How CD/DVD Discs Work & Fail

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How DVDs Work and Fail

This tutorial is for the average, non-technical person, layman, who wants to learn the basics about how DVDs work and more importantly, how DVD discs fail.

The following technical explanations are abbreviated and thus flawed -- for the sake of teaching "principles" -- not "mechanics".

For Example, fundamentally, "recordable CD discs" and "recordable single-sided DVD discs" are very much alike; so much so, that a DVD drive on most computers can play both a CD and a DVD however, a CD disc is simpler. Therefore we will use the diagram of a Compact Disc -- otherwise known as a CD -- to illustrate "in principle" how different DVD discs work.

Using this "simplified" diagram -- we will re-label the disc layers -- for the sake of explaining "working principles" of DVD discs. That way you do NOT have to "figure out" a new diagram every time we talk about different types of DVD discs.

A more technically correct discussion on DVD discs can be found at CD-R Optical Disc , DVD-R Optical Discs , Making a Commercial CD at Wikipedia.com

In keeping with the original intent of the diagram, the diagram of "CD layers" below correctly shows a "pressed CD" -- like a commercial "music CD" -- as follows:


"CD layers" by Pbroks13 - Own work by uploader. Disc IMAGE™ taken from IMAGE™:Etiquette cd-rom 01.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:CD_layers.svg#mediaviewer/File:CD_layers.svg

A. A "dye layer" has the data encoded by using "bumps" (i.e., pits and land).
B. A shiny layer reflects the laser.
C. A layer of lacquer protects the shiny layer.
D. Artwork is screen printed on the top of the disc.
E. A laser beam reads the CD -- that is to say, shines a laser on the bumps (called pits and land). The light is either reflected (bounced) back -- or not bounced back -- to a sensor, which converts the "bounce" or "NO bounce" Signal into electronic (binary) data -- ones or zeros.

What you do NOT see in the diagram (but will see below), is that the data is arranged as one long continuous spiral track -- much like a spiral track on a "vinyl record". Writing data to this "spinning spiral track" is the source of longevity problems.

A diagram of how optical reading works with a compact disc. Author :Hoikka1 This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license https://commons.wikimedia.org/wiki/File:Cd_optical_reading_technology.png

Above, in a "movie disc" or "pressed disc" a laser measures depth and duration of each physical "pit" data-feature and the depth and duration of each physical "land" data-feature on the disc. Each "land" data-feature triggers a signal at the prism. Each "pit" data-feature triggers a "no signal" at the prism. This on/off or ones and zeros is a binary code transmitted to a data processor. The purple square shows a pattern of "pit" and "land" data-features. The longer grooves would be side-by-side pits; and longer segments of "land" features would be side-by-side "land" features. Also notice there is a "no data zone" or "track" that parallels the "pit" and "land" data-featurs. This "track" is actually a "spiral".

In "dye based" recordable discs, the "land" is actually space occupied by a "dye layer" were "burned dye" -- "blocks light" and thus "simulates a land" data-feature. A "metallic foil layer" at a greater depth, in the same position of the bottom of the "pits", "simulates a pit" data-feature. Each "simulated land" data-feature triggers a signal at the prism. Each "simulated pit" data-feature triggers a "no signal" at the prism. This signal pattern is called, "a bounce" or "no bounce" Signal -- translated as a binary code.

The Two Basic Types of DVD Discs

Type 1: Movie DVD Discs (also called "Pressed" or "Pitted").

The name "Movie Discs" is many times used, because the content of these discs -- 99% of the time -- are "full length movies" -- made by the Movie Film Entertainment Industry -- like Fox or Disney Productions, etc. You are most likely very familiar with this type DVD -- which you have rented or purchased.

Movie DVD discs "fail", most commonly, due to "too many scratches" on the surface of the disc -- like an abused "rental" Movie DVD disc. Movie discs DVDs fact have longevity that spans decades -- if protected from scratches!

The "metallic foil layer" is where the "pits" and "land" data-features are imprinted and stored. The "metal foil layer" CAN, in fact, play a roll in the longevity of ONLY THESE type DVD discs. In theory, since "gold" will not tarnish and is soft, gold is an optimal "metal surface" into which to "press pits"; however, modern metal alloy foils have proven to be "near gold" rivals, and a lot cheaper.

