On 3/31/2010 6:59 AM, Invisible wrote:
> John VanSickle wrote:
>
>> I always knew that UV was bad for your skin!
>
> Notice how freckly she looks in UV though. That melanin really does mop
> up UV very effectively, eh? (Compare to how apparently freckle-less her
> skin appears under normal conditions.)

Interestingly, UV imaging can also be used in forensics to find evidence 
of an injury after the bruise has faded in visible, it will show up in 
UV wavelengths for quite some time.

Eventually I plan on buying an inexpensive P&S modified to capture UV, 
visible and IR. Its UV capability is one of the reasons. I've always 
wanted to capture what my eyes cannot.. .

-- 
~Mike

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Mike Raiford wrote:

> Interestingly, UV imaging can also be used in forensics to find evidence 
> of an injury after the bruise has faded in visible, it will show up in 
> UV wavelengths for quite some time.

If you believe NCIS and so forth, UV makes blood and other bodily fluids 
glow bright green. (I never did figure out why...)

> Eventually I plan on buying an inexpensive P&S modified to capture UV, 
> visible and IR. Its UV capability is one of the reasons. I've always 
> wanted to capture what my eyes cannot.. .

I've been watching Richard Hammond's Invisible Worlds. Some very cool 
stuff, but unfortunately the cool stuff is only on screen for, like, 2 
seconds, and then we get Hammond chattering some more.

I'd love to do the whole trip with time-lapse photography, high-speed 
photography, UV and thermographs, etc. In fact, I've often wondered what 
the world would look like if you would see radio waves. (I'm guessing 
that due to the absurdly long wavelength, most objects would be too 
blurry to see.) I've even wondered what the world would look like if you 
could see sound.

(Eyes and ears both detect waves. Eyes detect only three frequency 
bands, but with ludicrous spatial resolution. Ears detect waves with 
rubbish spatial resolution, but insane frequency resolution.)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Am 02.04.2010 12:40, schrieb Orchid XP v8:

> If you believe NCIS and so forth, UV makes blood and other bodily fluids
> glow bright green. (I never did figure out why...)

Because IIRC they also use some spray that somehow reacts with the blood.

Not sure about other body fluids though.

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Orchid XP v8 wrote:
> (I'm guessing 
> that due to the absurdly long wavelength, most objects would be too 
> blurry to see.) 

Indeed, that's kind of the point. That's why you can listen to the radio 
indoors.

-- 
Darren New, San Diego CA, USA (PST)
   Yes, we're traveling together,
   but to different destinations.

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Darren New wrote:
> Orchid XP v8 wrote:
>> (I'm guessing that due to the absurdly long wavelength, most objects 
>> would be too blurry to see.) 
> 
> Indeed, that's kind of the point. That's why you can listen to the radio 
> indoors.

You can see light indoors too. Not because it has a short wavelength, 
but because certain substances do not absorb it.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Orchid XP v8 wrote:
> Darren New wrote:
>> Orchid XP v8 wrote:
>>> (I'm guessing that due to the absurdly long wavelength, most objects 
>>> would be too blurry to see.) 
>>
>> Indeed, that's kind of the point. That's why you can listen to the 
>> radio indoors.
> 
> You can see light indoors too. Not because it has a short wavelength, 
> but because certain substances do not absorb it.

That's why you can listen to the radio in the dark.

-- 
Darren New, San Diego CA, USA (PST)
   Yes, we're traveling together,
   but to different destinations.

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>>>> (I'm guessing that due to the absurdly long wavelength, most objects 
>>>> would be too blurry to see.) 
>>>
>>> Indeed, that's kind of the point. That's why you can listen to the 
>>> radio indoors.
>>
>> You can see light indoors too. Not because it has a short wavelength, 
>> but because certain substances do not absorb it.
> 
> That's why you can listen to the radio in the dark.

I'm not sure what you're talking about, but I know what I'm talking 
about: Different materials absorb different wavelengths. There are 
materials that absorb visible light, and others that let visible light 
pass through it unaltered. Presumably the same thing applies to *every* 
wavelength - which ought to include radio waves. You can listen to radio 
indoors because not all of your house is made of metal (AFAIK the only 
thing that absorbs radio waves). That's nothing to do with the size of a 
radio wave, it's to do with what materials do or don't absorb it.

