How to Choose
the Best Media
The
number one issue is compatibility between the writer and the
media. There isn't necessarily a "best" writer or
"best" media; some combinations work better than others.
The writer’s laser power and write strategy must match the type
of media used. Both media and writers have evolved over the
years, so there are many opportunities for mis-match.
Most
major brands will produce good results with modern writers.
But anybody can make a bad batch. The media is pretty
consistent within a batch, so if there is a manufacturing defect,
every disc will be bad. We have received bad batches from
most major manufacturers. Defects range from stamper defects
to bubbles in the plastic, to dirty discs and dye layer coating
problems. We have had to change brands several times over
the years when a particular brand’s process changed.
You
can use CDX or DVX to determine the best type of media for your
application: Record a
full disc at the speed you wish to use in production. It is important to record the whole disc, because media
defects are more common at the outside of the disc.
Then measure error rates and analog signal parameters.
Low error rates (Grade A or B) are a good indication of
recording quality, but you can learn more by also making analog
“pit geometry” measurements.
Beta, for instance is a function of the writers laser
power. Negative Beta indicates that the laser power is insufficient
for the type of media or the speed used.
It is commonly seen that the Beta will drop as the
recording radius increases, due to the increased writing speed.
In this case, you can try a lower recording speed.
Reflectivity and I3 (or Resolution) are also good
indicators of recording quality.
Recordable discs have low reflectivity due to adsorption of
the laser beam by the dye layer.
If reflectivity is too low, the resultant signals will be
too small to be decoded reliably.
Recording
speed also makes a big difference. Again, your results will
depend on the combination of writer and media, but try each type
of media at different speeds to see what works best. In
general, the lowest and highest speeds will not work best.
Recorders are optimized for high speed, so some compromise is made
at the lowest speeds. The highest speeds are also
problematic, as higher laser power is required and faster servos. With our writers and media, we get the best results at 16X on
CDs, and 6X on DVDs. Check out http://www.mscience.com/hispeed.html
for more information about the risks of high-speed recording.
You
can also discover which is the lowest cost media that gives
satisfactory results. Try several different types of media
in your burners and see which works best. Cost is not
necessarily an indicator of good media quality. If you can
get as good or better results with lower cost media, you can save
money.
Major
Causes of Disc Failure
The most common cause of
disc failure is scratches and dirt due to handling.
With molded discs that have
not been handled, the most common cause of defects is a defective
master or stamper. This
is true because any defect on the stamper will be faithfully
reproduced on every disc. This applies to recordable discs too, as a stamper is used to
mold the pre-groove into the blank disc.
A small defect on the stamper used to make recordable media
will create a spot where proper recording cannot take place. Stamper defects can be so small or subtle that they are not
visible to the naked eye, so you can’t always tell by looking at
the disc.
With recordable discs, the
most common problem is incompatibility between the writer and
the media. Often the recording speed is too high for the type of media
being used. Generally,
higher recording speeds produce poorer results.
This problem usually manifests itself as high error rates
across the whole disc, or increasing error rates toward the
outside of the disc. Most
recording is done at Constant Angular Velocity (CAV), so the
record speed increases with the radius.
As a result, more laser power is required.
If the writer cannot provide the required power, the error
rates will increase and quality will decrease with radius.
Another common cause of
problems is a speck of dust or dirt on the disc (readout) surface
when recording. This blocks the recording beam, producing a void, where there
are no “pits”. Likewise,
bubbles or black spots in the disc substrate can cause the same
problem. These type
of errors show up as local defects and can cause serious errors.
Dirt, scratches, or stamper
defects produce double trouble on recordable discs because first
the writing of the disc is disrupted, causing a bad spot, and then
the reading is disrupted in the same spot on playback.
The pits on a DVD are much
smaller than CDs, so a speck of dust or small scratch has a much
bigger effect. For this reason, DVD error correction is more powerful than
that of a CD, but many errors are produced, and the disc can
eventually fail.
