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Re: [OM] Nope, It missed it by few hundred miles

Subject: Re: [OM] Nope, It missed it by few hundred miles
From: "Jeffrey Keller" <jrk_om@xxxxxxxxxxx>
Date: Sun, 21 Sep 2003 15:06:01 -0700
I think I might understand where your argument is coming from.

When a lens + film records an image of line spacings that can not be resolved the image shows up as a blur. If there were an optical filter that would block light from unresolved spacings, you could call the recorded image more accurate. I don't think such a thing exists. The antialiasing filter is a very different thing.

A peson's eyes and brain respond the same way as the lens. It's not seen as a radiacally foreign noise artifact. Its just seen as something that might be blurred. If you see a fuzzy grey thing, your brain is open to the possiblity that it might be closely spaced black and white lines that can not be resolved. Blocking the light might not even be the best thing.

When reconstructing something from samples when the sampling frequency was not high enough a radically foreign artifact can appear. With sound a steady 2 kHz signal could result from sampling a 6 kHz signal at 10 kHz.

An anti aliasing filter on a digital camera sensor is probably just trading off blur against strange artifacts.

-jeff

I don't follow your comparison to electronic equipment.

Records have more small defects than large which creates more high frequency noise than low. The signal recorded has its high frequencies boosted so that the high frequency noise can be attenuated when the music is played back. Similar thing for dolby and tapes. Nothing comparable to digital aliasing filters.

Beating really has nothing to do with high frequency content. Two low frequencies can beat just as two high frequencies can beat. However two high frequencies that can't be heard can beat and be demodulated by an AM radio to produce a noise that can be heard.


From: "Julian Davies" <julian_davies@xxxxxxxxxxxxxx>

Analogue recording systems don't form beating patterns when a particular
frequency is reached (for the purists this is only true when the recorder
works without modulating a base frequency). They suffer from an increase of
noise with frequency. In photographic film, this is base fog.

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