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Author Topic: View through the LP lathe microscope  (Read 2617 times)
Stephen W. Desper
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« on: December 07, 2017, 06:38:50 PM »

COMMENT for fans who are technically minded:

You may have heard me refer to looking at the groove of a Beach Boy LP I was mastering, by looking through a microscope.



All LP cutting lathes come with microscopes that are focused on the groove itself. If you view the spinning disc at 33⅓ RPM it’s a blur. By slowly spinning the lathe platter by hand while looking through the ‘scope, you can view various characteristics of the vinyl walls of the groove. This will tell the trained eye whether the cut will track well when played and if the sound will be faithfully reproduced by the LP mechanical medium.

We’ve all seen images of LP grooves . . .



For the technical fans, I found this example video showing what I see when looking through the ‘scope. I thought you might find it of interest too.


>>> https://www.youtube.com/watch?v=GuCdsyCWmt8

~swd


PS

Here are some numbers for the LP Sunflower:

Playing time: 36.5 minutes

Revolutions per side: 1218

Length of groove: 2581 feet or about 1/2 mile

Equivalent average inches per second: 13.5  IPS

Number of samples & bit rate:  Infinite

(( Actually all things in Nature have a limited bit rate.
For sound in the air the bit rate is limited by the size of the air molecule.
Magnetic tape is limited by fineness of the magnetic oxide.
And the LP is limited by the size of the carbon grind used in the vinyl.
In practical terms this is so far removed from any digital format it is considered infinite. ))
 
 
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Custom Machine
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« Reply #1 on: December 07, 2017, 11:49:43 PM »

Thanks for the technical info! Nice to learn that the groove length of my all-time favorite album is 2581 feet or about 1/2 mile. Basically that means my various styli over the past 47 years have traveled around a thousand miles listening to Sunflower, not to mention laser travel on CDs, and tape head travel on reel to reel and cassette tapes.

Concerning the resolution limit of LPs, since the ground bits of vinyl are melted together, wouldn't the limit be the size of a vinyl molecule?

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« Reply #2 on: December 08, 2017, 01:11:18 AM »

Wonderful information, as always, Mr. Desper.

And a great read, even for the non-technical types among us. Grin   
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Stephen W. Desper
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« Reply #3 on: December 08, 2017, 08:33:51 AM »

Thanks for the technical info! Nice to learn that the groove length of my all-time favorite album is 2581 feet or about 1/2 mile. Basically that means my various styli over the past 47 years have traveled around a thousand miles listening to Sunflower, not to mention laser travel on CDs, and tape head travel on reel to reel and cassette tapes.

Concerning the resolution limit of LPs, since the ground bits of vinyl are melted together, wouldn't the limit be the size of a vinyl molecule?



COMMENT to Custom Machine:  The bit rate limit in nature is really more complex. I just simplified it into a few statements that are generally true and make the point about analog resolution. The LP vinyl molecule is smaller than the black carbon molecule, so the carbon molecule (being the larger size) is the theoretical limiting factor.

The point I was trying to make was that in theory everything is subject to being sampled, even the sound in the air, which we think of as analog (sample free) . But even the most flowing of mediums are subject to a limit in their seeming seamlessness. At some point, and if you look close enough, everything is connected by a series of frozen events. The eye is ofttimes considered to be organ with the highest resolution, but actually the ear has a resolution or resolving ability thousands of times greater.  Provide the eye with 24 pictures per second and it "thinks" in terms of motion. Do that with the ear and you have what sounds like a buzz with the music. Eight thousand times per second is just adequate. You must up the sample rate from 24 bits per second a thousand times before the fluidity of fidelity is perceived. The dynamic range of the ear runs circles around the eye. But that's a whole other topic.

You will find this video enlightening >>> https://www.youtube.com/watch?v=4I5Q3UXkGd0


~swd
« Last Edit: December 08, 2017, 09:41:59 AM by Stephen W. Desper » Logged
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« Reply #4 on: December 08, 2017, 07:16:18 PM »

Wow, that's pretty cool! I had always thought that on a stereo LP, one side of the groove stored the left side of the sound, and the other side of the groove stored the other side of the sound, but I guess not.
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Stephen W. Desper
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« Reply #5 on: December 09, 2017, 09:07:57 AM »

Wow, that's pretty cool! I had always thought that on a stereo LP, one side of the groove stored the left side of the sound, and the other side of the groove stored the other side of the sound, but I guess not.

COMMENT to SMiLE-addict:  You are CORRECT.  The inside side of the groove stores the right channel information and the outside side the left channel. You can see it on the electron-microscope video cited in my first post.

Did you know that the LP cutting head actually cuts a Sum/Difference (S/D) signal, not a Left-Right signal.
 
Further that the LP is intentionally cut out-of-phase by 180 degrees.

Here's why.

When the Stereo LP was first introduced in 1959, it was important that all the previously cut mono LP's not become obsolete but remain playable with the new stereo equipment. So here's what the engineers did.

In order to make the Stereo LP compatible with the Mono LP or in other words, so that you can play a Mono LP on a Stereo player, the signal is first converted to the Sum/Difference format, i.e., Sum = (L+R) and Difference = (L-R). This would normally put the sum signal (both channels added) moving the stylus vertically. But that would cause mis-tracking or in loud cases, throw the styles up and out of the groove. To avoid this the polarity on one channel is inverted when cut so that a sum signal is now lateral motion, just like the groove of the mono LP. The stereo playback cartridge is wired out-of-phase also, reverting the signal to normal. Thus, playing a mono disc on a stereo player places the image in the center without removing bass. Bass is usually centered (and never out-of-phase) so it is cut laterally. The stereo information is cut vertically. Sending a Sum/Dif signal to the cutting head causes the cutting stylus to engrave the left channel on one side and the right on the other side of the groove by the natural vectorial outcome of physical movement when the two signals are causing motion of the one cutting stylus.

