Dan's Data letters #71
Publication date: 8 November 2003.Last modified 03-Dec-2011.
Long(er) range USB
I've been trying to connect three USB printers to a powered USB 2.0 hub, and then run a 10 meter USB extension ("A" male on one end and "A" female on the other) to a USB 2.0 port on a PC on the other side of the room, so that we would not have to run three long cables. Of course, it doesn't work! Windows XP does not recognise the hub (little tooltip in the corner says "USB device not recognised"), and the printers are not recognised either (yes, even if only one of them is plugged into the hub). Yet if the hub is plugged directly into the PC, or a printer is connected directly to the extension cable, all works absolutely fine.
I've tried with a different hub, different USB ports on the PC, with the same results. All I can think about is some weird unwritten rule of USBdom that says you can't have a hub at the end of an extension, but I've never heard of that...
Thomas
Answer:
A plain ten-metre extension cable is well over the five-metre
length limit for full speed USB
(version 1 or 2); no such object should exist. USB leads don't work perfectly when they're
495 centimetres long and instantly stop entirely if they're 505cm, but you can't cure the problem
by just using better wire; the limit has to do with signal timing, not cable loss. It's actually
rather surprising that even one printer could be made to work on the end of that much cable.
To get more than five metres out of a USB lead you have to use at least two cables, and at least one of them has to be one of the more expensive repeater leads (think of them as one port hubs; hubs are repeaters too), not just a cheap dumb cable. Repeaters re-time the signal and reset the maximum cable length limit.
There is no such rule. You can actually daisy-chain up to five hubs or repeaters, provided you're not trying to run bus-powered devices and don't have enough power in the chain; in that case, you'd need a self-powered hub (with its own wall-wart power supply) on the end of the chain. Normal desktop printers have their own power supplies, so that's not going to be a problem here.
Oddly, USB devices that run at less than full speed (things like mouses and keyboards and such) can only have three metres of cable between repeaters. Full speed devices (USB 1.1 or USB 2.0) can have five metres.
NotDirectEnoughSound
It's a fairly well-known fact that DirectSound sound fidelity does not measure up to Waveout or ASIO under Windows. You can test this by getting a decent pair of headphones (and a decent soundcard) and switching between DirectSound and waveout as the output device in Winamp.
One theory I've heard is that while normal sound-out sends two 16 bit integers along a 32 bit bus, DirectSound "squashes" three into it by removing a few least-significant-bits on each? Can you verify this?
Matthew
Answer:
This depends on the drivers, I think; some sound cards' drivers are lousier for DirectSound than
others, and I think WDM
drivers in Win2000 and WinXP currently give you no real choice at all.
Windows' standard KMixer component (which, as I understand it, is unavoidable at the moment if you're running XP and using WDM sound drivers) can connect sound card hardware that supports only particular sample rates to audio data that's in some other format. Which is great, except the resampling process hurts sound quality, and KMixer will, I think, often resample when it isn't necessary. If you're using DirectSound, I think it'll do it all the time; if you're using any format at all under XP with WDM drivers, I think you're also stuck with it. Even if the sound hardware would be perfectly happy to play the data direct in plain old WAVEFORMAT mode, KMixer still bit-diddles it on the way. This effect is not necessarily audible, though.
When you switch to the Waveout output plugin in your player program, you may well bypass at least some of this stuff, and may get better sound quality as a result.
Note, however, that you're practically certain to get a different volume setting as well when you switch output modes. A slight increase in volume, with no other changes, is commonly mis-perceived as a slight increase in sound quality.
I don't think there's any data-squashing happening, though obviously sound resolution does suffer when the data's resampled. KMixer can apparently chop off LSBs from 24 and higher bit audio in some situations, but that's not the issue here.
Here's a thread about all this.
More Power!
We are experimenting with rare earth magnets for an industrial safety device. We have a 50 by 50 by 25mm 5500 Gauss magnet in an iron core, but it's not giving us quite the capacity we need. I was wondering if you knew if there are larger capacity magnets available?
Geoff
Answer:
Well, there are physically larger rare earth magnets, and the peak magnetic field of high quality
rare earth magnets is around one Tesla (10,000 Gauss). That's the theoretical maximum strength,
though; in reality, few magnets manage much greater field strength than the one you've got now,
even right on the surface at the poles. Magnet shape and external flux-guiding ironwork are important,
as you no doubt already know; bigger magnets can also manage a stronger field at a given distance,
all other things being equal.
