Aliasing is an artifact of digital sampling. Things 'wrap around' the Nyquist frequency, which is half the sampling frequency. It's strange, but it comes out in the math, and is very real.
So, sampled at 44 kHz, with a Nyquist frequency of 22 kHz:
20 kHz -> 20 kHz
39 kHz -> 5 kHz
A proper recording process should cut off everything above the sampling frequency _before_ the A-D converter, otherwise aliasing will occur. And you'd definitely be able to hear it.
Often I can spot spelling mistakes after just a glance at an entire page of print - they just stick out. And sometimes at a glance, my brain notices that there's an unusual word somewhere, and I become aware of it, but just don't know where I saw it on the page (but just a brief search and I'll find it). I think there must be editors (real ones not slashdot ones) out there who do this much better.
I get that too -- it can be pretty annoying at times.
Why ignore E&M on the galactic scale? It's because plasmas are quasineutral -- the distribution of charges balances out. Electromagnetic forces in a plasma have a reach that scales like the Debye length. So if the size of the system is much larger than the Debye length, electromagnetic forces will be a small effect in comparison to forces like gravity. Of course to create the most accurate picture, all forces should be taken into account, but that situation quickly becomes computationally intractable (try running OOPIC sometime). Practical physics is about making the right simplifications.
It looks like a lot of/.-ers have made (or thought about making) FPS maps of our communities and schools.
I've talked about making CS maps with my friends, but invariably those conversations would end with "we'd get in trouble". And given the events of this story, that's the sad reality in today's kneejerk world.
On their website, you can sign up to receive some docs by email. The "datasheet" devotes one page each to the transmitter and receiver chips.
The transmitter is a 12-pin package; mostly ground pins, plus serial clock/data, vdd, and rf out. It operates on 5V.
WPT series Powercaster(TM) modules are programmable frequency sources for use in RF power harvesting applications. The modules encapsulate proprietary algorithms which extend the effective range of power transmission without increasing average power. Numerous standard units are available, with customization available upon request. The WPT9066 module is optimized for operation in the 902-928MHz ISM band. A highly accurate and stable oscillator centered at 905.8MHz is utilized to minimize wander, distortion, or other ill effects that could cause interference with other devices. Custom frequency ranges are available upon request.
The receiver chip is a four pin package with rf in, gnd, dc out, gnd. It claims 70% efficiency, no external components required, 1uW power consumption, and is available in voltages from 1.2 to 6.0 volts. It operates in the 900 MHz and 2.4 GHz bands.
But what's the available power? The health document says "Of the many consumer applications for Powercast technology, the highest-power transmitter identified was just under 2 Watts." An isotropic radiator will produce an energy density of P / (4 * pi * R^2), and an isotropic receiver has aperture lambda^2 / (4 * pi). At 900 MHz, with a distance of 1m, we're talking about 1.5 mW. Directional antennas will raise (or lower) that figure a bit. Close the distance and the power goes up.
I wonder how they minimize the interference these things must create.
You're right, I didn't address the critical part of your argument. You say that audio that cuts off at 12kHz will sound different at 24kHz and 44kHz. That's not right; if you do the sampling properly they will definitely sound the same. Now, that may seem like a brush-off again, but I speak from experience with DSP, whether you believe me or not. If they sound different, the re/sampling isn't being done properly, or the source file doesn't have the right frequency cutoff. Or, perhaps, the signal reproduction chain has a weakness.
[Sorry about my math comment; I didn't mean it that way. I just don't like it when people say, "oh, this stuff is too complex" when they are perfectly capable of understanding and benefiting from it. When you're talking signal processing, a certain amount of knowledge is assumed.]
Actually, SACD uses one-bit sampling at 2.8 MHz. This scheme is fine for lower frequencies, but suffers at higher frequencies (worse performance than CD, even).
Nyquist is very real. I don't see how you've debunked it; you've only reinforced it. Study some basic digital signal processing (the math isn't that complex at the introductory level, really), and you'll see.
As I said, others have done it. I'm sure there are many variations in recording/playback chains.
Why would harmonics be lost in the transfer to CD? You mean, just due to the Nyquist cutoff? Vinyl's frequency response is typically quoted as 15 kHz. And the human ear is hardly perfect.
I was (obliquely) hinting at flaws in your reasoning. You shouldn't make strange blanket statements that lead onto paths which end up at wrong answers.
Aside from mass, a potassium ion carries the same amount of charge as an electron (though opposite, of course). The rebuttal to your original conclusion is simply:
j = qv
There are charges moving, so there is a current. It's a current that results from chemical processes, of course, but to deny that it is a current is inane.