Another reason the LP sounds as good as it does --- look at the equivalent IPS speed of the groove past the stylus. It's close to 15 IPS which is the same speed as the master tape. I say "equivalent" because although the speed of the LP platter remains constant at 33 1/3 RPM the velocity of the groove moving past the stylus changes as you move into the disc. The velocity is greatest at the outside of the disc and reduces as the tracking moves toward the center of the disc. Since this will cause the fidelity to be compromised as the velocity slows, a compensation EQ is applied to the signal at the time of cutting to make up for the loss in speed. This is all automatically done so you never hear it.

The opposite thing happens with the CD. The laser starts reading the CD at the center and moves outward. It starts at the center because this is where the surface of the CD is most flat. Manufacturing tolerances of the CD hold it flat all over its surface to a given tolerance, but as the laser moves toward the outside of the disc, the CD becomes less flat and thus more susceptible to mis-tracing due to the disc not being as flat or changing in distance from the laser due to warping. Taking advantage of the flatness toward the center, all music (or pictures, or files) start with the best tracing at the center and as the song advances, along with the tracing laser advancing, the tracing becomes more susceptible to error correction due to the un-flatness of the outer edges of the disc.

Further, unlike the steady RPM of the LP, the CD changes rotational speed as it the laser moves outward. The speed of the CD is faster when the laser is tracing at the inside of the disc, but as it advances toward to outside the CD is slowed. All this is very precisely controlled by the player mechanism. 

You may also find it of interest that the CD or DVD actually has three dedicated (so-called) tracks or pathways of information. That is, one song is actually represented by three independent pathways on the disc. Each is read and the processor, called a comparitor, looks to see if the information from each of the three readings is the same. If not (due to dust or a scratch) the comparitor will take any two sets of information and discard the third. Since these pathways are in different parts of the surface of the CD, a scratch at one point on the surface will not effect the reading of the same information from one of the other pathways since they are at different areas of the disc. So the comparitor takes any two matching sets of information (bits). If the comparitor cannot read two signals that are equal, it will makeup the missing bits by a process call extrapolation. For extrapolation, the processor looks at the bits right before and right after the missing information and assumes or creates bits that are kind of an average between the two. Thus, the missing data is added back by assumption. All this is call "error correction." There are programs which tell you the error correction rate as you play a CD. On a dirty CD, or one that has be mis-handled by its user, the error correction rate can be quite high, but you never hear it.

Error correction takes time. We humans never hear it, but it does take microseconds to be processed. Error correction requires that the tracing information first be stored so it can be compared. Back when files were being traded for free a case was brought against the file traders with the ruling of the court that no CD could be copied as it violated the copyright law. Technicians and engineers laughed at that ruling since it would mean that you could not play a CD. The court had to change the law to accommodate error correction, because for error correction to work, the CD data must be stored in the player (or recorded). 

Sometimes our law makers don't quite realize what they are doing. Like the case against Microsoft in which they ruled that information had to be erased. It took Bill Gates testifying in-front of a committee of Senators for them to be educated that their ruling did little. As Gates said, erasing in computer terms is just changing the first bit (or letter) of a given document into a bit that tells the computer to disregard the following string of data. Removing a file or document from a computer is more than erasing. It take a special program to wipe all the data from memory.  This is why computer experts can go back into a computer and retrieve date the user thinks was erased. In cases of top-secret information or child pornography being stored on a computer and then erased, actually it is still on the computer and can be retrieved by disregarding the first bit. The same for a cell phone.  This is how hackers can look back into files and get information, like passcodes or your social security number from files you thought were erased.  So be careful.

 
~swd
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SloopJohnB
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« Reply #6 on: December 09, 2017, 01:44:50 PM »

You may also find it of interest that the CD or DVD actually has three dedicated (so-called) tracks or pathways of information. That is, one song is actually represented by three independent pathways on the disc. Each is read and the processor, called a comparitor, looks to see if the information from each of the three readings is the same. If not (due to dust or a scratch) the comparitor will take any two sets of information and discard the third. Since these pathways are in different parts of the surface of the CD, a scratch at one point on the surface will not effect the reading of the same information from one of the other pathways since they are at different areas of the disc. So the comparitor takes any two matching sets of information (bits). If the comparitor cannot read two signals that are equal, it will makeup the missing bits by a process call extrapolation. For extrapolation, the processor looks at the bits right before and right after the missing information and assumes or creates bits that are kind of an average between the two. Thus, the missing data is added back by assumption. All this is call "error correction." There are programs which tell you the error correction rate as you play a CD. On a dirty CD, or one that has be mis-handled by its user, the error correction rate can be quite high, but you never hear it.

Error correction takes time. We humans never hear it, but it does take microseconds to be processed. Error correction requires that the tracing information first be stored so it can be compared. Back when files were being traded for free a case was brought against the file traders with the ruling of the court that no CD could be copied as it violated the copyright law. Technicians and engineers laughed at that ruling since it would mean that you could not play a CD. The court had to change the law to accommodate error correction, because for error correction to work, the CD data must be stored in the player (or recorded). 

I never knew this. Fascinating and clever. Thank you!
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