Now, what kind of a picture you could make with a "light" having a 2 Km 
wavelength, I have no idea. I vaguely gather that there's some sort of 
relationship between the wavelength of something and the size of object 
you can see with it. (Hence electron microscopes have better resolution 
than light microscopes, for example.)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Orchid XP v8 wrote:
>>>>> (I'm guessing that due to the absurdly long wavelength, most 
>>>>> objects would be too blurry to see.) 
>>>>
>>>> Indeed, that's kind of the point. That's why you can listen to the 
>>>> radio indoors.
>>>
>>> You can see light indoors too. Not because it has a short wavelength, 
>>> but because certain substances do not absorb it.
>>
>> That's why you can listen to the radio in the dark.
> 
> I'm not sure what you're talking about, but I know what I'm talking 
> about: Different materials absorb different wavelengths.

I'm being sarcastic, is all.

> There are 
> materials that absorb visible light, and others that let visible light 
> pass through it unaltered. 

Yes.

> Presumably the same thing applies to *every* 
> wavelength - which ought to include radio waves. You can listen to radio 
> indoors because not all of your house is made of metal (AFAIK the only 
> thing that absorbs radio waves).

I'm pretty sure a couple kilometers of concrete would do the trick too.

> That's nothing to do with the size of a 
> radio wave, it's to do with what materials do or don't absorb it.

It has to do with both. Any amount of non-conductive surface smaller 
significantly than a radio wave is unlikely to absorb it, unless you happen 
to be very unlucky in the choice of your sizes. The reason that far fewer 
materials absorb radio waves is that they're very long and hence low energy. 
You need a substance where the electron gap bands are very close together 
(like free electrons in a conductor's surface) to capture radio waves.

> Now, what kind of a picture you could make with a "light" having a 2 Km wavelength,
I have no idea.

http://universe.chimons.org/contents/Radio.jpg

-- 
Darren New, San Diego CA, USA (PST)
   Yes, we're traveling together,
   but to different destinations.

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Invisible wrote:
> John VanSickle wrote:
> 
>> I always knew that UV was bad for your skin!
> 
> Notice how freckly she looks in UV though. That melanin really does mop
> up UV very effectively, eh? (Compare to how apparently freckle-less her
> skin appears under normal conditions.)

Those aren't good freckles, if they only show up in a UV filter. UV
photography is a good way to spot skin damage from too much UV exposure.

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Invisible wrote:
> http://en.wikipedia.org/wiki/File:UV_Vis_IR_Portrait.jpg
> 
> I wonder how much of the difference is due to the wavelength, and how 
> much is due to how the camera responds to it.

That's actually a really cool picture.  I assume that the shorter/longer 
wavelengths are what accentuates her wrinkles in the ultraviolet and 
smooths them in the infrared, but I'm surprised the effect is so visible 
at those length scales.  Maybe subsurface scattering is playing a large 
role in this.

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> Now, what kind of a picture you could make with a "light" having a 2 Km 
> wavelength, I have no idea. I vaguely gather that there's some sort of 
> relationship between the wavelength of something and the size of object 
> you can see with it. (Hence electron microscopes have better resolution 
> than light microscopes, for example.)

The issue is diffraction.  Once the objects get to the same scale as the 
wavelength then things get blurry.  For example normal light goes through 
your doorway in straight lines, if something is geometrically obscured by 
the wall, the light from it won't reach you (directly at least).  However if 
your "light" has a wavelength of 1 metre (or your "doorway" has a width of 
100 nm) then you'll be able to see things that are geometrically "blocked" 
by absorbing materials.  This is because the wave will diffract as it goes 
through the gap.

IOW I suspect if light had 2 km wavelength then everything would be a big 
blur.

http://en.wikipedia.org/wiki/Diffraction

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>> Now, what kind of a picture you could make with a "light" having a 2 
>> Km wavelength, I have no idea.
> 
> http://universe.chimons.org/contents/Radio.jpg

Well, yes, they have various radio telescopes. But I wonder... these 
images are in colour, but does the colour actually represent different 
wavelengths? Or is it just the intensity of radiation being recieved?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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On 4/2/2010 5:40 AM, Orchid XP v8 wrote:
> Mike Raiford wrote:
>
>> Interestingly, UV imaging can also be used in forensics to find
>> evidence of an injury after the bruise has faded in visible, it will
>> show up in UV wavelengths for quite some time.
>
> If you believe NCIS and so forth, UV makes blood and other bodily fluids
> glow bright green. (I never did figure out why...)
>

Only if treated with Luminol, the I think it's more of a dull blue....

>> Eventually I plan on buying an inexpensive P&S modified to capture UV,
>> visible and IR. Its UV capability is one of the reasons. I've always
>> wanted to capture what my eyes cannot.. .
>
> I've been watching Richard Hammond's Invisible Worlds. Some very cool
> stuff, but unfortunately the cool stuff is only on screen for, like, 2
> seconds, and then we get Hammond chattering some more.

