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Testing audiophile claims and myths

Discussion in 'Sound Science' started by prog rock man, May 3, 2010.
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  1. bigshot
    That is a variant on "We can't know anything because we don't know everything."

    I use a DVD burner to read CDs and DVDs. It rips and burns them at high speed. I do checksums and have verify turned on. I've never had any problems.

    There are few things in this world that are as foolproof and free of error as CD players. I wish my knees were that dependable.
     
    Last edited: Sep 11, 2019
    JazzVinyl and colonelkernel8 like this.
  2. gregorio
    1. Exactly and how many times have we seen that (or a variant on it) in this thread, other threads and audiophilia in general, for decades? And because "we can't know anything", we can therefore make-up "anything" as an audible fault/weakness and then sell audiophiles some snake oil to fix it or just state it can't be fixed and therefore vinyl/cassette/wax cylinders/two tin cans and a bit of string are better. Furthermore, we can support these assertions with made-up analogies, anecdotes, outright lies and self contradiction which proves they must be true, while poor old science can't absolutely prove anything, case closed!

    2. Reed-Soloman error correction was remarkably efficient even on those rare occasions when there were any errors. Errors which did slip through all the nets were incredibly rare and the result of serious abuse or even more rarely, a faulty product.

    Here's a great example of self-contradiction:
    So the reason why both companies went digital in the late '70's/early 80's was not because of sound quality but because of "digital photoshop" which hadn't been developed at that time (and wouldn't be for a decade or more)!

    G
     
    colonelkernel8 likes this.
  3. Mark74
    Found this a very worthwhile read - thanks for posting it.
     
    bigshot likes this.
  4. bigshot
    The OP is still doing its job well.
     
  5. 71 dB
    I have never heard distortion caused by a CD player interpolating errors it can't fix. The problem I have with some CDs I have bought used cheap is that my player can't play "through" the damaged parts of the dics and gets stuck where the serious errors are. When this happens I try the toothpaste trick. Sometimes it fixes the problem if the damage on the disc isn't too bad, sometimes it doesn't in which case I ask for a refund for the faulty disc and try to buy another functioning copy of it.

    So, there's three degrees of CD data errors:

    1 - correctable errors
    2 - uncorrectable errors that do not make the damaged section of the disc unplayable
    3 - uncorrectable errors that do make the damaged section of the disc unplayable

    Of these only number 3 is a problem for me and if I am lucky the toothpaste trick solves the problem.
     
  6. KeithEmo
    For most people, it is uncommon for this to happen, and you may never experience it.
    However, whether it is likely to occur depends on the particular disc player you have, and on how you treat your discs,

    Data is stored on a CD in the form of a track comprised of alternating spots that are shiny and not-so-shiny.
    As the disc spins a LASER beam follows the track, and reads it as an alternating series of ones and zeros.
    (The exact mechanisms involved are quite elegant... and quite complicated... and there are actually a few different ways of doing it... but the results are the same.)
    The track itself, and the spots that store the data, are both tiny (their size is expressed in millionths of an inch.)
    And, unlike a vinyl album, where a needle actually rides in a groove, the tracking is accomplished optically (the reader actually "follows the line").

    The error correction mechanisms used in CD players operate by detecting and correcting gaps or errors in the data.
    The data is verified mathematically - using a sophisticated variation of a checksum - so a gap is treated the same as a sequential string of incorrect numbers.
    (Also note that data is read and tested in blocks... so, for a given block of 1024 numbers, that block is either "good" or "bad", and there is no qualitative determination of "how bad is it".)
    However, it's not nearly as simple as storing duplicate copies of the data (for example printing each page of a book twice).
    The duplicate data is spread out across the surface of the disc in a very particular way - designed to optimize the chances of being able to correct errors while using the least space for extra data.
    (So, hopefully, if a scratch destroys some data, the extra data you need to correct it will be somewhere else, and will not also be destroyed by that same scratch.)

