Dan's Data letters #186

Publication date: 4 July 2007.
Last modified 03-Dec-2011.

 

Actual VT100 probably unsuitable

I've been looking around for a portable terminal device that will allow me to configure Cisco routers without the hassle of carrying my laptop around. Something that I can throw into my work bag and forget about until I need it.

Such a device would need a small screen, built in keyboard, RS-232 serial port and obviously be VT100 or similarly compatible. The smaller and lighter, the better. Ideally I would like to be able to recharge it, but I can deal with replacing batteries if I have to.

Unfortunately my (admittedly not very thorough) searches have yielded a lot of Point Of Sale machines and barcode scanners which look cool and appear to be the right size, but aren't designed for my purpose.

I'd also like to stay away from old laptops as I loathe the idea of booting up Windows just to debug a PPP session, and most old notebooks are too "chunky" to have floating around in my kit.

You have an affinity for old hardware so I thought I should go straight to the source. Is there something that springs to mind?

Jason

Answer:
How small a screen is too small?

If 80 characters by as little as eight lines is good enough for you, then one of those "mail slot" devices I mention on this page might suit you quite nicely, provided of course you can lay your hands on a working one at a decent price.

The Amstrad NC 100, for instance, has a basic terminal program in ROM (with Xmodem!), a decent basic word processor too, and an 80 character by eight line screen.

If you don't need even that many characters, a TRS-80 Model 100 gives 40 by 8 characters, and will probably be easier to find on eBay.

Both should run fine from NiMH AAs for quite a long time per charge.

(The TRS-80 Model 200 has a 40 by 16 character screen, and the rather bulkier and more seldom seen Amstrad NC 200 has 80 by 16 characters.)

If you want something really small, a Psion (3? 5? 7? Revo?) might work.

You'll need an adapter cable to connect any of those, I think; they don't have room for standard ports, but I think they do all have RS-232 compatibility.

And yes, blog readers, an Apple eMate 300 would work too. It's got a round serial port, but again a simple adapter will fix that. But you'd probably have to expend some effort to get it running.

(A reader's also suggested a less old school option, the Dell Axim X51v PDA from a couple of years ago. It's got a CompactFlash slot that can accept an Ethernet card, and a full VGA resolution screen. If you need a proper keyboard, there's Bluetooth for that.)

 

Dog-whistle hi-fi

In your canalphones comparison, you state "Very few adults can hear a 20kHz tone, and very few recordings even contain such frequencies."

Which ain't the case at all. Very few recordings will have primary tones in the 20kHz bracket, but almost all recordings have overtones up there - or they would if they hadn't been filtered when they were digitized to stop the frequencies wrapping around into the bass.

Overtones are what make one instrument sound different to another when they're playing the same note. It's also what the audiophile hears missing when they diss digital recordings and praise the warmth of old 1/4" tape or stamped vinyl records. Do a spectral analysis of some old AAD CDs, tons of frequencies all the way up there till they hit the knee of the filters (really good ones are above 20 kHz) and then whop, they're all gone. I've tried to tell audiophiles they can't possibly hear those frequencies, they keep on about the warmth of older media.

Jonathan

Answer:
As you say, many recordings could have 20kHz-ish tones - if they weren't filtered out, one way or another.

Yes, it is physically possible to record 20kHz on a CD. Assuming a given recording takes it all the way to the Nyquist limit and a magic brick-wall filter on input at 22.05kHz, it can indeed perfectly reproduce everything up to that limit.

Since there is no such thing as a magic brick-wall filter, the realistic limit for recordings of real instruments is only about 20kHz. But that is, in principle, achievable.

I think you'll find that there really isn't anything but spurious signals at 20kHz in the vast majority of recordings, though, because very few recordings are made with microphones that have significant response above 18kHz. A limit of about 15 or 16kHz is more common.

(Old AAD CDs may well have significant spectral content up around 20kHz, but it's likely to all be spurious rubbish created by crappy filtering.)

