Oh, ya, der "industrial stuff".
He probably went to the same college as the pointy haired manager in Dilbert.
You know, "the Einstein one."
You don't have to buy it, just believe it (oh wait, beliefs....)
How and why shielding works as it does.Yes.
No, it IS a circuit. Every object in the universe is capacitively and inductively coupled to every other object. Fortunately, these effects fall off rapidly as a function of distance.
It goes to ground (why we call it "the ground").
It's not a pin-ball machine, or a trampoline.
Think of the (as in a good) ground as a “massive” plane that is not influenced much at all by the signals that are connected to it. It IS NOT just a “wire thru the amp” that is part of a circular circuit, but THE reference and safety point that is the local reference for all signals. In an amp, with the steel chassis fabricated from thick sheet metal and plated to resist corrosion, this ground appears as a very low resistance and impedance (for the frequencies of interest in the audio spectrum) PLANE, with the effect of very nearly constant or equal potential (as in zero) regardless of where one contacts it.
The signal ground may just be a common point within the amp where ALL local signals are referenced. An amplifying circuit amplifies a signal with respect to its local ground, or reference point.
The world land speed record is in reference to the, uh, ground. While the Earth takes 365 ¼ Solar days
(not sidereal days) to circle the sun, at an average distance of 93,000,000 miles it is orbiting the sun at a speed of 33,352 miles per hour. The solar system is traveling thru space-time as a velocity considerable higher than that. Therefore, the record is in reference to the “local ground”.
Now, the extrapolation of this local ground “down the cable shield” is, from a signal instrumentation perspective, fairly horrid. However, it apparently is just good enough for most folk a’planet. To purloin my analogy, think of it as the Outer Banks in North Carolina. While stable enough fer the idle rich to build vacation shacks at government insurance risk, as a sand bar, it does tend to move aboot a bit.
Oh yeah, antennas.
All points are inter-coupled. Distance is distance. The rays from Alpha Centauri, as much as some would like to believe otherwise, due to sheer distance, do not influence us to any fathomable degree.
Allowing the third to “leak in” is what started this whole “m” theory mess anyway (no, not that
“m” theory).
(Well, since “according to Witten himself, "'M' stands for "magic," "mystery" , or "matrix", according to taste.”, maybe it IS that “m” theory.)
OK, we gots a ground. It gots a lot of “mass” (whatever that means). It takes a really big electron hammer to nudge/budge/fudge it (if one must wax Deitical, think of it as Thparky’s Hammer). Did I mention that it was hard to jostle aboot?
An antenna-based circuit (it IS a closed loop circuit) is comprised by a strong reference between the two antennas (the ground plane) and a weak EMF/charge coupling reference. Since it is well understood and demonstrable, it is NOT magic (but then, magic isn’t either, it’s just “parlor tricks”). Now, to be honest, a good ground isn’t
required for antennas otherwise that whole Apollo thing was staged in the Arizona desert (near the defunct Bisbee copper mines, but you didn’t hear it from me, oh the rays, the rays….bzooorrrrp).
Getting back to that “proper signal instrumentation thing”, a proper guitar would have galvanic isolation similar in quality to medical patient instrumentation (there’s a concept, let’s actively try to NOT kill the patient – oh wait, that would be the admission of past negligence), a differential balanced LOW impedance signal output (two wire - Dang, tone is), and a third wire shield conductor back to that there shielding/grounding concept thing, that carried NO signal other than the ones that we want to shunt/convey to the shield/ground (the ones that we DON”T want amplified).
The oatmeal box coil in the crystal radio (so, why is it called a crystal radio and how does it work?) is a parallel tuned circuit (in conjunction with that there tuning capacitator). In essence, the parallel RLC circuit shunts all signals received by the antenna with respect to that local in-radio ground (which hopefully is the Earthly good ground since the radio signals are transmitted with respect thereto), except (to a Q-related varying degree) for those of interest as a “carrier” (hint, hint), which are “consumed” downstream. It’s NOT a vibrato (hint, hint, my dear Watson).)
A pickup connected to the local in-guitar ground (as in signal return/reference point) and the guitar’s output signal (at the, er, output jack) is NOT the antenna.
When the pickup is in circuit (both wires are connected and just not shorted together), it primarily functions as a generator (the AC analogy to a DC magneto), with current flow induced by the variations in the magnetic field due to the varying magnetic path coupling thru the locally sensed string masses. (When the leads are shorted together, it still functions as a
shorted generator, but this effect is not germane to this tone, er tome.)
Unfortunately, it also forms a local secondary on that humongous gigantic air-core (with local ferrous core) meta-transformer comprised of ALL EMF generators in the universe. Fortunately, distance rules and most are moot. Not all, just most.