How
to Test Blank Media
Although some things can be
measured on a blank disc, none of these things will predict how
the disc will work. The recording action is in the dye
layer, and the only way to test the dye is to record on it.
We recommend that you record
the full disc (74 or 80 minutes) at the speed you wish to use in
production. This is important because defects are more
common at the outside of the disc. Then do the Error Test to
measure the results. Analog pit geometry measurements are
especially useful for qualifying new media.
Since the media is generally
quite consistent within a batch, it is only necessary to test one
or two samples from each batch of new media.
You will also find that the
results vary according to the recording speed. Generally,
results are worse at the highest speeds. Ideally, you should
make full test recordings at several different speeds to discover
what is the highest speed that will give satisfactory results.
DVX will measure Focus Error
and Tracking Error on a blank disc (CD or DVD). To do this,
load the disc, and wait for it to initialize. Then select
the "Blank Test" checkbox in the main screen, then press
"Start".
Note that there is no
calibration for focus error and tracking error on blank discs.
Look for a uniform level, with no spikes or increasing trend.
How to Tell
if a Disc is “Good”
Most problems are traceable
to the disc. Often
people will encounter a situation where a disc works in one drive,
but not another. The tendency is to blame the drive, concluding that “the
disc must be good because it plays in this other drive”. However, almost certainly it is the disc that is at fault.
Since there are no standards for players (other than that
they must play a disc that meets the standards), a disc that is
outside the Red Book or DVD specs may play on some drives, but not
others.
The solution is that all
discs must conform to the standards.
Discs that are made well within Red Book or DVD standards
should play on any player.
A disc that behaves
erratically is invariably bad.
A disc that is outside the specs (or marginal) will behave
in a way that is not predictable.
If a discs produces different results each time you test
it, that tells you that the disc does not meet Red Book (or DVD)
specs.
How CIRC Error Correction Works
This scheme uses two
principles to achieve a remarkable ability to detect and correct
errors. The first is
redundancy. This
means that extra information is added, which gives you an extra
chance to read it. For
instance, if all data were recorded twice, you would have twice as
good a chance of recovering the correct data.
The CIRC has a redundancy of about 25%; that is, it adds
about 25% additional data. This
extra data is not just a repeat of the data, but is cleverly used
to record information about the original data, which
provides the ability to deduce what the missing information must
have been.
The other principle used is
interleaving. This
means that the data is distributed over a relatively large
physical area. If the
data were recorded sequentially, a small defect could easily wipe
out an entire word. With
CIRC, the bits are interleaved before recording, and
de-interleaved on playback. What
happens is that the bits of individual words are mixed up and
distributed over many words. Now, to completely obliterate a
single byte, you have to wipe out a large area.
Using this scheme, local defects destroy only small parts
of many words, and there is always enough left of each sample to
reconstruct it. To
completely wipe out a data block would require a hole in the disc
of about 2 mm in diameter.
The CIRC error correction
used in CD players uses two stages of error correction called C1
and C2, with de-interleaving of the data between the stages.
The error correction chip in this unit can correct two bad
symbols per block in the first stage and up to four bad symbols
per block in the second stage.
Types of Errors
The error type E11 means one
bad symbol (byte) was corrected in the C1 stage.
E21 means two bad symbols were corrected in the C1 stage. E31 means that there were three or more bad symbols at the C1
stage. This block is
uncorrectable at the C1 stage, and is passed to the C2 stage.
Because of the de-interleaving of the data between the
stages, those three (or more) bad symbols are now in separate
blocks, and so can be corrected by the C2 stage.
Because of the interleaving, one uncorrectable symbol at
the C1 stage can be turned into as much as 28 bad symbols at the
C2 stage. This is why
E12 is typically much larger than E31.
E12 means one bad symbol was
corrected in the C2 stage and E22 means two bad symbols were
corrected in the C2 stage. E32
means that there were three or more bad symbols in one block at
the C2 stage, and therefore this error in not correctable.