These "Movie DVD discs" are also called "Pressed" or "Pitted" DVD discs" -- which describes the way "movie DVDs" are made.

Below: An over simplified diagram of How a "Movie DVD Disc" works.

The "metal foil" layer (B) of a disc -- where the video data is "pressed"(i.e., stored) -- as little "pits" (i.e., depressions) and "land" (i.e.,non-depressed areas). The Laser (E) can read through the clear protective layer (A) to read the video data from the "metal foil layer" (B). This example is technically flaw for simplicity.

These DVD discs can NOT be "burned" on a DVD recorder or deck.

Microscopic picture below
shows "pits" (i.e., depressions) and
shows "landings" (i.e.,non-depressed areas).

NOTE: "Dye layer" discs (see Type 2 DVDs below)
SIMULATE "pits" and SIMULATE "landings"
using "dark dye" and "clear dye".

Also note the "data tracks". DVD track are actually one "spiral track" wrapping the surface of the disc (note the slight angle) -- much like tracks on a "vinyl record". This design has caused havoc for DVD RECORDERS for Type 2 DVDs, more below.

"Compact disk data layer 2d 3d" by Valacosa and Blair Lebert. - Own work. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Compact_disk_data_layer_2d_3d.PNG#mediaviewer/File:Compact_disk_data_layer_2d_3d.PNG

Movie DVD discs are manufactured using molding machines -- that physically "stamp" video data (tiny little pits or indentions) onto a "metal foil layer" on two halves of the DVD. THE DATA IS MADE OF METAL. The 2 halves of the DVD are then "PRESSED" (bonded) together. These DVDs are mass-produced -- like cookies -- thousands at a time. Note: This explanation is over-simplified -- as Pressed DVD have dual data layers.


Type 2:Recordable DVD discs (DVD-R)

YOUR PRICELESS PHOTOS AND VIDEOS ARE ON TYPE 2 DVD DISCS:

Type 2 DVD discs were developed to "simulate" Type 1 DVD discs. Today ALL DVD players read BOTH types of DVD discs.

Because, to a layperson, they "look alike" and "play alike", most laypersons think "Movie DVD discs" and "Recordable DVD discs" ARE alike. In fact, they are entirely different data storage technologies -- both of which share a common "reader/player deck" technology.

"Recordable" discs "store data" in a light sensitive "dye layer" -- "NOT in the "metal foil layer". When a "high temperature" laser "burns" (i.e., records) data. The laser turns the dye, in the "dye layer", a different color -- to block light. "LOOK LIKE" pits and landings -- that is to say, to "LOOK LIKE" millions of little holes and grooves -- which ARE the actual video data.

The "metal foil" ONLY REFLECTS LIGHT (OR NOT) -- depending on whether the "dye layer" is "dark" (NO-bounce data) or "clear" (Bounced data). The "metal foil layer" does NOT store any data and therefore the color of the foil or the disc (silver or gold) does not effect data longevity.

Problem 1: DVD RECORDERS, are NOT standardized in the way they "write data" onto the tiny little -- fast spinning -- spiral tracks of a "Recordable" DVD -- thus creating many "data errors" or "false data". "Data errors" are "like" tiny little (digital) scratches in the "dye layer" in a DVD. These (digital) scratches interrupt light (like real scratches) -- by creating "false data".

Problem 2, is that once activated, the "dye layer" is "unstable" and continues to change colors over time -- a process which is influence by "light", "moisture", "air", "heat", "cold", "handling"." Simple aging" ALONE continues to degrade each "block of data" on the DVD -- reducing DVD DATA longevity -- from (manufactures claims of" Shelf-life of) "decades of longevity" -- to real life experience (backed by longevity tests of) 2 to 5 years -- even on "gold recordable discs".

IMPORTANT NOTE:If the DISC IMAGE™ of a DVD is created on a hard drive" first, then there are NO "recording errors" in the DVD disc IMAGE™. This is also called a "virgin" disc IMAGE™. Burning a "virgin" disc IMAGE™ to multiple DVD give a much better DVD -- than "STRipping" one physical DVD onto another physical DVD.

