Theoretical question about how CD's work


Theoretically, can the contents of a CD be printed out onto sheets of paper in 1’s & 0’s, re-entered digit by digit (say, by a generous helper monkey with an infinite lifespan) into some sort of program, and the same sound will be replicated? Just trying to understand how CD’s work (though I’ve been trying for 25 years and it still seems like magic to me).
sealrock
Short answer - the laser “reads” data on the CD metal layer as a series of reflective and non-reflective areas (lands and pits). The lengths of pits and lands are variable and represent digital strings of data according to a pre-determined scheme. The actual determination of what the laser reads is determined after the laser reading operation is complete. I.e., it has to go through an interpreter.

The photodetector detects only the reflected signal (land) but doesn’t detect the signal when it is on a pit due to light wave cancellation produced by clever design of the geometries involved and the wavelength 780 nm of the CD laser. Unfortunately, the photodetector also detects scattered laser light. Since the photodetector is not too bright it thinks it’s real signal. The vibration of the CD itself produces errors during the laser reading process as the laser servo system is unable to keep up with wobbly and floppy disc.
The short answer: Yes!

A longer answer:

First, the monkey would indeed be busy for a very long time, as a typical CD contains something like six or seven billion 1’s and 0’s (referred to as "bits," which is short for "binary digits"). The bits being physically represented by transitions or lack of transitions between the pits and lands Geoff referred to.

Some of those bits don’t represent data, but rather are provided for control purposes, i.e., to make it possible for the laser mechanism and associated circuitry to identify, track, and read the data. Numerous bits are also provided in the form of "error correcting codes," which allow most and in many cases all erroneous reads that may occur for various hardware-related reasons to be mathematically corrected to bit perfect accuracy by the subsequent processing circuitry in the player.

The rest of my answer, though, pertains just to the bits which represent musical data. We are all familiar with decimal numbers, where each digit in the number can range from 0 to 9. Computers and most digital circuitry uses binary numbers instead, where each digit in the number is represented by either a 0 or a 1. Any decimal number has a binary equivalent; decimal and binary are simply different ways of expressing the same quantities. (It’s actually a little more complicated than that, as numbers expressed by 0’s and 1’s can be in various formats such as "2’s complement," or "offset binary," or "straight binary," but you needn’t get into those distinctions for the purposes of your question).

The musical data on a CD conforming to the standard "Redbook" format contains for each of the two channels 44,100 samples per second that are proportional to the amplitude of the music signal at the instant the sample was taken. While "amplitude" can be thought of as volume, keep in mind that for an audio signal it can be either positive or negative.

Each of those 44,100 data samples that are present for each channel during each second of the recording consists of 16 bits, i.e., a group of 16 digits each of which can be either a 1 or a 0, representing a number that if expressed in decimal form would be integers (i.e., whole numbers) ranging from -32,768 to +32,767. With each such number, as I said, being proportional to the amplitude of the music signal in the corresponding channel at a given instant of time.

So, yes, given enough time and given appropriate and accurate playback hardware and software, an accurate transcription of those numbers by the monkey would allow the music to be reproduced consistently with the data on the CD.

Regards,

-- Al
Hey sealrock -


Imagine a record. On it, you can see the sound waves of the recording. The vinyl track gets deeper, or taller depending on the signal.


Now imagine you could measure the depth of the grove in precise increments and you did this every 1/1000 of an inch of grove length. Write that depth down. That is your digital signal.


Best,

E
The encoded data - the pits and lands do not actually represent digital data, not really. The laser reading process is, frankly, strictly an analog process. The series of pits and lands, their various lengths and the transitions from pits to lands and lands to pits are converted to meaningful digital data downstream. So, since the lengths of pits and lands is variable precise timing is critical to recover the data as it appears on the disc. It is not a difficult task to demonstrate that (1) Reed Solomon is only effective for certain errors, but definitely not all errors. For example it’s extremely ineffective dealing with circular scratches. It’s very good for predictable errors like scratches that are radial. Reed Solomon codes and the laser servo function are both rather ineffective for correcting errors due to vibration, including the fluttering and wobbling of the disc during play as well as seismic vibration, etc., or scattered light detected by the photodetector. Reed Solomon did the best they could under the circumstances, I guess. But that was 40 years ago, for crying out loud. 😢