I hope that show is available in the States soon, I've been following 
the guy who did the UV Photography's blog for quite some time.

> I'd love to do the whole trip with time-lapse photography, high-speed
> photography, UV and thermographs, etc. In fact, I've often wondered what
> the world would look like if you would see radio waves. (I'm guessing
> that due to the absurdly long wavelength, most objects would be too
> blurry to see.) I've even wondered what the world would look like if you
> could see sound.

Interesting thought. Not sure how to build a detector for radio waves, 
though (in terms of forming an image on a plane ...) it's exceedingly 
difficult to detect thermal IR, I can only imagine the difficulty in 
focusing radio waves onto a plane.

> (Eyes and ears both detect waves. Eyes detect only three frequency
> bands, but with ludicrous spatial resolution. Ears detect waves with
> rubbish spatial resolution, but insane frequency resolution.)
>

It's amazing when you look at how your auditory system works. Your brain 
essentially gets the Fourier transform of what you're listening to. Also 
interesting how color vision interpolates between 3 broad bands of 
wavelengths to give the perception of a huge amount of hues. Though, 
because there's only really 3 different frequencies detected, you get 
some metameric failure for certain colors. Different frequency spectrum, 
but they look identical. A good example of this is when you photograph 
something whose color is in the violet end of the spectrum. Our red 
receptors are a little sensitive to the very short wavelengths of 
visible light, giving us the perception of purple. The red elements on 
most digital cameras' Bayer arrays don't usually have the same ability 
to pass the short wavelengths, giving certain dyes and flowers a more 
blue color than what your eyes perceive.

It's fascinating how we process information from our world. I've often 
wondered why we perceive blue as-- well-- BLUE, rather than red. I'd 
love to know what the world would look like if I had tetrachromatic or 
even pentachromatic vision. Some of the subtleties of some colors would 
definitely be more obvious. Fun stuff.

-- 
~Mike

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On 4/12/2010 1:17 PM, Orchid XP v8 wrote:
>>> Now, what kind of a picture you could make with a "light" having a 2
>>> Km wavelength, I have no idea.
>>
>> http://universe.chimons.org/contents/Radio.jpg
>
> Well, yes, they have various radio telescopes. But I wonder... these
> images are in colour, but does the colour actually represent different
> wavelengths? Or is it just the intensity of radiation being recieved?
>

depends... sometimes it is colored by wavelength, sometimes just a scale 
of intensity. In the case of that image, it looks like intensity mapping.

-- 
~Mike

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On 4/5/2010 10:51 AM, Kevin Wampler wrote:

> That's actually a really cool picture. I assume that the shorter/longer
> wavelengths are what accentuates her wrinkles in the ultraviolet and
> smooths them in the infrared, but I'm surprised the effect is so visible
> at those length scales. Maybe subsurface scattering is playing a large
> role in this.

Right. the UV light does not penetrate as deep, so will reveal more 
surface detail. IR on the other hand penetrates deeper, giving the skin 
the milky appearance.

-- 
~Mike

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>> If you believe NCIS and so forth, UV makes blood and other bodily fluids
>> glow bright green. (I never did figure out why...)
> 
> Only if treated with Luminol, the I think it's more of a dull blue....

Yes, you never get to see that...

>> I've been watching Richard Hammond's Invisible Worlds. Some very cool
>> stuff, but unfortunately the cool stuff is only on screen for, like, 2
>> seconds, and then we get Hammond chattering some more.
> 
> I hope that show is available in the States soon, I've been following 
> the guy who did the UV Photography's blog for quite some time.

The other problem, of course, is that I missed it while it was on TV, so 
I had to watch it on BBC iPlayer. So it's very blury. (Oh, and I watched 
two episodes but seem to have somehow missed the rest...)

>> I'd love to do the whole trip with time-lapse photography, high-speed
>> photography, UV and thermographs, etc. In fact, I've often wondered what
>> the world would look like if you would see radio waves.
> 
> Interesting thought. Not sure how to build a detector for radio waves, 
> though (in terms of forming an image on a plane ...) it's exceedingly 
> difficult to detect thermal IR, I can only imagine the difficulty in 
> focusing radio waves onto a plane.

Presumably you need some sort of shielding to block out waves except 
through a specific opening, and then use magnets to focus the waves that 
come through the opening?

Alternatively, just don't bother trying to focus it at all. You'd still 
get *something*, if not a sharp images.

> It's fascinating how we process information from our world. I've often 
> wondered why we perceive blue as-- well-- BLUE, rather than red. I'd 
> love to know what the world would look like if I had tetrachromatic or 
> even pentachromatic vision. Some of the subtleties of some colors would 
> definitely be more obvious. Fun stuff.