    As you've noticed, if you have a spot that is damaged so badly that the drive simply "loses its place", then the disc simply stops playing.
    This is one of the standard hardware specifications for drive mechanisms... and there are special test discs, with increasingly large gaps on them, for testing how large a gap a specific drive can read past.
    Reviews of disc players used to often include the results of this test... but, since most modern drives are pretty good, and most people handle their discs reasonably well, you don't see it much any more.
    However, if you really abuse your CDs, you will find that some drives do far better than others with damaged discs.
    (All of the discussion about error correction assumes that the drive has been able to continue to read the disc.)

    Whether you ever encounter an interpolated error will depend on whether your drive is capable of reading past bigger flaws than the first two levels of error correction can fix.
    (If you have a drive that is especially good at mechanically reading past bad spots you are more likely to encounter significantly serious, and even uncorrectable, data errors.)

    The other relevant issue is that the ability to correct errors depends on the size of each error, the total number of errors, and the relative locations of the errors present.
    (The system has been carefully designed to give you the best chance of fixing the sort of errors that are likely to occur on consumer audio discs due to physical abuse.)

    In theory.....
    - A single gap in a track up to 2.5 mm in length can be corrected perfectly (assuming the drive doesn't lose its spot).
    - A single gap longer than 2.5mm will require interpolated correction (which is a nice way of saying that you get a patched-over gap that contains incorrect information).
    - The damage caused by a single scratch, even a deep one, that runs radially (from center to edge) is usually correctable
    - The damage caused by a single scratch that runs circumferentially , and may cause damage over a significant length in a single track, is often NOT correctable
    (This is why we are always instructed to clean discs by rubbing from center to edge....so that any scratches we may cause are radial).
    - The damage caused by a large number of small scratches can add up to a situation that causes uncorrectable errors
    (If you cause enough damage, in enough different spots, you increase the chance that the data you need to correct one error will be the victim of another error.)
    (This is why a disc that looks like it's been hit with sandpaper is more likely to be unreadable than a disc with a few isolated scratches.)

    Also remember that the LASER used to read CDs is a specific color, and focuses slightly below the surface of the plastic, which allows some types of physical surface damage to be largely ignored.
    And, yes, if you have a disc that is sufficiently damaged, and in the correct way, you may well end up with a disc that can still be played, but will have multiple uncorrectable errors.
    And, if you do, you may hear ticks and pops... or, if the individual errors are short, you may not hear the individual gaps as ticks, but they may be audible as distortion.
    (A series of short ticks or gaps may not be audible as separate ticks... and will simply sound like distortion or noise.)

    However, if you treat your discs well, this is a rare occurrence.

    Out of many hundreds of discs I've ripped....
    Which amounts to several thousand tracks....
    I've encountered only a total of three or four uncorrectable errors that didn't disappear if I cleaned the disc....
    (These are discs that "played through to the end with reported errors on one track".)
    (One of those was a flaw in the pressing master which was present in the same spot on multiple copies of the same CD....)
    (I don't know if those errors would have resulted in ticks or audible distortion since I wasn't playing the discs on an audio drive which had interpolation.)
    (I use dBPowerAmp for ripping CDs... and it actually uses checksums, compared to an online database, to verify the accuracy of each track it rips, as do many other modern ripping programs.)

     
    Mark74 and analogsurviver like this.
  7. bigshot
    Damaged sections of CDs will either skip and click or not play at all. If a disc plays through without obvious artifacts, it is a safe bet that you are getting the correct sound. If you're ripping, just turn on verification. It certainly isn't something to lose sleep over or belabor with lots of paragraphs of discussion. As a format, CDs are remarkably dependable. The only real problem is if they get scratched. Blu-rays are considerably more scratch resistant.
     
    colonelkernel8 and PhonoPhi like this.
  8. KeithEmo
    That is INCORRECT... interpolation (otherwise known as error concealment) always results in artifacts, simply because it does not deliver the correct data, but those artifacts may or may not be obvious or audible.