A lot of errors get past the goalie. For for many people the whole thing works good enough. 🙄

All these issues with CDs and especially the player, have always been there. And there are very important reasons why CDs often sound thin, compressed, congealed, honky, metallic, brittle, bass shy, thuddy, synthetic, like paper mache.
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Think of it as the world's most sophisticated player piano. 😄
Well, at least the world’s most sophisticated synthetic piano player. 🤭
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Short answer: no. Because like (gulp) geoff said the data is not on the disc in 1's and 0's, its encoded in pits and lands of varying length. So the data is not there in digital form but rather digital form encoded into pits and lands that are decoded. Which for those following closely means its not digital at all. But not quite analog either. But enough so variations in speed, vibration, scatter, etc all introduce noise and accounts for why analog type tweaks produce improvement in what is supposed to be digital.
From the same reviewer who explained USB transmission so wonderfully, here's a great explanation of how CDs work:
https://6moons.com/audioreviews/psaudio7/perfectwave.html

As stated, it's not done digitally but ends up in the digital realm.

All the best,
Nonoise
@Millercarbon, it seemed to me to be implicit in the OP’s question that the monkey would have to be provided with some means of converting the physical representation of 1’s and 0’s that is used by the CD medium (namely the transitions between pits and lands that I referred to in the first paragraph of my previous post, and that has also been referred to by Geoff, you, and others), into actual 1 and 0 numbers that could be written out.

One way in which that could be done would be to provide the monkey with a computer having a CD/DVD drive and a program such as EAC. By using that program he would assure bit-perfect reads of the data into the computer. (Or in the relatively unusual case that a CD is simply unreadable after many successive attempts, without uncorrectable errors occurring, the program would flag an error). All of that is of course no different than what many of us do when ripping.

The computer could then be provided, if desired, with a simple program that would convert the ripped data into a visible/printable series of 1’s and 0’s, and of course that could be accomplished with bit-perfect accuracy.

Providing the monkey with those provisions would make my previous short answer of "yes" to the OP’s question entirely applicable, as well as saving the monkey a good deal of time by automating much of the process.

Regards,
-- Al
Short answer: yes.

If you want to get a better grasp of the process you would need to understand the basic concepts of ANALOG and DIGITAL. Sampling is the process that takes you from Analog to Digital. You take a "sample" of an analog event and turn it into a number, a digit. This is sampling.

There are 2 key elemnts to how sampling is done: how often do you take the sample and how precisely you do that. The Red Book audio standard (the one CDs are made out of) calls for samples to be done 44.100 times a second and with a precision of 16 bits. So within those boudaries you can build an excat replica of the input signal by any mean, including the Monkey you mention. This is what would be called "bit perfect" in that the bits are exactly copied. The copy the monkey would have made would be absolutely non recognizable form the original and if played on the same playback system would sound 100% the same.

There are many misconceptins around, I'll mention a few:

QUITE The encoded data - the pits and lands do not actually represent digital data, not really UNQUOTE

Yes they absolutely do !

QUOTE The series of pits and lands, their various lengths and the transitions from pits to lands and lands to pits are converted to meaningful digital data downstream. So, since the lengths of pits and lands is variable precise timing is critical ... UNQUOTE

Precise timing is indeed critical in that the samples need to be payed back at exactly 44.1 KHz. But that is implicit in the standard, NOT HARDCODED in the digital domain !

BTW your it's an intersting question

You can start here:  https://en.wikipedia.org/wiki/Sampling_(signal_processing)

And spend more or less the rest of your life digging ! Enjoy !
Mark.
Christ Almighty, Geoff and Millercarbon! Get a grip! A "1" or a "0" is an abstract human invention to describe there being a thing or the lack of there being a thing. No medium records anything called a "1" or a "0" because they don't actual exist in reality. They're figurative terms to describe a charge, polarity, or material presence of a thing, or the lack of a charge, presence of a thing, or an opposite polarity. That's how ALL digital recording mediums work. There's nothing analog about how CD's, DVD's, BlueRays, NAND memory, or digital magnetic media save data. There's either a thing representing the abstract value of "1" or there isn't. The thresholds for what determines a "1" is defined and it doesn't have some gray area. That's why digital data is so reliable. That's the entire premise that digital computing is built upon. 

