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PostPosted: Thu Sep 30, 2010 3:31 pm 
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Boot Polisher

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Hi. My daughter (15yrs) is doing physics and looking at gramaphones. We found resources on you tube and wickipedia...but found no where to simplify the complex dark art of transducing mechanical impressions, not so much back into sound, but into sound that is "recognisable" as the original!! We looked at a recent new scientist article about generated sound of vivaldi's four seasons purely from photos of the grooves in a disc....not hi fi but you could catch a note or two. Her teacher says electricity is a subtle spiritual force.....i am begining to think he may be right?
any thoughts appreciated
cheers


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PostPosted: Thu Sep 30, 2010 4:44 pm 
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Robbie The Botkiller
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I could write a complete chapter about this.

Problem is the translation to english and the fact I'd have to use a number of drawings.

Waves have a shape. The value of the signal varies with time and depends on it. That is the way it was "written" to vinyl. The needle that is in the track moves along with the variation of location of the track. Because of the inertia of the record player's arm the needle will move accordingly compared to the arm. Two little coils are induced by little magnets (which is only one of a few ways to translate a mechanical signal to an electric signal) so the displacements get translated into electric signals with the same shape. The signal gets amplified by a preamp and then further amplified by whatever it is you want the signal to go to.

In stereo, there are two signals: Left and Right. The L signal is coded into the track by a motion of the needle that moves from the upper right to the lower left. The R signal is coded into the track by a motion of the needle that moves from the lower right to the upper left. These two motions are 90 degrees apart so they won't influence each other. In theory that is, because there will be some unavoidable mechanical cross talk. They are supposed to be at least 45 dB apart to be able to provide a decent stereo image.

Let me know what information you're looking for. I might be able to be more specific.

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PostPosted: Fri Oct 01, 2010 11:49 am 
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Boot Polisher

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i am toggling between complex and weird. One aspect: i would grasp sound as a result of friction from variable (analog) etching (?) in a continuous vinyl groove with variable frequency reproduction as a result and thence via transducer to new energy form and then to amplification and so on........but how.....is the timbre and specific nature of a source imposed on a physical analogue representation and more or less faithfully reproduced regardless of fidelity? Are we seduced into seeing a physical etching in vinyl as a crude analogy and not actually a micro complex analogue of the freuncies inherent in any source. that must mean that every iota of sound in the universe is distinct from every other......


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PostPosted: Fri Oct 01, 2010 12:06 pm 
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Robbie The Botkiller
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Sound is nothing more that air in motion. It happens in waves. Air is all around you, it has a high pressure (one kilo on every square cm = somewhere around 12 pounds on every square inch). Air is immensely flexible as the volume decreases by a factor two when its compression is multiplied by two (OK - that's only for ideal gases but the displacements are so small that you can leave out that air isn't an ideal gas).

When air moves, also the air pressure varies. With any sound, you could make a graph that represents the displacement of "air particles" related to its idle state. These are motions that will push the membrane in your ear accordingly, and it will move the membrane in your mic accordingly. This sound is, by electrical and mechanical processing, written in the groove of a vinyl disc. It is not about friction. It is displacement from the center. Take a vinyl record and look real close, you can see that it's not a straight line (obviously the record being a circle the track is bent into a spiral) but you can see the separate waves. They are best visible on a 12" maxi single. The tracks are wider apart, the displacement is bigger and the tracks are deeper (to get a better s/n ration) than a 33 rpm 25 minute per side disc.

You could connect a record player to the sound card input of your computer (there has to be a preamp in between) and record a part of a song. Importing the recording into Sound Forge or any other sound processing software, it'll put exactly the same graph on the screen as the tracks shape on disc. It is the same signal.

Here's a vinyl groove seen through a microscope. Recording that sound and showing it in Cubase or whatever you happen to use will reveal the exact same shape of curve.
Image

Image

You can see you have a left slope and a right slope. The left slope varies from bottom left to top right, and the right slope makes the needle move from top left to bottom right. So, these motions are 90 degrees apart and theoretically totally independable. That means you have two separated signals: Left and Right. Creating a stereo image.

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PostPosted: Sat Oct 02, 2010 2:56 am 
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Mr. Electonica Dude
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http://www.co-bw.com/Audio_Turntables_And_Cartridges.htm

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PostPosted: Sat Oct 02, 2010 7:08 pm 
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Boot Polisher

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Thanks for these thoughts......we will read them carefully. I appreciate the effort put into the answers...
cheers
steve


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PostPosted: Sat Oct 02, 2010 10:04 pm 
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Mr. Electonica Dude
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Not 100 % brain surgery .

Although circuits that detect capacitance can get pretty deep .

Conductors moving through or creating magnetic fields and piezo crystals bending to produce a voltage are pretty much electricity 101 stuff .

Quote:
electricity is a subtle spiritual force.....


Electricity is one of the four fundamental forces outside of a black hole that has been defined .

Strong and weak nuclear forces have been unified .

The remaining big mystery is the quantum solution of gravity . Lotsa theorys but still unswered questions .

Might really be a spiritual force , black magic , the magic fairy princess , wicked witch of the west .........

No one knows for sure .

msg

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PostPosted: Sat Oct 02, 2010 10:56 pm 
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Robbie The Botkiller
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mrskygod wrote:


Cool stuff, Geno.

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