Since the pickup coil and circuit is comprised of an inductor, a capacitor (within and without), and a resistor, it is a series RLC circuit as it applies to where/how we sense its output. These characteristics of the pickup, which have a response in the frequencies of most interest to musicians, also result in the sensing of these exact unappreciated asynchronous frequency components characterized as noise (“weeds” if you will – when you fertilize your yard, you fertilize your yard).
If a pickup is only connected to the local ground, it acts as a loading coil (one of them there radio things) attached to the base of the short antenna comprised of the wire going to the selector switch. Ok, so it’s not well shielded and leaky and is an antenna.
See, it pickups signals but is connected to the local ground.
The shield.
Shielding does two things; it surrounds stuff to prevent charge coupling to the stuff therein(side) and it surrounds stuff to prevent EMF coupling to the aforementioned stuff therein. Think of it as a raincoat (I’m not sure of what it is called in England (slicker or divot-winger), but I’ll bet it’s funny). (Note: Babelfish does NOT translate between the languages of the British-speaking and English-speaking people.)
Raincoats have openings. This helps prevent suffocation and walking into lamp poles, as well as enabling the use of opposing thumbs and walking into lamp poles. (With flexibility comes responsibility.)
Shields have openings too. They are not 100% effective even when they don’t.
And yes, it’s an antenna (prophylacticly speaking, an out-tenna if you will – but then so is everything to something).
And, like the sheaves of old and new (baa-ganic and refined) they prevent neither pregnancy not disease, just liability.
Primarily, it shields the innards.
theory
There are two types of signals afoot here; common mode and differential mode.
The true differential mode signals are those of musical interest. These are what we want to properly convey to the amplifier, and have the amplifier, uh, amplify. These are the good ones (minds, legends, etc.).
The common mode signals are the ones that are equally induced on every wire. A two-wire balanced signal cable (think of the CAT-5 100bT with four differential twisted pair, no shield required) will convey all signals quite well. The differential ones are properly fed into the amplifier and the common mode ones are, well, common and rejected by a proper amplifier design.
/theory
So where are we. Ah, yes, the present (sad) state of guitar electronics. The amplifiers are not properly differential and discriminating (hint, hint this is not a tremolo), but it wouldn’t matter anyway due to the cables in use.
The cables, even in their best form (forget the cryogenic aligned molecule [this is a pinball machine] oxygen-free hyper-crap, that only works on your wallet) with low capacitance (the “hidden” tone control – the entire 60’s and 70’s sounds were defined by the mud, the blood, and the goo of a 25 foot coil cord) are still SINGLE-ENDED and
intrinsically incapable of properly conveying high impedance low energy passive guitar signals.
The guitars, designed in an era where the (really crappy tube) amplifiers couldn’t regenerate a signal higher in frequency than a mild fart, had no need to be shielded.
(Trivia question: Why did the Stratocaster not have a tone control on the bridge pickup? Because it was the treble pickup and the tube amplifiers couldn’t regenerate a signal higher in frequency than a mild fart.)
Today’s guitars, cables, ‘n amps are based on technology that, for the innumerate afoot, is almost 70 years out of date. It wasn’t generally developed for the electrical guitar industry, it was available (and cheap). It sucks. It has always sucked.
The pioneers of the industry were adaptive buggers, They became cheap buggers.
See, we’d like to have a separate shield around our signals that affected them in no manner, but we can’t.
Now, regarding the sandbar, since it conveys ALL signal references from the guitar (wire-fully speaking), ALL signals are thereby inter-referenced. ALL signals connected to the cable shield/guitar ground contribute to the current flow in it and hence develop potential across its length that is added (subtraction is just the addition of a negative) to the differential signal of interest. This creates an offset voltage in the signal of interest. It gets amplified too.
Think of it as a star ground in the amp tethered to a local planet(oid) ground(’ish) in the guitar.
See, we should be using a two wire plus shield cable with amps designed for this proper practice. While we’re at it, we could add galvanic isolation to protect (and serve) the musician, thus ensuring that they will (with GAStro-intestinal frequency) buy more (really stupid, we just changed two shiny things, but they’re, uh, yeah that’s the ticket, vintage-like) gear (marketing is).
Or, we could just use DI boxes or wireless (gasp).
Bass players have been using DI boxes for eons. Since a bass guitar is incapable of generating a lasting signal higher in frequency than a mild fart, unless something SBD (silent, but deadly) was unleashed on an unsuspecting world, they want said world to know that it
really wasn’t them…………..
No, it takes ALL possible paths ALLWAYS, with a resulting current thru each (and every) path equal to the impressed potential divided by the path impedance. Or, if one prefers a first principles model, the charge induced develops a potential drop across the sum of all paths equal to the instantaneous charge times the resulting impedance.
And that's why a LOW IMPEDANCE PATH to the amp ground (reference) point for all signals is just peachy.
If'n ya can't find yer slicker, grab yer bumbershoot! (Go thearch for that one Thparky.)