Apparently most mammals see only 2 wavelengths. Humans and a few 
primates have a duplicate gene that mutated very slightly, yielding a 
new pigment that responds to a very slightly different wavelength band, 
giving us 3-colour vision. (Go look at the CIE spectrum diagrams. The 
"red" and "green" wavelengths are quite close together, but "blue" is a 
mile away.)

Apparently mad scientists tried introducing the mutant gene into mice. 
Normally mice see only 2 wavelengths, but these genetically-engineered 
mice see the same 3 bands that we do. And, as best as the scientists can 
tell, their brains were able to process this new information. Just 
adding the new pigment was all that was necessary for the brain to adapt 
and use it.

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Am 29.04.2010 14:34, schrieb Mike Raiford:

>> I'd love to do the whole trip with time-lapse photography, high-speed
>> photography, UV and thermographs, etc. In fact, I've often wondered what
>> the world would look like if you would see radio waves. (I'm guessing
>> that due to the absurdly long wavelength, most objects would be too
>> blurry to see.) I've even wondered what the world would look like if you
>> could see sound.
>
> Interesting thought. Not sure how to build a detector for radio waves,
> though (in terms of forming an image on a plane ...) it's exceedingly
> difficult to detect thermal IR, I can only imagine the difficulty in
> focusing radio waves onto a plane.

It's being done, not only for IR, but also for radio/microwave.

Thermal IR cameras are pretty well established as equipment for e.g. 
police helicopters to search for or track people (particularly at night).

Modern jet fighter planes like the F22 Raptor are also equipped with 
electronics that can detect the direction of both IR and radar (i.e. 
microwave) emitters, to warn the pilot of potential threats. And AFAIK 
they're not limited to a single emitter, but can detect multiple at once.

Sound "images" are common for sonar systems. And ultrasound imaging 
devices, for that matter.

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> Thermal IR cameras are pretty well established as equipment for e.g. 
> police helicopters to search for or track people (particularly at night).

I'll bet they're damned expensive though. ;-)

> Sound "images" are common for sonar systems. And ultrasound imaging 
> devices, for that matter.

That's not "taking a picture of sound". That's "using sound to take a 
picture". If you want to split hairs, anyway... ;-)

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Invisible <voi### [at] devnull> wrote:
> > Sound "images" are common for sonar systems. And ultrasound imaging
> > devices, for that matter.
>
> That's not "taking a picture of sound". That's "using sound to take a
> picture". If you want to split hairs, anyway... ;-)

Interestingly, you can kind of 'take a picture of sound'. I've seen a couple of
videos on youtube of extremely high-speed film footage (i.e., extreme slo-mo
when played back at 25fps), including one of a car being blown up. As the charge
goes off, you can clearly see the shockwave propagate as a glassy shell.

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>>> Sound "images" are common for sonar systems. And ultrasound imaging
>>> devices, for that matter.
>> That's not "taking a picture of sound". That's "using sound to take a
>> picture". If you want to split hairs, anyway... ;-)
> 
> Interestingly, you can kind of 'take a picture of sound'. I've seen a couple of
> videos on youtube of extremely high-speed film footage (i.e., extreme slo-mo
> when played back at 25fps), including one of a car being blown up. As the charge
> goes off, you can clearly see the shockwave propagate as a glassy shell.

Yeah, Invisible Worlds did this. They detonated a big stack of 
gunpowder, and you actually can see a visible shockwave. It's vaguely 
visible even at normal speed, but in super slo mo it's very clear.

It looks Photoshopped.

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Invisible <voi### [at] devnull> wrote:
> > goes off, you can clearly see the shockwave propagate as a glassy shell.
>
> Yeah, Invisible Worlds did this. They detonated a big stack of
> gunpowder, and you actually can see a visible shockwave. It's vaguely
> visible even at normal speed, but in super slo mo it's very clear.
>
> It looks Photoshopped.

Do you think it only *looks* 'shopped, or do think it actually *is* 'shopped?
I thought it looked as one would expect it to, personally.

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>> Yeah, Invisible Worlds did this. They detonated a big stack of
>> gunpowder, and you actually can see a visible shockwave. It's vaguely
>> visible even at normal speed, but in super slo mo it's very clear.
>>
>> It looks Photoshopped.
> 
> Do you think it only *looks* 'shopped, or do think it actually *is* 'shopped?
> I thought it looked as one would expect it to, personally.

I'm rather surprised that it's a real effect. I thought it only happens 
in Hollywood action films. But, apparently, it's real. Even if it does 
look hard to believe...