Particle physicists measure mass in units of electronvolts/c^2, which is written without the c^2 for convenience. The electron is about 0.5 MeV, and the proton/neutron are about 1 GeV. So this experiment supposedly found axions with mass ~10 MeV, whereas theory says they should be on the order of eV -- big discrepancy.
6.4MHz fits nicely into this, yielding a Q of 2.5.
Actually, a Q of 2.5 is atrociously bad -- see the sentence right before the 1.6e7/f one, which says "A typical Q for a quartz oscillator ranges from 1e4 to 1e6." (I have no idea where the equation comes from, or if it's valid at all.) In frequency space, your oscillator output power looks like a hump centered on the oscillator frequency, and the width of the hump at half-power is f/Q. So, a Q=2.5 crystal at 6.4MHz is going to have significant output from ~5.1 to ~7.7 MHz, not exactly a stable frequency reference.
Q is not a typical metric for applications. Manufacturers generally specify frequency tolerance, stability, and aging.
As for 6.4MHz, it's not a particularly special frequency. You can get crystals in all sorts of frequencies, and it's not hard for the factory to make you a custom one. The BBC article doesn't give many details, but I expect it's a convenient frequency in terms of engineering. We'll have to wait for the talk to get the whole story.
Brandt has a long-standing (well, year-old) beef with Wikipedia. You can read about it, ironically enough, in the Wikipedia article about him.
He got into a dispute because he didn't like having his biography on WP (though it was constructed from publicly available news sources). He was generally combative and belligerent, and so was blocked and banned various times; check out the Talk archives for details. Afterwards he started a webpage where he attempted to list the real-world identities of the editors involved in the dispute.
Brandt is also the guy responsible for outing the anonymous editor in the Seigenthaler controversy.
Slashdot Mirror
Aliasing is an artifact of digital sampling. Things 'wrap around' the Nyquist frequency, which is half the sampling frequency. It's strange, but it comes out in the math, and is very real.
So, sampled at 44 kHz, with a Nyquist frequency of 22 kHz:
20 kHz -> 20 kHz
39 kHz -> 5 kHz
A proper recording process should cut off everything above the sampling frequency _before_ the A-D converter, otherwise aliasing will occur. And you'd definitely be able to hear it.
I endorse this summary. The only thing I liked about Halo was the vehicles.
Actually, in this case, it's more accurate to think of the solar wind blowing past the earth. The bow shock faces the sun.
http://www.agu.org/sci_soc/cowley.html
Thanks for the link; I may have to get my hands on this.
Often I can spot spelling mistakes after just a glance at an entire page of print - they just stick out. And sometimes at a glance, my brain notices that there's an unusual word somewhere, and I become aware of it, but just don't know where I saw it on the page (but just a brief search and I'll find it). I think there must be editors (real ones not slashdot ones) out there who do this much better.
I get that too -- it can be pretty annoying at times.
Why ignore E&M on the galactic scale? It's because plasmas are quasineutral -- the distribution of charges balances out. Electromagnetic forces in a plasma have a reach that scales like the Debye length. So if the size of the system is much larger than the Debye length, electromagnetic forces will be a small effect in comparison to forces like gravity. Of course to create the most accurate picture, all forces should be taken into account, but that situation quickly becomes computationally intractable (try running OOPIC sometime). Practical physics is about making the right simplifications.
It looks like a lot of /.-ers have made (or thought about making) FPS maps of our communities and schools.
I've talked about making CS maps with my friends, but invariably those conversations would end with "we'd get in trouble". And given the events of this story, that's the sad reality in today's kneejerk world.
Resistive MHD? Two-fluid MHD? PIC? Magnetic reconnection?
The transmitter is a 12-pin package; mostly ground pins, plus serial clock/data, vdd, and rf out. It operates on 5V.
The receiver chip is a four pin package with rf in, gnd, dc out, gnd. It claims 70% efficiency, no external components required, 1uW power consumption, and is available in voltages from 1.2 to 6.0 volts. It operates in the 900 MHz and 2.4 GHz bands.
But what's the available power? The health document says "Of the many consumer applications for Powercast technology, the highest-power transmitter identified was just under 2 Watts." An isotropic radiator will produce an energy density of P / (4 * pi * R^2), and an isotropic receiver has aperture lambda^2 / (4 * pi). At 900 MHz, with a distance of 1m, we're talking about 1.5 mW. Directional antennas will raise (or lower) that figure a bit. Close the distance and the power goes up.
I wonder how they minimize the interference these things must create.
