comes to mind is if we substituted magnetism for heat and tape for water. One can apply a hot flame to a pan of water, but it takes a while for the water to actually heat up. Same thing after you remove the heat, it takes a while for the water to cool down again.

Chazba brings up a good point about the bias current being used to help overcome hysteresis on the recording or "attack" side. It's kinda like pre-heating the water so it doesn't take as long to bring it to a boil. Though in this case it's keeping the magnetic domains on the tape "pre heated" so that it responds to transients better and actually sticks to tape better.

But it's still not a perfect copy, there is still some slew in the recording to tape. This slew in the recording is, I beleive, what the "Hysteresis" control on a tape emulator plug tunes.

Again, if I'm off on that, someone will come along and correct me, but I think that's pretty much what's up with that plug control.

G.