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>> Thermal IR cameras are pretty well established as equipment for e.g. 
>> police helicopters to search for or track people (particularly at night).
>
> I'll bet they're damned expensive though. ;-)

Not *that* expensive, we use one in the lab to see the hot-spots on LCDs.  I 
don't think it was more than a few times the cost of a good visible-light 
camera (ie about 3-4K).

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Invisible <voi### [at] devnull> wrote:
> >> Yeah, Invisible Worlds did this. They detonated a big stack of
> >> gunpowder, and you actually can see a visible shockwave. It's vaguely
> >> visible even at normal speed, but in super slo mo it's very clear.
> >>
> >> It looks Photoshopped.
> >
> > Do you think it only *looks* 'shopped, or do think it actually *is* 'shopped?
> > I thought it looked as one would expect it to, personally.
>
> I'm rather surprised that it's a real effect. I thought it only happens
> in Hollywood action films. But, apparently, it's real. Even if it does
> look hard to believe...

Nah, hollywood only tends to do it in 2 dimensions, a ring-shaped wave (which I
believe you can get on water surfaces etc, but that's not what I've seen in
movies!). They also make it too prominent and slow-moving. I'm more inclined to
believe the slo-mo because it is spherical, very fast, and fades very rapidly.
Although of course it might be 'shopped!

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scott wrote:
>>> Thermal IR cameras are pretty well established as equipment for e.g. 
>>> police helicopters to search for or track people (particularly at 
>>> night).
>>
>> I'll bet they're damned expensive though. ;-)
> 
> Not *that* expensive, we use one in the lab to see the hot-spots on 
> LCDs.  I don't think it was more than a few times the cost of a good 
> visible-light camera (ie about 3-4K).

Sure, it's not expensive for suitably large definition of "expensive". ;-)

To me, £500 is pretty expensive.

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>>>> It looks Photoshopped.
>>> Do you think it only *looks* 'shopped, or do think it actually *is* 'shopped?
>>> I thought it looked as one would expect it to, personally.
>> I'm rather surprised that it's a real effect. I thought it only happens
>> in Hollywood action films. But, apparently, it's real. Even if it does
>> look hard to believe...
> 
> Nah, hollywood only tends to do it in 2 dimensions, a ring-shaped wave (which I
> believe you can get on water surfaces etc, but that's not what I've seen in
> movies!). They also make it too prominent and slow-moving. I'm more inclined to
> believe the slo-mo because it is spherical, very fast, and fades very rapidly.
> Although of course it might be 'shopped!

Well, on the example I say, it wasn't spherical at all. The shockwave 
seemed to propogate more quickly along the ground (presumably because 
it's a hard surface?), and was vaguely cylindrical like the tower they 
detonated.

Then there was the smoke cloud reigniting briefly as it escaped the 
shockwave area, "due to the extra oxygen"...

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Invisible <voi### [at] devnull> wrote:
> Well, on the example I say, it wasn't spherical at all. The shockwave
> seemed to propogate more quickly along the ground (presumably because
> it's a hard surface?), and was vaguely cylindrical like the tower they
> detonated.

Ah, I've not seen this one. Sounds about right, tho.

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Bill Pragnell wrote:
> Nah, hollywood only tends to do it in 2 dimensions, 

There was a video showing the real-time shockwaves going thru the Iceland 
volcano ash as it was coming out of the cone.

-- 
Darren New, San Diego CA, USA (PST)
   Linux: Now bringing the quality and usability of
   open source desktop apps to your personal electronics.

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On 4/29/2010 8:13 AM, clipka wrote:
> It's being done, not only for IR, but also for radio/microwave.

I'm supposing that's what the radio telescope images some from....

> Thermal IR cameras are pretty well established as equipment for e.g.
> police helicopters to search for or track people (particularly at night).

Yep. But those cameras are very expensive pieces of equipment, and have 
to be specially cooled in order to work properly.

> Sound "images" are common for sonar systems. And ultrasound imaging
> devices, for that matter.

Well, that and Doppler radar (used for weather) also shows an image of a 
radio wave of sorts. (A very similar concept, actually)

I was thinking more along the lines of a "CCD" for radio type device, 
though. I think the radio telescopes generally examine a small are of 
the sky for a certain frequency, then composite the information to build 
a picture.



-- 
~Mike

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On 4/29/2010 9:27 AM, Bill Pragnell wrote:

>
> Do you think it only *looks* 'shopped, or do think it actually *is* 'shopped?
> I thought it looked as one would expect it to, personally.
>

Watch Mythbusters enough and you'll see one ...


-- 
~Mike

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On 4/29/2010 9:33 AM, Invisible wrote:

>> I thought it looked as one would expect it to, personally.
>
> I'm rather surprised that it's a real effect. I thought it only happens
> in Hollywood action films. But, apparently, it's real. Even if it does
> look hard to believe...