I wish you guys would stop injecting your nonsensical snake oil beliefs into otherwise sensible conversations. If your stupid ideas about how digital media worked reflected reality digital computing would never have gotten off the ground. 
Nurse! Thorazine! Man down! Costco hasn’t been paying attention. In the case of CDs the 1s and 0s are represented by transitions not by the ON or OFF. As I already explained it’s more complicated than you apparently think. The link I provided yesterday includes the allowable sets of data determined by both transitions AND the length of pits and lands.

Also, you are incorrect that - in the Case of CDs - the digital process is reliable. It is only *partially* reliable 🤭 within limits that are set by failure of the CD playback system to deal properly with scattered laser light and vibration, internal and external. Not to mention the obvious failure of Reed Solomon error codes and the laser servo feedback system to correct all errors. Wake up and smell the coffee! ☕️

Even CD players that are *isolated* have difficulty with laser reading the data on the disc because the *CD itself* is wobbling and fluttering. The CDs are often not perfectly round and the disc is often not absolutely level during play, which exacerbates this CD vibration problem. Yes, CD Compact Disc “works” well enough for those who don’t mind generic, thin, bass shy, brittle, honky, synthetic, congealed sound.
Ah snake oil accusations took their time to get into this thread ... amazing! Next ... 0 and 1 sound the same in any cable no matter what ... stand by ... oh dear
About CDs did you know they binary code is pressed onto the polycarbonate disc and yes a fine powered lubricant is used which does go into the surface layer , a product the cable company sells  called Art de sone. You sprayon not messy and wipe off removes the residue and alllows the laser to read better 
and music sounds for sure cleaner, and I do thisalso before ripping a disc .
you can hear the difference .
one thing for sure I experimented with a properly burned  in copy sounds better 
then the original copy especially when using top blanks like a maxel blue color 
cd. ,why forburning the code in ismuch more defined and easier for the laser to read and theblue make up allows less light scatter. It isnoticably better sounding 
but a pain in the butt considering I have everything on a Solid State drive.
i have proventhisto others Many timesin the past .today I just look for quality 
remasters and apply the Art de son to everydisc before I rip it 
and use either Wav, or the highest uncompressed #8 on db power amp 
for ripping which is a great program and great for converting any disc format 
even from DSD  to high quality standard if your player don’t read DSD recordings.
Actually they don’t use mold release compound MRC during the manufacture of CDs. That’s what they call an old wives tale.
I would emphasize at this point that the intent of the OP's question is that he is "just trying to understand how CD’s work."

Discussions of why a given CD can sound different when played on different players, or why different physical CDs containing the same 1's and 0's may sound different when played on the same player, or why CDs, ripped files, and CDs burned from those ripped files and containing the same 1's and 0's can sound different, are separate (and complex) subjects that are unrelated to the stated intent of his question.

Regards,
-- Al
 
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The OP is clearly trying to make the point that if it’s all 1s and 0s on the CD how can anything make a difference - the data on the CD cannot be changed. Hel-loo! It’s not exactly rocket science. It’s the same argument regarding digital cables. They’re both Strawman Arguments. Can we please try to be a little more chill?
Since we've already gone pretty far down the rabbit hole I've got a related question. @almarg mentions EAC and FLAC. When I rip a CD to FLAC using db Poweramp what does the data on the FLAC file look like. Since it can be compared to a perfectly accurate copy (whatever that means) it shouldn't need the error correction wizardry. Is it just a stream of 0 and 1's or is it more like internet packets?
8th note
The Flac file is exactly like any other file, a bunch of 0s and 1s. It is a "non lossy" compressed format, meaning that its size is smaller than the original but no information is lost. BTW an internet packet again is a stream of 0s and 1s !!! 
I for one do not believe any error correction is needed. You copy a file you get a perfect copy unless someone can show me why a copy of a Data file from a CD should be different from a copy of an audio file.
Differences may and will come up when you "render" that file. I.e. when you get back to the analog world via a DAC; that might be slightly different (the rendering I mean) from one DAC to another resulting in a slightly difrernt analog signal. Same digital input, different analog output.
marklings
I for one do not believe any error correction is needed. You copy a file you get a perfect copy unless someone can show me why a copy of a Data file from a CD should be different from a copy of an audio file ...
You need error correction because there are often errors when playing a CD in real time, which is quite different than simply making copies of a data file.
8th-note 6-27-2019
@almarg mentions EAC and FLAC. When I rip a CD to FLAC using db Poweramp what does the data on the FLAC file look like. Since it can be compared to a perfectly accurate copy (whatever that means) it shouldn’t need the error correction wizardry.

