Feurio Questions & Answers - Feurio! CD-Writer
general questions:
Why doesn't Feurio! make exact 1:1 copies?
Question: I used Feurio! to copy
a CD. If I read the tracks from the original CD and from the copied CD, then
I can see that the data within the tracks has been shifted by a small offset.
When will this error be corrected?
Answer: (Unfortunately) this is not
an error made by Feurio, but a principle problem with audio CDs. On an audio
CD there are no sector marks (in contrast to data CDs) which make it possible
to determine the data the be read exactly. All the data is in a long spiral.
Several "Subchannels" are coded in to this stream of data. One of these subchannels
contains a position reference; unfortunately this position reference is only
accurate to 1/75 seconds. This subchannel normally controls the time display
of a CD player.
The sectors which are now "read" by the CD-ROM actually do not exist, they are
"interpreted" by the CD-ROM from the subchannel data.
And this is just the problem: "Cheap" CD-ROMs just read the first data for which
the time code of the subchannels "fits", this is what causes the notorious "Jitter
errors", i.e. the drive provides data shifted by a certain offset for every different
time the CD is accessed to read.
"Good" CD-ROMs already carry out a jitter correction in the drive, i.e. certain
measures are taken to make sure jitter errors do not occur (e.g. by accurately
determining the position where a "Subchannel block" finishes) - if for example
sector 246 is read, then in future read operations the same data is read.
As however, these sector marks are not really on the CD but are created by the
CD-ROM and the CD-ROMs have got different algorithms to determine these "pseudo
sector marks", these CD-ROMs always read exactly the same data if a certain sector
is to be read but not necessarily that which was written exactly at this position.
If the data is read by another CD-ROM (or even just using different firmware),
then it can often be seen that the data is the same in principle but has been
shifted by a certain offset.
Different CD writers also create a different pattern of subchannel data (as before:
The position references and thus the structure of the subchannel data is only
defined accurate to 1/75 of a second), so that the correction algorithm of the
CD-ROMs can provide different results depending on which writer was used to write
the CD.
With a CD copy in principle Feurio! simply reads sector 0 from the CD-ROM and
writes it in sector 0 of the CD-R, then sector 1 to sector 1 etc.
So in principle an exact 1:1 copy is created here if the CD-ROM really reproduces
the data exactly.
As this however is not usually the case with most CD-ROMs (as already mentioned
not even an exact position is defined) then a certain shift of the data occurs.
In practice this shift however generally remains less than 1/75 of a second and
is thus practically not audible.
Unfortunately there are also CD-ROMs, which have a greater shift, i.e. if you
compare the written data (or that read by another CD-ROM) with the data read
by this CD-ROM, then the shift is greater than 1/75 of a second. In this case
the algorithm of the CD-ROM to "create" the sector marks is not correct - in
this case you can either hope for an update by the vendor OR use a special function
in Feurio:
Under "Program parameters" -> "Device parameters" -> "Special parameters"
in the field "Sector offset" an offset can be entered which Feurio! automatically
adds to every sector. I.e. if sector 0 is actually read, with an offset value
of e.g. "2", "Read sector 2" is sent to the CD-ROM. Therefore by giving the "correct"
offset value this error in the CD-ROM can be corrected.
This offset can currently only be found out by manually comparing
the data but I am working on a function which will enable you to "calibrate"
the CD-ROM by reading a test CD you have burned yourself.
See also:
Questions and Answers - Feurio! CD-Writer general
questions
Questions and Answers - Feurio! CD-Writer problem solving
Questions and Answers - Main Page
You are the 1039. visitor of this page (today: 3)