DVD RECORDABLE DISCS:

(DVD-R, DVD+R, DVD-RW, DVD+RW) are "dye based" -- meaning, a "hot" laser beam discolors a "layer of dye" in the disc -- to "look like" pits and landings. Because they "look like" Pitts and landings, the "dye layer" DVD can be played by the same DVD player used to play Pressed DVDs (also called Movie DVDs).

Illustration Below: Simplified diagram of How a "Recordable" disc (also called "CD-R or DVD-R") disc works.

DATA IS STORED IN THE "DYE LAYER" (A). When Recording Data, the "dye layer" (A) is "burned" by a hot laser -- which makes the dye turn darker.

Later, "Reading the Data", the light from a cool (less intense) laser will NOT "bounce" thru the "dark dye" -- from the "metal foil layer" (B). Where the "dye layer" (A) is clear, the laser DOES "bounce" off the "metal foil layer" (B).

The "metal foil" layer (B) of a disc -- reflects light ONLY. NO DATA IS STORED IN THE "METAL FOIL LAYER" (B).

The Laser (E) measures "bounce" or "NO bounce" Signals as binary data -- just like "Pitts" and "landings" in "pressed DVDs".

IMPORTANT: For our diagram above to be "more" technically correct, there is a protective coating over the "dye layer" -- to protect it from environmental factors;

HOWEVER, as you will learn below, one problem with readable discs, IS that chemical reactions within the "dye layer" and environmental factors outside the "dye layer" DO AGE the "dye layer" -- rather rapidly -- after the "dye layer" has been activated by "burning". Thus "shelf-life" and "data-life" of at "recordable DVD" are two very different lies.

For Example: Expose one of these disc to sunlight on the dashboard of your car for an hour or so, and see what happens to YOUR data!

The picture below shows they different kind of "burn patterns" and "laser technologies" used. Notice the (dark dye) dot and dash type patterns and the "clear dye" (white, reflective) areas between the tracks. These same patterns are created in a "Disc IMAGE™" -- prior to being "burned" onto a physical DVD.

Notice

  1. how wide the tacks and data are for the CD, and
  2. how small they are for DVD, and
  3. how tiny they are for Blu-ray.

As you will learn below, ONE factor to "data longevity" is the SIZE of the data and size of the tracks. Looking at the SIZE of the data and tracks, it is easy to see why a CD last 3 times longer than a DVD!

"Comparison CD DVD HDDVD BD" by Cmglee - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Comparison_CD_DVD_HDDVD_BD.svg#mediaviewer/File:Comparison_CD_DVD_HDDVD_BD.svg

THE DATA IS MADE OF DYE. Once activated by a hot laser, "layers of dye" do NOT appear to "chemically stabilized". They appear to continue to degrade (change colors) due to many factors -- heat, moisture, air pollution, and just time.

As long as these discs are "virgin" -- not burned -- they have "the potential" to "last a long time", however, the "recording process" Stimulates a light sensitive chemical chain reaction -- just like movie film exposed to light!

In laymen's terms, DVDs are made with "Disappearing Ink". Like disappearing ink, once a chemical chain reaction starts, it never completely stops -- until the chemically stimulating agent is exhausted. When photons (light) is the stimulating agent, chemical control gets tricky -- especially in the chemical areas where "dark dye" meets "clear dye" -- and light is still floating around.

Environmental factors -- that permeate the protective layer -- also cause the clear dye to turn dark.

WARNING: "Recordable CD-R discs" last up to 3 times longer than "Recordable" DVD-R discs! This is partly because the "grooves" are much closer together and the physical data patterns are bigger and fatter. Do NOT equate CDs with DVDs them.

CD-R with Degraded Dye - 20080220" by Retron. - Own work.. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:CD-R_with_degraded_dye_-_20080220.jpg#mediaviewer/File:CD-R_with_degraded_dye_-_20080220.jpg

The readable surface of a compact disc (or DVD) includes a spiral track wound tightly enough to cause light to diffract into a full visible spectrum. The white areas are where the this disc has failed -- light refraction has ceased because the "dye layer" totally blocks all "bounced light" over the entire section of that area.

Ultimately, enough of the dye keeps changing - to a point of "data block" failure and then total disc failure -- like too many scratches on the surface of a rental DVD.