What always surprising to me is how a real detonation is so 
underwhelming compared to a Hollywood detonation. No fireball, just a 
loud report, the shockwave and a dust cloud.

-- 
~Mike

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Mike Raiford wrote:
> What always surprising to me is how a real detonation is so 
> underwhelming compared to a Hollywood detonation. No fireball, just a 
> loud report, the shockwave and a dust cloud.

I think that depends on whether it's a controlled explosion or not. If 
you're intentionally blowing up a building, you don't *want* much more than 
that.  If the propane tank catches on fire, yeah, it's probably gonna be louder.

-- 
Darren New, San Diego CA, USA (PST)
   Linux: Now bringing the quality and usability of
   open source desktop apps to your personal electronics.

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Darren New <dne### [at] sanrrcom> wrote:
> Mike Raiford wrote:
> > What always surprising to me is how a real detonation is so
> > underwhelming compared to a Hollywood detonation. No fireball, just a
> > loud report, the shockwave and a dust cloud.
>
> I think that depends on whether it's a controlled explosion or not. If
> you're intentionally blowing up a building, you don't *want* much more than
> that.  If the propane tank catches on fire, yeah, it's probably gonna be louder.

Has anyone seen The Kingdom? There's a large carbomb towards the end that was
actually done for real with the explosives they said were used in the script...
looked terrifying. And not at all typical hollywood :)

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> Sure, it's not expensive for suitably large definition of "expensive". ;-)

Usually "expensive" means expensive for the apparent/usual value.  eg a new 
car for 5000 pounds is not expensive, but a phone for 1000 pounds is 
expensive.

> To me, £500 is pretty expensive.

Not for a specialist camera.

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>> Sure, it's not expensive for suitably large definition of "expensive". 
>> ;-)
> 
> Usually "expensive" means expensive for the apparent/usual value.  eg a 
> new car for 5000 pounds is not expensive, but a phone for 1000 pounds is 
> expensive.
> 
>> To me, £500 is pretty expensive.
> 
> Not for a specialist camera.

What I'm saying is that it's far more expensive than anything *I* could 
ever afford.

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Mike Raiford wrote:

> It's amazing when you look at how your auditory system works. Your brain 
> essentially gets the Fourier transform of what you're listening to.

Be careful with that...

Audio systems generally represent sound in the time domain. The Fourier 
transform moves data from the time domain to the frequency domain. But 
humans don't hear sound in the frequency domain; they hear it in a 
*combination* of both domains - i.e., as a spectrum that changes over time.

You can take a 5-minute song and take the Fourier transform of it, and 
that will only tell you what combination of [fifty zillion] sine waves 
you need to add together to get back the same waveform. That's not how a 
human would perceive it; they hear notes and beats and things - sounds 
*changing* over time, but one static combination of frequencies.

It turns out this is one of the most tricky things in DSP. Working with 
signals that have a meaning in *both* domains...

(You might think, for example, that you could just snip your 5-minute 
song into, say, 50 ms chunks and take the Fourier transform of each 
chunk. Alas, snipping it up introduces phantom frequencies that aren't 
really there.)

I've spent a significant amount of time trying to come up with some 
mathematics for analysing sound the way that the human auditry system 
does... So far, nothing works.

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> (You might think, for example, that you could just snip your 5-minute song 
> into, say, 50 ms chunks and take the Fourier transform of each chunk. 
> Alas, snipping it up introduces phantom frequencies that aren't really 
> there.)

Use something other than a rectangular window function then, that helps with 
the phantom frequencies.

> I've spent a significant amount of time trying to come up with some 
> mathematics for analysing sound the way that the human auditry system 
> does... So far, nothing works.

If you don't need it in realtime then just run a band-pass filter over the 
whole song at varying frequencies, you can then read off the amplitude at 
any frequency at any time during the song.

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scott wrote:
>> (You might think, for example, that you could just snip your 5-minute 
>> song into, say, 50 ms chunks and take the Fourier transform of each 
>> chunk. Alas, snipping it up introduces phantom frequencies that aren't 
>> really there.)
> 
> Use something other than a rectangular window function then, that helps 
> with the phantom frequencies.

The [discrete] Fourier transform reduces a signal to a finite number of 
frequencies by assuming that the signal is periodic - i.e., the samples 
you've fed it repeat forever. If the endpoints don't match up, that 
introduces a discontinuity, which causes phantom frequencies to appear. 
(They represent the discontinuity.)