You're already using a magnet about as big as most cheap NIB suppliers sell, but there are some ring magnets that are larger.
(A reader told me about that site the other day. Six hours later, they'd agreed to send me some freebies to review. My job sucks, really it does.)
Forcefield have two inch diameter one inch thick cylinder magnets, too, which have a maximum flux density spec of 1.21 Tesla; I don't know what the real surface field strength is.
They sent me a couple of those monsters the other day. I'm trying to figure out how to review them without ending up being called "Lefty".
Less Power!
I tried to find a 2.5 inch notebook drive that would be OK in my portable USB case, but all of them need more than the 2.5 watts USB can supply, especially at spin-up time. I've looked around and around (with hours spent reading IBM, Hitachi and Fujitsu manuals), but found no suitable drive. Strange, as I'm sure I've seen external 2.5 inch USB drives that ran from the bare USB port.
Lorenzo
Answer:
Most 2.5 inch drives actually will work fine from USB power, despite the fact that most
(if not all) of them want around twice as much current on spin-up as USB can provide. This is
probably a combination of more tolerant design of the hard drive, and very short spin-up times
thanks to the low inertia of the tiny platter assembly. Anyway, the things generally do work.
A current Fujitsu or Hitachi 4200RPM 2.5 incher will generally be fine from only USB power in
an external box.
The best solution here is to find a local computer store that'll either let you install the drive and test it in the store, or at least guarantee that it'll work for your application.
FZAT, redux!
I was hoping you could shed some light on why one would be electrocuted if a home appliance fell into the bath.
Is it because the lower resistance path for the voltage would be through one's skin and not the water? Or is it a case of television blowing it all out of proportion?
I read your "Avoiding Electrocution" column, and was hoping you had a better idea of the cause.
Dawid
Answer:
Using the ol' hair dryer while you're in the bath is not at all a clever idea, but no, you're
not guaranteed to become a crispy critter if you drop the dryer in the water.
A double-insulated device (like most hair dryers, little plastic fan heaters and similar bathroom hazards) has only an active and a neutral wire going into it - no earth. Drop it in water, and electricity will pass through the water as well as through the device's circuits to get from active to neutral. The more conductive the water is (thanks to the addition of bath salts, soap, or whatever), the more current will flow through it.
Most of this current flow happens inside the device itself, though; only if the bath is quite well earthed will significant current flow from active to the bath's earth (probably just the plughole surround, since metal baths normally have an insulative enamel coating everywhere else). This current path may pass through you, and it'll do so fairly readily since your skin is your main protection against electrocution, and skin's insulative value drops massively when it's wet.
If the bath isn't earthed (if it's got plastic piping between the plughole and the drain, and if no other cold-water-pipe-connected metalwork is in the water), then not much current at all will escape the housing of the device, and you may be able to get out of the bath quite normally, unplug the device and remove it - especially if the water's clean and thus quite un-conductive.
Many people injure themselves when fishing still-on appliances out of the bath, rather than when they drop them in there in the first place. I dare say quite a lot more hurt themselves because they leap, terrified, out of the bath, and then hit their head on one of the many soft and harmless surfaces that abound in the average bathroom.
If you drop an earthed appliance into the bath, it'll have another good internal path to ground inside it (and attached to its body - metal toasters have earthed chassis, for instance), and will be less dangerous again.
[UPDATE: It turned out that the earthing of toasters is a more complex situation than I thought!]
If a significant amount of current is flowing to earth, by the way, and you have a "safety switch" (an ELCB or RCD) on your power circuit, it'll detect the earth leakage and cut the power, probably fast enough to save your life even in a worst case current-through-body situation.
If the device is double insulated and the bath isn't earthed, though, the safety switch will see nothing amiss at all.
More bolt-on magic
Debunking various "magic" products/inventions seems to be a hobby of yours so I wondered if you've ever heard of the "Tornado Fuel Saver" for increasing fuel efficiency and horsepower for carburetor and fuel injected cars.
One of its resellers even displays results supposedly done by "EPA and California's ARB's Licensed Emission Lab". I think that stands for the California Air Resources Board. I did a quick search on their site but didn't find any Tornado testing results.