Based on major optical disc longevity studies, the National Archives places a safety limit of 2 years for precision data records (where every data bit matters) and 5 years for IMAGE™ data -- before freezing and hanging occurs -- and it is too late for some images.

The type metal foil used in these discs has NO influence on the longevity of the "dye layer". A gold-foil DVD-R has the same "dye layer" as most other premium non-gold-foil DVD discs. Therefore, the dye layer (see disc above) WILL deteriorate LONG BEFORE the metal layer could EVER play a roll in "data longevity".

Summary, " Dye is for DATA; Metal is for Marketing" .


UNDERSTANDING THE HEART OF THE PROBLEM...

THE SPIRAL DATA TRACK

Diagrams Below shows the different "Track Format Patterns" for different disc technologies with tracks not-to-scale.

CHART OF DISCS AND THEIR DATA TRACKS:

Top Row:

Center Row (below):

Bottom Row (above):

"Comparison disk storage" by Cmglee - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Comparison_disk_storage.svg#mediaviewer/File:Comparison_disk_storage.DATAME

The Spiral Data Track on an Optical Disc serves two functions:

  1. A safe zone in which valid data is "stored" (the function of the disc manufactures)
  2. A target zone to which data is "recorded" (the function of the disc recorder manufactures -- which never standardized their "recording standards and specifications").

Criteria for a Perfect Recording:

  1. A perfect disc -- perfectly selected (culled within one disc brand) -- does NOT insure a perfect recording (but it helps).

  2. A perfect recorder does NOT insure a perfect recording (but it helps).

  3. A perfect recorder (off the shelf) and a perfect disc (off the shelf) do NOT insure a perfect recording (but they help)

  4. Only a perfect recorder -- perfectly calibrated to synchronize -- with a perfectly selected -- perfect disc -- insures a perfect recording.

The Library of Congress Optical Disc Longevity Study achieved criteria #4 above -- using DVD industry (insider) consultants, a team of engineers, specialized disc recorders, and only after "culling" the best of the best discs (from 2 brands) -- OVER A 5 YEAR PERIOD.

Since this technical level of accomplishment is beyond the financial and technical means of most businesses and common consumers -- NARA banned recordable DVD discs from being used in "National Archives" - including the Library of Congress -- one month after the "official" results of the Library of Congress Study.

This study asserted that it was "possible" for recordable DVD to retain complete sets of data for 40 years.

On the other hand, NARA says a recordable DVD could fail within 1 year, should be checked every 2 years, and should never be used as an archive medium.

This presents a problem to all those Wal-mart-LIKE" companies "selling video "archive services" -- on "recordable archive DVDs" -- calling them "archive worthy" -- since, as an industry, we "archivist" have ALL KNOWN ALL THIS -- since NARA rejected recordable DVD disc for archive purposes -- way back in 2007.

That these "archive services" and "archive products" were, and are, sold to mostly the poor, to woman, and to minorities -- is subject matter for another discussion -- possibly in a court of law.

I am just the little old messenger!

LONGEVITY: ONLY WITH STRICT RECORDING QUALITY CONTROLS

  1. (Top Row Above) Technically, it is easier to record to media that use concentric (circular) tracks. A concentric track is located at a fixed radial distance -- equal distance around the entire disc -- with a beginning and an end to every circle -- and with each circle segmented into fixed write/read zones. A concentric track offers fixed targets -- or fixed "data write points"-- on the platter.

  2. (Center Row) Recording data to a "spiral track" -- spinning at high speeds -- is much harder than writing to a fixed concentric (circular) track. A spiral track has NO fixed "data write point" -- because the write area is moving both horizontally and vertically -- at the same time.

    The "data write point" must be "predicted" -- for every bit of data written -- because "the next bit" -- the next "data write point" -- is a constantly moving target -- A) moving away from the center of the disc but also B) moving to "synchronize timing of recording data" with the angular velocity (speeds 4x, 8x 16x, etc.) of the per-formatted spiral track.

  3. The Problem with Spiral Data Tracks: A picture is worth a thousand words. Pick a point on the spiral -- and then try to touch it! It is a moving target.