    A section of a CD that is too badly damaged for the drive to track will either skip or not play at all.
    A section with correctable errors will deliver perfect data after those errors are corrected.

    And, if you're ripping that disc using a computer, and you use some form of active verification, any track with uncorrectable errors will be rejected or flagged.
    (Therefore, if you're ripping your CDs, and you use a program that supports verification, then you needn't worry about this, which is an excellent reason to rip CDs.)

    However, if you're using a separate "audio CD player".....
    And it encounters uncorrectable errors....
    It will then do its best to patch over the errors using interpolation and then proceed to deliver incorrect data to its internal DACs....
    Details are scarce - but different drives are claimed to use different methods of error concealment - with some claiming to be better than others.
    (This means that, while perfect data is perfect, and data with correctable errors is perfect after the errors are corrected, exactly what is done to conceal uncorrectable errors CAN NOT be assumed to always be the same.)

    Again, this is moot if you succeed in simply avoiding uncorrectable errors.....
    HOWEVER, if you do have uncorrectable errors, and interpolation is required, then how well interpolation succeeds in hiding the error will depend on several things:
    - the number and duration of the errors
    - the exact content that's playing when they occur
    - exactly how well the interpolation on that particular device works
    - the details of how the DAC in the particular device handles errors (some DACs will "lose synch" and deliver a loud pop; others will simply mute, which may be barely audible)

    This all happens because, unlike data CD drives, audio CD players place a higher priority on "playing the disc through" rather than on "delivering either perfect data or none at all".
    Interpolation is a compromise intended to allow a player to "play a disc imperfectly rather than not at all".
    This is not a quirk, or something unusual; it is the intent of the Red Book standard.

    And, yes, IF YOU ACTUALLY WANT TO KNOW HOW IT WORKS, it rates an entire chapter in a book.

    And, yes, if someone cared enough, they could actually test how well different drives conceal uncorrectable errors....
    (but it seems easier to simply suggest that discs with uncorrectable errors should be discarded and replaced).

     
    Mark74 and analogsurviver like this.
  9. 71 dB
    I do treat my discs very well, thank you. :)

    It's the used CDs I buy to save money or to get my hands on hard to find OOP discs I have problems with sometimes, because the prior owners of those discs DIDN'T know how to handle optical media! I do.
     
  10. bigshot
    Meguire's Plastic Polish will fix any scratch that hasn't deformed the mylar playing surface underneath. It works wonders. toothpaste is slow, but it works.

    Errors aren't a very serious problem with CDs. They are designed to be played and they do a very good job of it. No need to obsess over it. In fact, beat up old players that have fallen out of alignment probably cause more errors than CDs that have received normal care. Honestly, I don't know why people even waste energy discussing it, because anyone who owns a CD player and uses it regularly knows that most of the time they either play well or they don't play at all.
     
    Last edited: Sep 19, 2019
  11. analogsurviver
    First of all, I would like to thank @KeithEmo for the comprehensive covering of the subject. I knew it was not easy or even "perfect" ( FAR from it... ), but did not expect it to be that complex and problematic. That particular combination usually spells trouble in real life.

    How many of you have been working in CD retail - for over a decade ? I could bore you to tears with stories about latest CD release by XY that just would not play in a CD player(s) made by YZ company. Among the staff of the shop, we had a motley selection of CD players - and over the years, we learned - the HARD way - that all CD records ( officially issued, not CD-R piracy versions ) are just NOT playable with all players on the market. And would suggest the customer aching to hear latest from XY on CD he/she just bought to listen to it in another make of CD players - even IF that meant sending the customer over to the competitors' - because they were carrying that brand we knew from "staff experience" would play that particular CD without a hiccup.