You can reduce this by using another window function - but then, 
strictly speaking, you're multiplying your signal by a window, which 
convolves its spectrum with the spectrum of the window. In other words, 
by windowing the signal you're blurring its spectrum.

>> I've spent a significant amount of time trying to come up with some 
>> mathematics for analysing sound the way that the human auditry system 
>> does... So far, nothing works.
> 
> If you don't need it in realtime then just run a band-pass filter over 
> the whole song at varying frequencies, you can then read off the 
> amplitude at any frequency at any time during the song.

This, of course, is effectively what the human ear does. (As in, the 
physical organ, not the elaborate post-processing that undoubtably 
happens when the data reaches the brain.) Each hair in the cochlea is 
essentially a crude band-pass filter. (I don't imagine they actually 
have a great frequency response; much like the human eye isn't actually 
a very good camera.)

So far, everything I've ever learned about DSP seems to hinge on one 
single equation:

   sin X + sin Y = 2 cos (X-Y)/2 sin (X+Y)/2

This says many things. But most obviously, it says that the sum of any 
two waves is also the product of two *other* words, and vice versa. So 
the question is, at what frequency does a wave become "sound" rather 
than "variation in sound"?

The *other* question, of course, is "how many frequency bands do I need?"

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On 4/30/2010 7:02 AM, Invisible wrote:
> Mike Raiford wrote:
>
>> It's amazing when you look at how your auditory system works. Your
>> brain essentially gets the Fourier transform of what you're listening to.
>
> Be careful with that...

Yeah, I was simplifying greatly what is happening... If you want to 
display a voiceprint or Realtime spectrum, you'll need to move a window 
across the signal.

> (You might think, for example, that you could just snip your 5-minute
> song into, say, 50 ms chunks and take the Fourier transform of each
> chunk. Alas, snipping it up introduces phantom frequencies that aren't
> really there.)
>
> I've spent a significant amount of time trying to come up with some
> mathematics for analysing sound the way that the human auditry system
> does... So far, nothing works.

Yeah, someone said it before, but there are windowing functions for 
that. If you want to get quick and dirty, you can use a rectangular 
window, but this will introduce artifacts in the resulting spectrum.

Strangely enough the other day I was experimenting with windowing in a 
Fourier transform to analyze a rather complex FSK signal (Trying to get 
a grip on how the signal worked ...) The major problem I had with it was 
the fact that there was significant smearing between samples, that I 
couldn't tell when the frequencies were changing, not only that it made 
it exceedingly difficult to determine what the frequencies were. I tried 
various windowing functions and window sizes.

I should pick up my little DSP project I was working on and fiddle with 
it again.

-- 
~Mike

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Mike Raiford wrote:

> Strangely enough the other day I was experimenting with windowing in a 
> Fourier transform to analyze a rather complex FSK signal (Trying to get 
> a grip on how the signal worked ...) The major problem I had with it was 
> the fact that there was significant smearing between samples, that I 
> couldn't tell when the frequencies were changing, not only that it made 
> it exceedingly difficult to determine what the frequencies were. I tried 
> various windowing functions and window sizes.

Yeah, Scotts idea of using a bank of bandpass filters is probably the 
way to go for that. Split the signal into several dozen signals, each 
containing only a narrow band of frequencies. See how much energy each 
signal has.

(I have no idea how you measure signal energy...)

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> You can reduce this by using another window function - but then, 
> strictly speaking, you're multiplying your signal by a window, which 
> convolves its spectrum with the spectrum of the window. In other words, 
> by windowing the signal you're blurring its spectrum.

Usually a slightly blurred spectrum is the preferred outcome.  

> So far, everything I've ever learned about DSP seems to hinge on one 
> single equation:
> 
>   sin X + sin Y = 2 cos (X-Y)/2 sin (X+Y)/2
>
> This says many things. But most obviously, it says that the sum of any 
> two waves is also the product of two *other* words, and vice versa. So 
> the question is, at what frequency does a wave become "sound" rather 
> than "variation in sound"?

20 Hz?
 
> The *other* question, of course, is "how many frequency bands do I need?"

Experiment!

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> (I have no idea how you measure signal energy...)

You mean how to assign a number to "loudness" given the waveform?  I'd guess 
that taking the RMS over a few cycles followed by some log scaling would be 
a good start, but the ear is complex...

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scott wrote:
>> You can reduce this by using another window function - but then, 
>> strictly speaking, you're multiplying your signal by a window, which 
>> convolves its spectrum with the spectrum of the window. In other 
>> words, by windowing the signal you're blurring its spectrum.
> 
> Usually a slightly blurred spectrum is the preferred outcome.

Preferable to phantom frequencies, sure...