GIP: Animation thanks to GIPHY

Animated Spiral: Source: Giphy.com: http://giphy.com/gifs/spiral-viral-JN7rcwL4B9cDm

The spiral above shows the complexity of motion -- thus the complexity of writing data to a "moving target" on a spinning spiral tracks.

"Burning a disc" at slower speeds produces fewer "data errors".

Burning to a disc with larger spacing between the track, produces fewer "data errors".

What is a "data error"? "Good data" recorded "off-track" or "bad data" recorded "on track".

The animation (below) shows another "moving target" problem. If the center of the hole in the disc is "slightly off-center" -- or if the spindle inside the disc recorder is "slightly off-center" -- and IF TRACK IS SPINNING (which it is not in my example) -- even the best recorder produces many data errors!

Animated Spiral: Source: http://giphy.com/gifs/works-pump-spirals-miqh2rTtSksOk

Although this spiral does not spin, it does demonstrate the problem of targeting a specific point on spiral track that is off-center.

Wear and tear on DVD manufacturing equipment tolerances, wear and tear on a DVD player, variations in voltage, friction due to dust and to no lubrication on the drive train, and much more -- can affect the quality of the spin -- on a spiral data track.

For ALL the problems above, DVD recorders -- WERE NEVER STANDARDIZED! Each "DVD recorder manufacture" picked the problems THEY wanted to tackle -- their own way -- producing DVD recordings -- on the brink of failing.

The cheapest DVD discs and the cheapest DVD recorders -- have survived. Using the two, together, is why NARA has revised the "longevity" of DVD recordings -- DOWN -- from 2 to 5 years -- to NOW only 1 year.

NOTE: Burning an M-Disc on an "off-the-shelf" DVD or Blu-ray "recorder" -- WILL create an M-Disc -- full of "data errors"! Placing that same M-Disc on the hot dashboard or seat of a your car -- for a couple hours -- will tell you a lot about "longevity statistics".

LAST BUT ON LEAST:

All "optical discs" are "light sensitive" -- even M-Discs. Photons from random sources -- of random wave lengths -- WILL create "bad data" from "good data" on both the "spiral data track" and even more "bad data" -- between the spiraled track -- of all photo-sensitive discs.

This last "fact-of-light", pushes optical discs, most of which are on the very brink of failure from "data errors" - over the edge -- sometimes in less than a year -- after being burned.

Add to this fact, that DVD "players" are going the way of CD "players" -- due to Netflix-LIKE internet services.

I love my CD and DVD discs, but they are looking more like my VHS tapes and vinyl records -- every day!

Still want your hone movies video archived to DVD?

Unfortunately, mass bombardment of both particles and photons -- is an argument (in quantum physics) against optical discs -- and the equipment needed to play them -- as protection against an EMP or Mass Solar Flare Event.

Truth is, we really do not know "the truth" -- even with Faraday protections!

Bottom line, now you know, why a spiral data track is much easier to read right -- than it is to write right; and why optical discs -- of any kind -- are NOT "archive worthy" -- because "recording techonology" was never "standardized".

PROOF: (Quote from NIST/Library of Congress Optical Longevity Study)

"There are many formats and speeds of discs (for example, 4X, 8X, 16X…) in the market and yet there is no unified standard for the drive write strategy. Therefore each recorder drive manufacturer develops their own drive to satisfy the write/read requirements of recordable CD and DVD discs. As such, there is no guarantee that the recorder used to burn a disc is fully compatible with that disc. This can, and often does, lead to very high error rates immediately after recording the disc,.."

"... the compatibility of the (DVD) recorder and the (DVD) media will have a critical effect on the LE (Life Expectancy) of (DVD) media, including very stable (DVD) media, since (DVD) error rates immediately after recording will impact the length of time before (DVD) failure.

For this study, great care was taken to ensure compatibility between the (DVD) recorder and the (DVD) media, thus ensuring low initial (DVD) error rates.

Expiring DVD discs:

Recordable DVD disc are used by everyone -- in the

These DVD disc are called "Expiring-DVD™ DISCS".

Custom made DVD discs should always come with Copyrighted Assets -- which includes the original "virgin" DVD "disc IMAGE™" -- from which your physical DVD discs are burned. Clients can then make or re-make new DVD discs -- as desired -- with all DVD discs being as good as the original DVD disc made.