I made no mention of FLAC, which as stated above is a lossless format for storing audio data in compressed form. EAC ("Exact Audio Copy") is a software program that is widely used for "ripping" (copying) the contents of audio CDs onto computer hard drives. It provides the capability of re-reading data on a CD multiple times that depending mainly on the condition of the disc and the drive mechanism may not be captured accurately on the first pass (i.e., on the fly).

The "error correction" that I have been referring to is invisible to the user, is performed by circuitry associated with the drive mechanism, and makes use of error correcting codes that are on the CD and are an inherent part of the CD format. My understanding is that **for a CD and a drive mechanism that are in good condition** something like hundreds of bits or even more will typically be misread by the laser mechanism during a single pass, among the billions of bits that are on a CD, and all or very nearly all of them will be routinely corrected by that circuitry to bit-perfect accuracy, on the fly. Use of a program such as EAC, which can make multiple passes if necessary, provides additional assurance that will happen, and will flag an error if for some reason it does not happen.

When I rip a CD to FLAC using db Poweramp what does the data on the FLAC file look like. Since it can be compared to a perfectly accurate copy (whatever that means) it shouldn’t need the error correction wizardry.

I’m not familiar with db Poweramp, but even if it only rips using a single pass chances are that all or nearly all of your rips are bit perfect (assuming discs and drives are in good condition), with the necessary error correction having been performed by the hardware invisibly, ***prior to db Poweramp even seeing the data.*** EAC, as I said, just provides added assurance, especially if disc or drive condition may be marginal.

Regards,
-- Al

I suspect things are much worse than represented by almarg and his statement that perhaps 100s bits are in error. However, if that were actually true it would probably be inaudible. So, things must be worse than that. Since vibration isolation, CD disc beveling, disc coloring, CD liquid treatments, disc damping, disc demagnetization, disc static electric charge dissipation and other steps obviously improve the sound. Hell, just getting rid of the laser scattered light problem alone doubles performance! So, gentle readers, I think I’ll stick with my original evaluation that the whole CD playback system is FUBAR. I’ll leave it to others to speculate on how many errors there are on high end CD playback systems.
Responding to nonsense from kost_amojan, geoffkait writes:
Nurse! Thorazine! Man down! Costco hasn’t been paying attention. In the case of CDs the 1s and 0s are represented by transitions not by the ON or OFF. As I already explained it’s more complicated than you apparently think. The link I provided yesterday includes the allowable sets of data determined by both transitions AND the length of pits and lands.

Also, you are incorrect that - in the Case of CDs - the digital process is reliable. It is only *partially* reliable 🤭 within limits that are set by failure of the CD playback system to deal properly with scattered laser light and vibration, internal and external. Not to mention the obvious failure of Reed Solomon error codes and the laser servo feedback system to correct all errors. Wake up and smell the coffee! ☕️

Even CD players that are *isolated* have difficulty with laser reading the data on the disc because the *CD itself* is wobbling and fluttering. The CDs are often not perfectly round and the disc is often not absolutely level during play, which exacerbates this CD vibration problem. Yes, CD Compact Disc “works” well enough for those who don’t mind generic, thin, bass shy, brittle, honky, synthetic, congealed sound.


Right. Exactly. Could hardly have said it better myself.

About the only thing left unsaid is that yes indeed one could get a string of 1’s and 0’s from the output of a CD player. Just not the same one every time. And so no, the same sound cannot be replicated. Otherwise a CD would always sound the same. Which they don't. Because the CD is not digital.
OK, simple question
How do you realize one bit is wrong ?
How do you know this zero is a good zero but that one is a bda one ?
IMO the reason many of the tweaks mentioned above by Geoff may be beneficial in some situations has nothing whatsoever to do with bit errors or error correction.