    Regarding damaged discs - first, A WARNING. Toothpaste has been mentioned as remedy. CAREFUL with that one; it depends on the abrasive properties of the paste in question. Tooth pastes range from VERY mild ( usually for children or sensitive teeth ) to almost "sandpaper" - and which one to use depends on the degree of damage to the disc. If the scratches are deep, you will have to use "sandpaper" first, followed by at the very least VERY mild ( children ) variety. For the final polish, I found the paste sold for polishing screens on mobile phones to work wonders. In most normal cases, the use of the "sandpaper" paste would be detrimental - scratches it leaves may well be worse than what you have started with. Only practice can teach you how to proceed in each case.

    We have a great public library also carrying CDs - but people have - most obviously - decided to put to the test the claim that unless you use the CD as object on which you put your part of the body where back loses its proper name to slide downslope on the snow covered ground, everything would be OK.
    I only wish I met a friend with a CD ( disc...) polishing machine earlier... - it does it safer, faster and better than it can be done by hand. It, of course, polishes in a radial direction - and may, or may not, be still available somewhere online. As with any "dirt", heavily soiled/fingerprinted/scratched discs are recommended to be hand polished at least with baby toothpaste first - or else you will end up contaminating the polishing machine too much.
     
    Mark74 likes this.
  12. Mark74
    Thanks, this info can be useful to me - a family member's in-car CD player gouged quite a large number of my CDs over many months, so I'm reluctantly in the CD surface repair business.

    This fact leads me to wonder about my polishing technique. When using a coarse grained polish to remediate a deep scratch, the CD's "mirror" shine is lost and replaced with a dull, mat, ugly surface with eliptical swirls. I tend to feel obligated to keep on polishing with a finer grained polish to recreate a more (not perfect) shiny surface, but often wonder whether that may be just a waste of time. I just want to rip it with EAC and store it away.

    For a CD with no Accurate Rip data available, should you stop polishing the moment your drive reads the CD without skipping/stopping, or is there actual benefit in polishing the surface further ?
     
  13. KeithEmo
    Those are excellent questions...
    Your first question is actually trickier than it sounds.

    Because of how it works, the error correction on CDs is able to repair single relatively serious flaws, but can be "overloaded" by a large number of smaller flaws.
    This suggests that it's a good idea to avoid adding any more smaller scratches or flaws than absolutely necessary.

    It's also generally recommended, at least for a "final polish", to wipe radially (from the center out or from the outside in).
    Next best is small swirls or ellipses.
    Worst is to go around the disc parallel to the tracks.

    However, the data on a CD is actually stored on a layer below the surface, and read by a LASER that is focused below the surface, and operates in a certain color range.
    Because of this, the read mechanism is able to ignore many small surface flaws (again, depending on exactly what sort of flaws they are).
    Unfortunately, this means that the answer depends on nit-picky details like the exact size and depth of the scratches you're adding, and even the direction they're running in.
    All polishes work by making ever-finer scratches... the catch here is that what looks clearer to the human eye may not be exactly the same as what looks clearer to the read LASER.
    (So those "dull elliptical swirls" may look perfectly clear to a red LASER focused below the surface... or they may not.)

    In your case, though, your second question "informs" the answer that will apply to the first one.
    If you were playing your CDs on an audio player there would be cause to wonder if significant amounts of "error concealment" (interpolation) was occurring.
    However, because computer drives generally do little or no interpolation, as long as you're getting a "solid read", that probably means there are no uncorrectable errors, so there would be no point in going further.

    A good compromise, if you want to be as sure as you possibly can be, would be this...
    RIP the CD once and save the results...
    Then give it one last "final polish" and RIP it again...
    If you compare the two resulting digital files, and they are identical, then your "final polish" didn't make any difference...
    (A lot of file management programs can generate checksums... to compare two files, simply create a checksum of the first ripped file, then "test" the second one against that checksum.)
    Since minor scratches usually result in "soft errors" that change each time you attempt to read or polish them, if the results of two subsequent rips are the same, then the data you've read is probably perfect.
    This is sort of a variation on the idea, which some software still offers, of reading the disc multiple times and assuming that, if it reads exactly the same multiple times, then it's probably good.
    It's also usually a good idea to open the drive, then re-close it, between reads.
    This rules out the possibility that a slight shift in the position of the disc in the drive may affect the results. (
    Many programs do this automatically when they "verify" discs.)