>> This says many things. But most obviously, it says that the sum of any 
>> two waves is also the product of two *other* words, and vice versa. So 
>> the question is, at what frequency does a wave become "sound" rather 
>> than "variation in sound"?
> 
> 20 Hz?

That's approximately the lowest frequency that the human ear can 
mechanically detect. There's no particular reason to expect this to be 
in any way related to the rate of change in frequency spectrum that the 
human mind can percieve. (One is related to the physics of the ear, the 
other is related to the internal operation of the brain...)

>> The *other* question, of course, is "how many frequency bands do I need?"
> 
> Experiment!

Science: It works, bitches! :-D

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>> 20 Hz?
>
> That's approximately the lowest frequency that the human ear can 
> mechanically detect. There's no particular reason to expect this to be in 
> any way related to the rate of change in frequency spectrum that the human 
> mind can percieve.

Modulate a 1 kHz tone with a N Hz square wave.  At what value of N do you 
cease to be able to recognise the 1 kHz tone turning on and off?  I suspect 
it's somewhere around 20 Hz, you could easily figure it out with a sound 
editor program like Audacity.

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scott wrote:

> Modulate a 1 kHz tone with a N Hz square wave.  At what value of N do 
> you cease to be able to recognise the 1 kHz tone turning on and off? 

Yes, this is the experiment to perform.

> I suspect it's somewhere around 20 Hz, you could easily figure it out with 
> a sound editor program like Audacity.

I'd probably use my synthesizer actually, but anyway... ;-)

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Invisible wrote:
> at what frequency does a wave become "sound" rather 
> than "variation in sound"?

Around 10Hz to 12Hz.

-- 
Darren New, San Diego CA, USA (PST)
   Linux: Now bringing the quality and usability of
   open source desktop apps to your personal electronics.

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scott wrote:
> suspect it's somewhere around 20 Hz, 

Closer to 10Hz.  Between 10Hz and 20Hz, you detect sufficiently loud sound 
with different organs, such as your eyeballs vibrating or your sinuses 
resonating. If you play a tone in that range, you get the "spooky" feeling, 
or unaccountably sad, or mystical feelings, or things like that, depending 
on the frequency.

-- 
Darren New, San Diego CA, USA (PST)
   Linux: Now bringing the quality and usability of
   open source desktop apps to your personal electronics.

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On 4/30/2010 10:52 AM, Darren New wrote:

> Closer to 10Hz. Between 10Hz and 20Hz, you detect sufficiently loud
> sound with different organs, such as your eyeballs vibrating or your
> sinuses resonating. If you play a tone in that range, you get the
> "spooky" feeling, or unaccountably sad, or mystical feelings, or things
> like that, depending on the frequency.

I know with my headphones (Hybrid canalphones, actually) I can detect 
down to around 10hz, anything lower is inaudible. I designed my 
headphone amp with a ~15hz corner frequency high pass in it's input 
stage (AC coupling, to get rid of any DC bias before feeding the signal 
to the op-amp)

It sounds beautiful, very rich tones.

There is something about a big pipe organ with extremely low registers. 
Occasionally at church they'll pull what I think is the 16' stop, and it 
sounds incredible, there's something about that deep rumble the big 
pipes put out...

-- 
~Mike

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Mike Raiford wrote:
> There is something about a big pipe organ with extremely low registers. 

That "something about" is exactly what I'm talking about. :-) That's the 
subsonics kicking in.

-- 
Darren New, San Diego CA, USA (PST)
   Linux: Now bringing the quality and usability of
   open source desktop apps to your personal electronics.

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Mike Raiford wrote:

> There is something about a big pipe organ with extremely low registers. 
> Occasionally at church they'll pull what I think is the 16' stop, and it 
> sounds incredible, there's something about that deep rumble the big 
> pipes put out...

On a normal church organ, 16' is the lowest available stop. This is due 
mainly to size considerations. (Not to mention that building large pipes 
requires a lot of metal, and therefore costs money.)

A cathedral organ, however, would usually have at least one 32' stop.

Exactly two pipe organs on the face of the Earth have a 64' stop. But 
then, really, that's not a note, it's a small earthquake! o_O

(I should maybe point out that a 16' stop doesn't necessarily contain 
any pipes that are 16' long. There are ways to make the pipes shorter 
[while muffling the tone they generate]. Or the stop might simply not 
contain all the notes of the scale; only the *lowest* pipe would need to 
be 16' long; the next just one octave up only needs to be _half_ that size!)

I leave it as an exercise for the over-interested reader to figure out 
what the fundamental frequency of a 32' pipe is. (Remember that the 
fundamental wavelength is 2x the pipe length - or 4x if it's a closed pipe!)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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