BLER
(Block Error Rate) is
defined as the number of data blocks per second that contain
detectable errors, at the input of the C1 decoder. This is the most general measurement of the quality of a
disc. The “Red Book
” specification (IEC 908) calls for a maximum BLER of 220 per
second averaged over ten seconds.
Discs with higher BLER are likely to produce uncorrectable
errors. Nowadays, the
best discs have average BLER below 10.
A low BLER shows that the system as a whole is performing
well, and the pit geometry is good.
However, BLER
only tells you how many errors were generated per second, it
doesn’t tell you anything about the severity of those errors.
Therefore, it is important to look at all the different types
of errors generated. Just
because a disc has a low BLER, doesn’t mean the disc is good.
For instance, it is quite possible for a disc to have a low
BLER, but have many uncorrectable errors due to local defects. The smaller errors that are correctable in the C1 decoder are
considered random errors. Larger
errors like E22 and E32 are considered burst errors and are
generally caused by local defects.
As you might imagine, the sequence E11, E21, E31, E12, E22,
E32 represents errors of increasing severity.
Why
E32 is Considered Uncorrectable
Although it is possible
under some circumstances to correct up to four bad symbols at the
second stage, not all players can do this.
Until recently, most players could only correct two bad
symbols at the C2 stage. For these players, E32 would be uncorrectable.
In order to have a high probability of a disc working in
any drive, we consider E32 an uncorrectable error, even though
some drives may be able to correct it.
This is also the rationale
for not allowing E22 or E32 errors on a CD-ROM.
The earliest generation of CD players could only correct
one bad symbol at the C2 stage.
As a result, an E22 error (two bad symbols at the second
stage) would be uncorrectable on these drives.
In order to have the highest confidence in a data disc, it
should have no E22 or E32 errors. Also, keep in mind that this requirement is for new discs, as
made. Obviously, the
quality will degrade with use and age.
Making discs with E22 or E32 errors does not leave adequate
margin for future degradation.
This is not an onerous requirement, because with modern
equipment, there is no reason to make discs with E22 or E32
errors.
How DVD Error Correction Works
DVD error correction also
uses a Reed-Solomon product code for error correction.
The primary difference from CD is the size of the
correction block, and the lack of interleaving.
CD error correction uses an
error correction block of only 24 user bytes.
CDs were designed for audio, and you don’t want an
uncorrectable block to be too noticeable.
DVDs, on the other hand, use an error correction block of
32 kB. It turns out
that the error correction capability of Reed-Solomon product codes
increases with the size of the block, so this gives a much greater
error correction capability, which is required due to the small
size of the pits.
DVD error correction also
works in two stages, but instead of C1 and C2 as on a CD, DVD
error correction arranges the data into 208 rows and 182 columns.
Each row and column has it’s own parity bytes.
Correction of the rows is known as “Inner Parity” or
PI. Correction of the
column is known as “Outer Parity” or PO.
The rows are corrected first, so uncorrected bytes in a row
may still be corrected by the outer parity correction.
Therefore,
PI Fails (PIF) are not fatal. But PO fails cannot be corrected, and should not be
allowed.
Why
Analog Measurements are Important
Most people consider a disc “good” if
they can put it in a drive and read it. The problem with
this is that it doesn’t tell you if the disc will be readable in
all drives, or if it is on the verge of failing.
Measuring error rates gives a pretty good
picture of disc quality, but what you are measuring is how well
that disc plays in one drive. This is a good measure of disc
quality, but it doesn’t tell you if the disc is likely to work
in all drives.
One of the most common problems with DVDs
& CDs is discs that play in some drives and not others.
Often, the user will conclude that there is something wrong with
the drive that doesn’t play the disc. This is a natural
conclusion, but usually wrong.
The thing is, the ability of the drive to
read the disc is wholly dependent on the size and shape of the
“pits” on the disc. The drive’s laser beam must follow
the track and stay in focus while the disc is spinning. The
way that the laser beam bounces back from the disc determines the
signals that guide the beam along the track. The drive’s
servo systems are designed to work with pits of a specific size
and shape. Deviations in “pit geometry” will affect the
drive’s ability to play the disc. The cause of most
playback errors are a direct result of poor pit geometry.