It's exceedingly difficult to make a mic that can accurately capture those very high frequencies, since the length of the sound waves starts to match up with the size of various components and apertures in the mic. Since only audiophile nuts care, there's very little reason to compromise other elements of a mic design (including purchase price...) to push the treble that little bit further. Especially when you can just print "2Hz-40kHz" on the packaging of a 40Hz-14kHz mic and achieve very nearly the same effect.

It is, of course, often desirable to do some of the filtering mechanically at the microphone rather than electronically further down the chain. Pick a mic you like the sound of that doesn't deliver any useless high frequencies in the first place, and the production task is simplified.

And overtones certainly do characterise the sounds of different instruments, but those overtones are almost all far below the hyper-treble.

Look at a spectrum analysis of, say, a violin playing the highest note the player can manage without cheating by using harmonics in the first place (whereupon the "fundamental" is actually already at least half way through the instrument's harmonic series; see also Victor Wooten playing the melody line of Amazing Grace entirely on harmonics...), and you'll see the harmonics dribble away to nothing at a frequency only three or four times the fundamental, if that. So if you start on double high C, there'll be bugger-all left at 9000Hz, if not lower. And that's a violin, with little tiny soundboard resonator parts; you'll have a much harder time getting anything shaking at above 10kHz on most other instruments.

Quarter inch tape sure could capture better than 20kHz with no need to mess about with special filters, but it was seldom called upon to do so, particularly with the microphone technology available when domestic reel-to-reel was still popular.

You can forget about the "Elvis" ribbon mics (the modern Shure 55SH Series II still doesn't beat 15kHz!); even the "hi fi" condenser mics cut out well shy of 20kHz. A Neumann/Telefunken U 47 may have cost half as much as a Cadillac in 1953, but it still only made it to 16kHz, and that was if you cut it some slack at the plummeting end of its response chart - the sales graphs put 16kHz at -4dB, but 16001Hz looked to be about -25dB :-).

Brand new vinyl can also deliver very high frequencies, but few records are physically capable of delivering anything above 18kHz after being played several times. Apparently ultra-compliant styli can reduce the damage greatly, if we can believe the people who insist that their 30kHz-carrier CD-4 quadraphonic LPs still play correctly. Those discs needed very VERY special lathes to cut the masters, though.

Apparently those ludicrous ELP laser turntables, originally only specified at 20-20000Hz, are (after... reconsideration... by the manufacturers) meant to be able to reproduce 25kHz. Astonishingly, nobody seems to have empirically verified this claim.

Look, I can believe that some people can perceive a difference between music with stuff up there in the beyond-normal-adult-hearing range and music without. Humans are amazing creatures. I think that number is a lot smaller than the number of people who think they can perceive the difference, though. And I also think the number will fall again if you test to see if they can tell the difference between music with instrument harmonics above 16kHz, and music with random noise above 16kHz.

 

PVA glue not recommended

I enjoyed reading some of your thoughts on magnets. I’ve always been fascinated by them... good to get some more insight into their behaviour and capabilities.

We have a simple project at my office where I will be affixing some rare earth magnets (5/16" dia. X 1/4" discs) to an aluminum part. Do you have any advice on a good adhesive? Would epoxy work well, or is there another bonding material that is more suitable for my application?

Jordan

Answer:
I think epoxy's what people usually use. Make it strong epoxy, not five-minute stuff (this could be a job for the metal-filled J-B Weld/Devcon type), and make sure to scuff up the mating surfaces a little with some coarse sandpaper, to give the epoxy more to grab onto.

The people who make wind turbines with HUGE rare earth magnets on the rotors use epoxy, too. But they use a lot of it, and are quite picky about the kind of epoxy they use.

 

I hope he doesn't set the ocean on fire...

I've been emailed this thing a couple of times now, and the bullshit meter is spiking.

I'm sure you've seen it - an inventor with his magic RF generator is making salt water burn. Fuel for the masses, dependence on oil removed, great joy!