The main reason in most cases is likely to be related to electrical noise generated by the servo mechanisms and circuitry in the transport part of the player, as it tracks the disc, coupling into unrelated downstream circuitry in the player, causing jitter in the D/A conversion process, and/or intermodulation or other effects on the analog signal path. The degree to which that occurs will be dependent on the design of the particular player, of course, as well as on the condition of the disc.

From this Wikipedia writeup:
Reed-Solomon coding is a key component of the compact disc.... In the CD, two layers of Reed-Solomon coding separated by a 28-way convolutional interleaver yields a scheme called Cross-Interleaved Reed Solomon Coding (CIRC).... The result is a CIRC that can completely correct error bursts up to 4000 bits, or about 2.5 mm on the disc surface. This code is so strong that most CD playback errors are almost certainly caused by tracking errors that cause the laser to jump track, not by uncorrectable error bursts.

Note that the term "error correction," as properly defined in this context, refers to bit-perfect correction. "Error interpolation" is the term used to refer to the less than bit-perfect approximation that can occur (rarely) when bit-perfect correction fails.

And from a post by member Kirkus (who probably has more hands-on experience with the internal workings of CD players than the rest of us put together) in this Audiogon thread:

CD players, transports, and DACs are a menagerie of true mixed-signal design problems, and there are a lot of different noise sources living in close proximity with susceptible circuit nodes. One oft-overlooked source is crosstalk from the disc servomechanism into other parts of the machine . . . analog circuitry, S/PDIF transmitters, PLL clock, etc., which can be dependent on the condition of the disc.... One would be surprised at some of the nasty things that sometimes come up out of the noise floor when the focus and tracking servos suddenly have to work really hard to read the disc.

Apologies to the OP for the digression.

Regards,
-- Al
 
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It’s a theoretical question as clearly stated and the answer is yes. That simple! Short and sweet.

Many audiophiles (largely vendors like Geoffkait) are even more long winded than all those politicians up on the debate stage trying to convince people to vote them in as President.

Almarg as an example though , and some others, are different. Al does not waste words on speculation...just the technical facts that can help further understanding of how things work in the right hands. He is also not a vendor. No $$s to be made in this game for him specifically TTBOMK.

As if words can accurately describe what something, pretty much anything, actually sounds like.  Theory and reality are not the same.

Sound technical facts help for sure but even those words alone can’t do it.

Disclaimer:: I too am NOT a vendor......no $$$s in this game for me.
So it's settled. The monkey on the player piano is the correct outcome.
🐒 + 🎹 = 🎼

here is a somewhat related comment...i'm sure that many of us played a CD in the car while driving...i bet it sounded fabulous...regardless of the laser scattering, the bumps in the road, and all the motion the car experiences during a normal drive...athat cheap CD player sounded pretty good...
Well, the CD player in the car buffers the data, otherwise the CD would not play at all. However, data buffering cannot fix the damage already done by scattered light and the vibration CD, which occurs in the first picosecond as the laser reads the disc. So, you would have thought the sound was even more fabulous had the scattered light and CD vibration been eliminated. Everything is relative.

A lot of people think their system sounds fabulous. I get it. 😬

No matter how much you have in the end you would have had even more if you had started out with more. 😛
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Most of CDPs play in real time - there is no time to read particular sector again.  Ripping program can access each sector unlimited number of times to obtain right data (proper checksum).  Car or portable CDPs have to make up time to be able to return to troubled (bump) moment/sector and reread it.  In order to do this CD is rotating a little faster than necessary and the data is buffered.  I had once Panasonic portable CDP with a switch to enable/disable this mode.  I could clearly see thru the window CD rotating faster in bump-proof mode.  I'm only not sure what is considered a "bad data".  Reed Solomon code allows not only error correction up to certain number of missing data point (2.5mm scratch along the track per Al's post), but also data interpolation for a little longer scratches along the track.  CDs in really bad shape might still play fine, but a lot of data will be interpolated.  Ripping program can obtain right data to create new CD-R with better sound than original scratched CD.  I was even able to save that way few completely unreadable CDs (but it took long processing time).
I love that the OP has orphaned this thread, but people are still arguing about how CD's work.  :D :D :D
almarg
IMO the reason many of the tweaks mentioned above by Geoff may be beneficial in some situations has nothing whatsoever to do with bit errors or error correction.