Although the pits are sub-microscopic, it
is possible to infer what the pit geometry is by looking at how
the light bounces back from the disc. This is what the
analog measurements are all about.
Analog Measurements on DVDs
The primary measurements that are related
to pit geometry on DVDs are Reflectivity, Modulation, Resolution,
Asymmetry, Jitter, Tracking Error, and Focus Error.
Reflectivity
The reflectivity of the disc affects how
strong the reflected laser beam is.
A stronger beam allows the possibility of a better
signal-to-noise ratio.
Reflectivity
is a function of the metal reflective layer, and on recordable
discs, the absorption of the dye layer that is the active
recording medium. The
reflectivity of aluminum is typically about 80%, but some light is
absorbed by the plastic substrate, so typical values for good
molded discs are around 75%.
Reflectivity of recordable discs is usually lower due to
absorption in the dye layer. The minimum allowed on
recordable discs is 45%. Low
reflectivity makes the signal harder to decode.
Dual layer discs have lower reflectivity, because the beam
must “see” through the first layer to the second layer.
Therefore, the minimum reflectivity for dual layer discs is
18%, and the maximum is 30%.
True reflectivity is difficult to measure.
What we are doing is measuring the intensity of the laser beam
reflected from the disc. This is affected not only by the
disc’s reflectivity, but also the pit geometry, due to
diffraction of the beam by the pits or the pre-groove on a
recordable disc. As a result, discs with strange pit
geometry can produce erroneous reflectivity measurements.
I14/ I14H (Modulation)
This is a measure of overall signal
strength. It is the
difference between the brightness of the reflected beam when it is
over “land”, compared to the it’s brightness when over at
“pit”. Too small
of a signal makes it difficult to decode.
Modulation is primarily affected by pit depth and width.
As a rule, higher modulation is better, and will generally
produce lower error rates. Optimum
modulation is about 0.67 on molded discs, but can be considerably
higher on recordable discs.
I3/ I14 (Resolution)
I3 represents the shortest pits
and lands, and is the highest frequency part of the signal.
I3 is always smaller than I14 because
the smallest pits are at the limit of resolution of the optics. Pits that are too small can cause low resolution.
Again, larger resolution is better as a rule, although very
high resolution may indicate a problem too.
Minimum allowed resolution is 0.15 for single layer discs,
and 0.20 for dual layer discs.
Asymmetry and Beta
This measurement is a function of the
writing power of the laser during glass mastering (or writing a
recordable DVD). Asymmetry
and Beta both measure the same thing, but are measured in a
slightly different way. Higher laser power produces more positive asymmetry (beta).
Some positive asymmetry is often desirable, with optimum of
0 - 5%. The maximum
allowed is -5% to +15%.
Jitter
Jitter is a measure of the error in the
length of the pits and lands. The information on the disc is encoded in the
changes
between pit and land. Since
DVDs and CDs use a “self-clocking” modulation scheme, the
length of the pits and lands is critical. Jitter is a sensitive measurement that can have many
causes.
Tracking Error
Tracking error shows how faithfully the
laser beam can follow the track of pits. If the beam goes off-track, many serious errors can be
generated, so the drive’s ability to follow the track is
crucial.
Focus Error
Likewise, focus error shows how well the
laser beam can stay in focus. The laser beam must be focused down to a fraction of a
micron (10-6 meter), so this is also critical to
playing the disc.
Analog Measurements on CDs
The primary measurements that are related
to pit geometry on CDs are Reflectivity, I11/ Itop, I3/ Itop,
Asymmetry, Jitter, Tracking Error and Focus Error.
Reflectivity
The reflectivity of the disc affects how
strong the reflected laser beam is. A stronger beam allows the possibility of a better
signal-to-noise ratio, and therefore more accurate reading.