    I haven't used EAC in a long time, but older versions used to offer a "secure read" option, which was independent from AccurateRIP, and which involved reading each sector multiple times (I think at different speeds).
    If multiple reads produced the exact same data then it would assume the read was "good".
    And, if one or more of the attempts produced identical data, but one or more did not, it would assume the version that was identical across multiple attempts was the correct one.
    (I sort of remember some versions falling back to doing this if they failed to find the disc you were doing in the AccurateRIP database.)

    Either way, once you've got a good read using EAC, using its secure mode, there's probably no reason to keep polishing.

    Three other thoughts here....

    First, there are several products on the market for "making foggy headlights clear again", and for "repairing scratched sunglasses and safety goggles"...
    Rather than polishing out scratches, many of these work by filling in surface scratches with plastic, or with some sort of wax, which matches the optical properties of the original plastic.
    (Most of them you apply, let dry, then "buff off".)
    I tried this a long time ago with a commercial product for repairing the lenses on safety glasses - and it worked quite well (that particular product is no longer available).
    I suspect that some of the newer products may work even better (especially since CDs are made of a type of polycarbonate plastic rather similar to modern headlight lenses).
    I also suspect that some products designed to "remove scratches from the clear coat on cars" or "actually fill in the scratches" may work very well - but I haven't tried any of them.

    Second, there are a few commercial music CDs that incorporate various types of copy protection that works by deliberately storing some "bad data" on the disc.
    The idea is that they've incorporated "special errors" which won't produce audible flaws, or are "successfully" repaired by interpolation, but will cause the disc to be treated as a bad disc when you attempt to copy it.
    Some of these discs have trouble playing on some ordinary players, some won't play at all on a computer, and some end up with multiple loud audible ticks when you try to RIP them.
    There are several of these systems, none of which was very widely used, and most of which were used by only one or two manufacturers on certain discs, but the discs still turn up from time to time.
    So, if you find a disc that "just won't RIP but looks OK", before you go nuts, try doing a Google search on that particular title or ASIN.
    (This has been annoying enough people long enough that you will probably find it mentioned somewhere in reference to a particular disc if you look - and you can find lists of discs that use each.)
    (Also, some programs may be more or less successful with RIPping some of these, and many of them were eventually reissued or replaced with new versions without the protection scheme.)

    Finally.....
    On audio CDs, the recorded layer is sandwiched between two sheets of clear plastic which are actually quite tough...
    On CD-Rs, the recorded layer is in the top (label) side of a single clear sheet of plastic, with only a relatively thin layer of lacquer or plastic coating over it.
    (That's why it's dangerous to write on the label side of CD-Rs or to pull adhesive labels off of them.)

    The plastic on the front surface is actually quite thick.
    I actually tried to repair a few very badly scratched discs once using a buffing wheel attachment on a grinding machine.
    I was successful with one, but the other one overheated, the adhesive softened, and the disc actually fell apart.

     
  14. 71 dB
    1. Surprisingly Meguire's Plastic Polish seems to be available in Finland, 14 euros for a 300 ml bottle. The good thing about toothpaste is I buy it anyway. I don't know what you mean by "slow". Toothpaste treatment takes from start to finnish about 20 minutes, half of that is waiting for the toothpaste to do it's magic. I have that much time. How fast is Meguire's Plastic Polish? 27.43 seconds?

    2. About 99 % of my CDs are problem free. Whether they are bought new or they have been owned previously by people who know how to handle optical media.
     
  15. 71 dB
    Wiping optical media radially should be teached in kindergarden. It's common knowledge and more important that learning the names of the weekdays and months.

    Scratches that are parallel to the tracks are the WORST! :anguished: A deep inch long scratch like that makes the toothpaste repair prosess of the disc pretty much hopeless.
     
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