You're right, I didn't address the critical part of your argument. You say that audio that cuts off at 12kHz will sound different at 24kHz and 44kHz. That's not right; if you do the sampling properly they will definitely sound the same. Now, that may seem like a brush-off again, but I speak from experience with DSP, whether you believe me or not. If they sound different, the re/sampling isn't being done properly, or the source file doesn't have the right frequency cutoff. Or, perhaps, the signal reproduction chain has a weakness.
[Sorry about my math comment; I didn't mean it that way. I just don't like it when people say, "oh, this stuff is too complex" when they are perfectly capable of understanding and benefiting from it. When you're talking signal processing, a certain amount of knowledge is assumed.]
Actually, SACD uses one-bit sampling at 2.8 MHz. This scheme is fine for lower frequencies, but suffers at higher frequencies (worse performance than CD, even).
Nyquist is very real. I don't see how you've debunked it; you've only reinforced it. Study some basic digital signal processing (the math isn't that complex at the introductory level, really), and you'll see.
As I said, others have done it. I'm sure there are many variations in recording/playback chains.
d m/part12/page2.html
Why would harmonics be lost in the transfer to CD? You mean, just due to the Nyquist cutoff? Vinyl's frequency response is typically quoted as 15 kHz. And the human ear is hardly perfect.
http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/ian
You can probably guess that I hail from the Realist school of audio. I like vinyl because it's fun; I don't think it's inherently superior to CD.
Try recording an LP onto CD, and playing it back. People have done this, and they sound the same.
I was (obliquely) hinting at flaws in your reasoning. You shouldn't make strange blanket statements that lead onto paths which end up at wrong answers.
Aside from mass, a potassium ion carries the same amount of charge as an electron (though opposite, of course). The rebuttal to your original conclusion is simply:
j = qv
There are charges moving, so there is a current. It's a current that results from chemical processes, of course, but to deny that it is a current is inane.
May I point out that chemistry is applied physics? In a simple redox reaction, electric charge and potential energy are the things to think about.
And regarding your first paragraph, you should study transmission lines fiercely before making strange arguments.
What I do is co-opt the "favorites" menu for shortcuts.
1) Put shortcuts into the favorites menu.
2) If any two share the same first letter, rename.
Access with separate keystrokes
[WIN] (brings up start menu)
a (goes into favs menu)
[first letter] (runs shortcut)
So to get to Firefox, I type [WIN] a f, for Cygwin it's [WIN] a c, etc.
This method works best if you don't use IE, obviously. You can keep the "Links" folder from showing up in the start menu by making it hidden.
Uh, why would business deals in China be written according to Taiwanese law? China doesn't acknowledge Taiwan's sovereignty.
And Morano works for James "global warming is a hoax" Inhofe, the senior Senator from Oklahoma.
Nope, the first Perpendicular Seagate desktop drive was the 7200.10.
Sure there's banner ads, but do you go to hardware sites much? The Tech Report is one of the last honest places on the web, IMO.
That's pretty crazy. Here at Berkeley, each dorm has two third-levels. The whole campus is on a handful of second-levels.
Particle physicists measure mass in units of electronvolts/c^2, which is written without the c^2 for convenience. The electron is about 0.5 MeV, and the proton/neutron are about 1 GeV. So this experiment supposedly found axions with mass ~10 MeV, whereas theory says they should be on the order of eV -- big discrepancy.
Neutral Beam Heating has been around since the 70s, at least. Thirty-forty years of work can yield a lot of improvement.
Actually, a Q of 2.5 is atrociously bad -- see the sentence right before the 1.6e7/f one, which says "A typical Q for a quartz oscillator ranges from 1e4 to 1e6." (I have no idea where the equation comes from, or if it's valid at all.) In frequency space, your oscillator output power looks like a hump centered on the oscillator frequency, and the width of the hump at half-power is f/Q. So, a Q=2.5 crystal at 6.4MHz is going to have significant output from ~5.1 to ~7.7 MHz, not exactly a stable frequency reference.
Q is not a typical metric for applications. Manufacturers generally specify frequency tolerance, stability, and aging.
As for 6.4MHz, it's not a particularly special frequency. You can get crystals in all sorts of frequencies, and it's not hard for the factory to make you a custom one. The BBC article doesn't give many details, but I expect it's a convenient frequency in terms of engineering. We'll have to wait for the talk to get the whole story.
Brandt has a long-standing (well, year-old) beef with Wikipedia. You can read about it, ironically enough, in the Wikipedia article about him.
He got into a dispute because he didn't like having his biography on WP (though it was constructed from publicly available news sources). He was generally combative and belligerent, and so was blocked and banned various times; check out the Talk archives for details. Afterwards he started a webpage where he attempted to list the real-world identities of the editors involved in the dispute.
Brandt is also the guy responsible for outing the anonymous editor in the Seigenthaler controversy.