The main reason in most cases is likely to be related to electrical noise generated by the servo mechanisms and circuitry in the transport part of the player, as it tracks the disc, coupling into unrelated downstream circuitry in the player, causing jitter in the D/A conversion process, and/or intermodulation or other effects on the analog signal path. The degree to which that occurs will be dependent on the design of the particular player, of course, as well as on the condition of the disc.

>>>>Uh, I never said all the tweaks I mentioned affected bit errors or error correction. The ones I am pretty sure I can prove affect bit errors and error correction are New Dark Matter visible and invisible stray light absorption tweak, damping the CD (my Mystery Tweak) and coloring the CD, which absorbs visible stray light. Also, vibration isolation. All of those are my tweaks.

The demagnetizing of CDs and elimination static electric fields ON CDs is anybody’s guess. I don’t think anyone has explained them, at least not to my satisfaction.

note to self - I hate to judge before all the facts are in but it appears the Skeptics (capitalized for almarg’s benefit) would rather fight than switch. It certainly would not take a neurosurgeon or rocket scientist to do a little investigation and try to get to the bottom of these things. Isn’t investigation a primary part of the scientific method. And what about curiosity? Isn’t that part of the scientific method? Apparent it’s all too much trouble.
erik_squires
I love that the OP has orphaned this thread, but people are still arguing about how CD’s work. :D :D :D

Yeah, I’m starting to think it might have been a drive by. 😬
No reason to argue about how CDs work.   It's pretty well documented.  Books are still an inquiring mind's best friend.
P.S. to my previous post:

I recall seeing measured data a few years ago indicating that erroneous reads by the laser mechanism occur vastly more often when ripping at high speeds than when ripping (or playing) at normal (1x) playback speed. Although even when ripping at high speeds the hardware will usually correct at least the vast majority of those errors bit-perfectly (assuming disc and drive are in good condition).

That would suggest, though, that if the particular ripping program being used cannot detect erroneous data and perform multiple re-reads as necessary, and provide an indication to the user if and when uncorrected errors cannot be overcome by re-reading at the particular speed, it would be desirable to rip at low speeds, e.g. 1x or 2x or thereabouts.

Regards,
-- Al
mapman
No reason to argue about how CDs work. It’s pretty well documented. Books are still an inquiring mind’s best friend.

>>>>Ah, good one, moopman! The old Skeptics Society chestnut. 🌰 Since everything is known and can be found in Wikipedia or some online textbook we can dismiss any inconvenient arguments as not worthy of examination. Bravo, moopman!
Just for emphasis, anyone who thinks he’s listening to all the information that’s on the disc is badly mistaken. The scattered light problem itself without even going into any of the other problems associated with CD playback accounts for an apparent loss of at least 30% to 50% of performance, in terms of bass articulation and slam, treble performance, air, resolution and size of the soundstage, signal to noise ratio, dynamic range, distortion, compared to the case where scattered light is controlled. It’s not even close, gentle readers. Geez, even a green pen around the outer edge gives a glimpse of what you can get if you control all the scattered light, not just the visible red portion. Hel-loo! 
Geoffkait actually you are clearly the skeptic here. Don’t be such a sad sack! Why don’t you write a book and enlighten the world further then smart guy? I’m sure there are enough neurotics out there you can appeal to to make it a big seller and help your business.
Apparently mapman’s butt is still stinging from something I said. Get well soon, Moops.
Geoffkait- I have a CD transport the "47 labs shigaraki" the CD sits on top of the player out in the open, do you think this helps with the scattered light problem?


Thanks, TISH

Geoff, I've noticed that you have frequently posted about the scattered light problem since you started selling a product to help eliminate this problem.  There's nothing wrong with that if you discovered a problem and came up with a solution. 

It would be helpful to us and you, though, if you provided the evidence that shows that scattered light increases reading errors for cds that cd players don't correct.  If the evidence is simply that you hear a subjective improvement when playing cds treated with your product, it would be good to know that too.   


The light travels really fast and it all arrives at the sensor at the same time so there is no problem except that Geoff needs to sell stuff.
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