I didn't see any commentary on your site about this particular brand of hooey, but your familiarity with the breaking of the laws of thermodynamics is better than mine.

Eldwin

Answer:
The first reader to point out this story about John Kanzius and his mystic combustible water sent me to this news report. I was intrigued by the statement that "The APV Company Laboratory in Akron has checked out John's amazing invention. They were amazed."

Apart from this amazing journalist's amazingly amazing turn of phrase, this looked like a lead to me. So I looked up the only company called "APV" that exists in Akron. That turned out to be APV Engineered Coatings, which only a cruel person would describe as a paint company.

I e-mailed David R. Venarge, the boss of APV Coatings, and I invited him to - if the claims about his company in the news piece weren't just lies - look into the long, long, long history of "frequency" treatments for disease. For, yes, Mr Kanzius says he discovered his water burner incidentally, as a side-effect of his cure for cancer. I also invited Mr Venarge to check out the similarly dismal record of those who claim to have invented a way to split water into hydrogen and oxygen using less energy than can be recovered by combusting the gas again (as per the "Water car" letters in this column, and the water-to-gasoline letter in this one).

Regrettably, Mr Venarge has not replied.

After this page went up, I got an e-mail from Bill Beaty, whose Science Hobbyist site I have linked to on many previous occasions. It's one of the Web's leading sources of information about odd things you can make at home but in some cases shouldn't.

Bill mentioned that the plasma blobs so many people have managed to make by performing Unwise Microwave Experiments (Bill's term) appear to match the description of the "flame" John Kanzius thinks he's invented. Bill's only uncertainty is how Kanzius is managing to make such discharges in the open air, rather than in the resonant chamber of a microwave oven. It's perfectly possible to do that - have magnetron, will travel - but only with "tens of kilowatts" of microwave energy. That is not something you should just be shining around the place - even an ordinary microwave magnetron presents a significant health hazard if extracted from the oven.

Fortunately, some dude who thinks he can burn water is sure to know all about microwave beam safety. So there can't possibly be anything for Kanzius' neighbours to worry about.

 

Magnifying-glass heat ray experiments are ongoing

I've been playing with ants lately. They're very amusing little creatures and no longer do I crisp them up with Mr. Magnifying Glass, instead I have a little game I play with my friends. I live in a two storey house and thought it would be cool to drop an ant from the upper story onto the pavement below - albeit with gusto and background covers so I can see the ant flying down and track its flight.

I then went one further and thought it'd be even cooler to glue a matchstick/thin cloth parachute to the critters and see if they fair much better or worse.

Here comes the Dan part. On both attempts, they survived!

How in queen ant is this possible? How can they survive and us humans, taking into account scale thus increasing the height in which we fall, wouldn't survive 6/7 stories let alone what must be like 30/40 stories to them?

Is this a maths problem, logistics problem, science problem - I can't work it out.

P.S. If it helps: The ants that have come forth to sacrifice their life in the name of childish research herald from bull-ant-ville.

Julien

Answer:
No ant, even a big old bull-ant, is at all likely to be injured by any fall, parachute or no parachute.

The reason for this is connected to the famous incredible strength of ants, many species of which can indeed lift many times their own body weight using nothing but their jaws.

This all comes from one of the most important engineering principles, the square-cube law. If you scale something up by a factor of two - making it twice as high, twice as wide and twice as deep, while keeping all of the materials the same - you increase its surface area, and the strength of the parts it's made from, by a factor of four. But you increase its volume, and thus mass, by a factor of eight. Volume, by definition, changes with the cube of the scaling factor, while strength only changes with the square.

This principle explains why it's hard to make big things. Bigger things are much more massive and subject to much higher stresses - just from the pull of gravity, for a start - but the stuff they're made out of doesn't get stronger in proportion to that increase in mass. If a structure weighs one tonne and can exactly support its own weight, and you double it in size, now it can support four tonnes - but weighs eight. Crunch.