Reflectivity
is a function of the metal reflective layer, and on recordable
discs, the absorption of the dye layer that is the active
recording medium. The
reflectivity of aluminum is typically about 80%. Some light is absorbed by the plastic substrate, so typical
values for good molded discs are around 75%. Reflectivity of recordable discs is usually lower, and the
minimum allowed is 45%. Low
reflectivity makes the signal harder to decode. CD-RW discs have even lower reflectivity due to the
recording material used. Reflectivity on CD-RW discs can be as low as 15%.
For this reason, we do not recommend CD-RW media for
anything critical.
True reflectivity is difficult to measure.
What we are doing is measuring the intensity of the laser beam
reflected from the disc. This is affected not only by the
disc’s reflectivity, but also the pit geometry, due to
diffraction of the beam by the pits or the pre-groove on a
recordable disc. As a result, discs with strange pit
geometry can produce erroneous reflectivity measurements.
I11/ Itop
This is a measure of overall signal
strength. It is the
difference between the brightness of the reflected beam when it is
over “land”, compared to the brightness when over a “pit”.
Too small of a signal makes it difficult to decode. Brightness over land is Itop, which is
determined by the reflectivity of the disc and the laser power.
I11 is the peak-to-peak amplitude of the signal. The maximum level is determined by Itop, and the
minimum level is determined by the “darkness” of the pits.
On molded discs, the pit depth and width determines how
dark the pits are. As
a rule, higher modulation is better, and will generally produce
lower error rates. Optimum
modulation is about 0.67 on molded discs, but can be considerably
higher on recordable discs.
I3/ Itop
I3 represents the shortest pits
and lands, and is the highest frequency part of the signal. I3 is always smaller than I11 because
the smallest pits are at the limit of resolution of the optics.
Pits that are too small can be barely readable. Again, larger I3/ Itop
is better as a
rule, although very high I3/ Itop may
indicate a problem too. Minimum
allowed I3/ Itop is 0.30.
I11 and I3 are
measured relative to Itop so that the measurements are
independent of the reflectivity and the laser power.
Asymmetry and Beta
Asymmetry and beta are two different ways
of measuring the same thing. They are a measure of the relative amount of pit and land.
Zero asymmetry (or beta) means that the amount of land is
the same as the amount of pit. When they are not the same, this causes distortion of the
signal that makes it hard to decode.
Although they measure the same thing,
asymmetry and beta have opposite signs. So negative asymmetry is the same as positive beta.
CDX and DVX with AMM-1 measure Beta, so we use that
convention.
To add to the confusion, asymmetry on DVD
discs is defined with the opposite sign as for CD, so on CDs,
asymmetry and beta have opposite sign, but the same sign on DVDs.
This measurement is useful because it is a
function of the writing power of the laser during glass mastering
(or writing a CD-R). Higher
laser power produces more positive beta. Some positive beta is desirable, with optimum of +5% to
+10%. The maximum
allowed is -5% to +15% for molded discs, and -10% to +15% for
CD-R. Beta must not
vary by more than 2% over the disc. CD-Recordable discs with low I3 and negative
beta are often found to give unreliable results.
Jitter
Jitter is a measure of the error in the
length of the pits and lands. The information on the disc is encoded in the
changes
between pit and land. Since
DVDs and CDs use a “self-clocking” modulation scheme, the
length of the pits and lands is critical. Jitter is a sensitive measurement that can have many
causes, such as distortion of the pits, or poor focus of the laser
beam.
Tracking Error
Tracking error shows how faithfully the
laser beam can follow the track of pits. If the beam goes off-track, many serious errors can be
generated, so the drive’s ability to follow the track is
crucial.
Focus Error
Likewise, focus error shows how well the
laser beam can stay in focus. The laser beam must be focused down to a fraction of a
micron (10-6 meter), so this is also critical to
playing the disc.
As a rule, most serious errors on a disc
are caused by focus or tracking errors. Inability to properly follow the track can result in large
numbers of errors over the whole disc.