I wouldn't even consider anything "as seen on TV", but my auto detailer (whom I trust) works with a small business that swears by the product on their van fleet. They seem to have verified the product on their own (for the fuel efficiency at least), but I'm still a little leery.
I would think if the car manufacturers found a way to increase fuel efficiency and horsepower by a little swirling of the gas/air mixture, they would have already done so.
Gerald
Answer:
Imagine what you could achieve if you used a Tornado Fuel Saver and a
Fuel Saver Pro!
The US EPA has tested all sorts of fuel/emission improving gadgets, but the closest I think they or anybody "licensed" by them has actually come to came to reviewing this thing was when they looked at the "Environmental Fuel Saver" (PDF review here). They concluded that it didn't do anything, just like the rest of these gadgets. They've gone on record as saying that they've never tested a fuel saving gadget that turned out to work.
If you pack more air/fuel mixture into the combustion chamber with a turbocharger, supercharger or big old air scoop, then you can get more power, though not necessarily better economy. Just swirling the air/fuel mixture (or only the air, in the case of fuel injected cars) around before it gets sucked into the chamber, though, doesn't do diddly. What, pulling the air through the air filter doesn't swirl it up already?
There's not much in the way of reviews of this particular gadget on the Web, but car magazines have looked at these darn things over and over in the decades they've been on the market under various names, and invariably concluded they don't do a thing but slightly restrict air flow.
Here's a TV review transcript for the Tornado Fuel Saver.
Why does your detailer's friend's cousin's hairdresser think they work? Because you can think all sorts of things work if you don't test them scientifically. If you tell all of your van drivers that you're concerned about their fuel economy and that's why you're fitting magic swirly gadgets to their vehicles, maybe they're likely to ease off on their traffic light drag racing as a result, whether or not you really modify their vans in any way.
That's just one possible explanation, of course, and it's only off that top of my head; I don't know the details of the situation. But people who don't properly control for other variables in complex systems (a van sitting on a dynamometer is one thing; a van being driven around the city by a human being is quite another) can end up believing that all sorts of crazy things work. People without scientific education are often surprised by how many ways there are to get things wrong.
(More on the Tornado here and here.)
Getting To Know Me...
Just out of curiosity, how well did you do in English when you were at school?
You've obviously had success in journalism, but looking back now, do you think those school results were an accurate representation of your English skills?
Did you have anything specific in mind that you wanted to do (career wise) at the time, or were you just trying for the best results you could get?
Matthew
Answer:
I got 99% in the HSC English
exam.
I get so few opportunities to boast about that these days.
My assessment task score was only 86% or something, though.
Yes, those results did happen to reflect the fact that I'm a bit of a silver-tongued (or at least keyboarded) devil, but the same doesn't necessarily apply for everyone. Tests that try to reduce complex things like knowledge of and effectiveness within a field to a simple score are seldom actually particularly informative. The broader the test's reach, the more this applies; IQ tests are very deceptive.
The final-high-school-exam system in which kids who want to get into popular university courses go nuts trying to score that elusive ninety-nine-point-nine-repeater that's necessary for freakin' Veterinary Science at Sydney University (Wanna be a nurse instead? Hey, anybody who can sign their name right two times out of five gets in!) is severely flawed. This doesn't mean I've got a blueprint for a better system, mind you.
While you can generally say that a 17-year-old who scores in the top few per cent in a particular subject is likely to have a solid grasp of the high school syllabus for that subject - so what? The syllabus doesn't necessarily actually give you a solid pre-university or pre-real-life grounding.
(Insider secret: People who get 99% in English, get some you-beaut Communications diploma or other and then get a job in a newspaper office may well find themselves as subeditors rather than journalists - and "subeditor" does not mean "only one rank below the editor".)
An exam also can't test your ability to use, or even retain, your knowledge in the future.
Great exam scores don't mean you're temperamentally suited to a job in that field, don't mean you even like the subject, and certainly don't mean you'd actually end up being a better journalist, doctor, lawyer, vet or whatever than someone who managed a lowly 80%.
This doesn't mean I want the universities to open 50% of their pre-med places to the dudes who missed half of the exams because they were pulling cones down at the skate park and lost track of time. But outside of getting you a job or a university placement in the year or two after high school, those exam scores really don't mean a thing.
As regards "trying for the best results I could get" - Trying?
TRYING?
I'll have you know I never tried once while I was at school.
Honestly. The very idea.
Next you'll be asking me whether I studied.