This sets some pretty solid limits for the size of particular kinds of things.

Take wooden ships, for instance. Isambard Kingdom Brunel's famous SS Great Western was, at 65 metres in length, the biggest properly seaworthy oceangoing wooden steamship ever built. We couldn't make one much bigger today, even after another 170 years of engineering progress, because at that scale wood (in the thicknesses that're possible in something that has to float) starts to get too bendy for the ship to stay watertight. Without iron strapping to keep her planks together, the Great Western wouldn't have made it across the Atlantic once, let alone 68 times.

A few larger wooden ships have been made over the centuries, but they sagged in the middle even sitting in calm water, and visibly wiggled around on waves, like an inflatable mattress. Except leakier. They had a disturbing tendency to sink with all hands if they ventured out on the open ocean.

(This, by the way, is one of the many arguments against the "1421 hypothesis", which holds that Chinese fleets including multiple wooden vessels twice as long as the Great Western sailed all over the world.)

Getting back to insects - they benefit from the square-cube law, because they're so small. Halve the size of something and its structural strength is 0.25 of what it was, but its mass is only 0.125 of what it was. So, proportionally, it's now stronger.

A small ant may weigh only one milligram. I don't know the average weight of a bull-ant off the top of my head, but unless it's a real giant it's probably still under 0.1 grams.

At these very small scales, you can make immensely sturdy structures (by the standards of things of similar size) out of all sorts of materials that aren't useful at all for large scale construction. Insect exoskeletons are very thin and insect muscles are very weak, when compared with the comparatively enormous human equivalents, but the forces they have to cope with are also exceedingly small.

The terminal velocity of an ant falling in air (the speed at which the pull of gravity is matched by the air resistance) is surprisingly low. This is partly because smaller things, in a situation analogous to the square-cube law, have a higher ratio of cross-sectional area to mass - make something twice as big and it's eight times as massive, but only has four times the cross-sectional area to present to the passing air. But it's also because on small scales the viscosity of air becomes high compared with the pull of gravity. This is why dust can hang in the air for ages - at that scale, air is like honey.

(If you're dropping things in a vacuum, there is no terminal velocity, because there's zero drag and zero air viscosity. Everything, be it a cannonball, a feather or an ant, will then accelerate towards the ground at exactly the same rate and continue to do so until it lands.)

For these reasons, the terminal velocity of a falling human in air is at least 200 kilometres per hour, but you probably can't make even a big ant fall much faster than about 30 km/h.

0.1 grams travelling at 30km/h only has only has 0.0035 joules of kinetic energy. This is a tiny, tiny amount of energy, and dissipating it over a small fraction of a second when the ant hits the ground is easy.

The kinetic energy of any moving object scales proportionally with the object's mass, so a one-gram object hitting the ground at 30km/h has 0.035 joules to deal with.

But the energy also scales with the square of the object's velocity, and terminal velocity increases rapidly as you move up from the insect-scale where air is all gooey.

If your one-gram object's terminal velocity is 60km/h, and it falls far enough to reach that velocity, its energy is now 0.14 joules. A 75-kilogram human, 750,000 times the mass of the theoretical large ant, at 200km/h is carrying 115,741 joules of energy - 33 million times the energy of the falling ant.

Combine this great increase in energy with the relative decrease in the strength of the materials the body's made from, and you see why people go splat (or, if they haven't fallen far enough to get to terminal velocity, at least get badly injured) while ants go boing.

This also, by the way, is a good reason why giant monsters exist only in movies. An ant scaled up to the size of a bus would collapse into a smashed mess under its own weight, long before it discovered that it was now unable to breathe. Similarly, Mighty Joe Young could just about exist, if his bones and muscles and joints were significantly upgraded, but King Kong would find it very difficult to even breathe.

(Whales can be as big as they are because their mass is supported by the water. If a large whale's beached, it too will have a hard time breathing.)

If you want to ask me a question, feel free - but please read this and this first.

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