What is jitter?

On a CD the music data is written in a single, spiral-shaped track (the same as a record).
Thus the data is given as a long chain of bits that ideally can be read continuously.
In contrast to e. g. hard disks or disks, the data itself is not divided into sectors!

A kind of "logical sector structure" is created on CD-ROMs by writing additional "sector marks" so the CD consists of single sectors for the computer again that can be accessed directly by the computer.

On an Audio-CD these sector marks are missing. There is only a "time-code", that is coded into the data. This time code indicates the position with a resolution of 1/75 seconds. (This time code is also used for the information on the display of the CD-player).

If the computer now wants to read a certain position of an Audio-CD, the CD-ROM can not locate the position precisely (i.e. only accurate to within 1/75 of a second) because of the missing sector marks.

Normally this isn't a problem because when reading out you usually start at the beginning of a track; the start of a track consists of silence anyway.
In the course of reading on, the CD-ROM simply follows the track and transfers the data to the computer.

The problem (the "jitter errors") will only occur the moment the computer can not take up the data quickly enough and thus the CD-ROM has to "start up" again. (As the CD continues to spin in the drive, the reading head initially "loses" the track and has to look for it again and then has to restart the read operation).

Because it is not possible to locate the exact position again (using a time code) at which the read operation stopped, some samples are skipped or read again on the "restart" of the read operation!
Just imagine the effect of this behaviour!

The solution (jitter correction)

"Good" CD-ROMs already carry out jitter correction "internally", i.e. they already take internal measures to transmit correct data after a "re-start".
In this case set the switch "jitter correction" to "Off" .

Particularly IDE drives are no "good" CD-ROMs (they don't have an internal jitter correction).
On one hand it often helps to reduce the read speed so less data is transmitted and the probability that the computer can not process the data is reduced. (As long as the data is read continuously, there is no problem).

On the other hand Feurio offers a jitter correction per software.

The way the jitter correction works:

If, for example the read block size is set to 20, Feurio would normally read the sectors in the following order :
1-20, 21-40, 41-60, 61-80, 81-100, .....
If jitter correction is activated with 5 sectors then the next block read is set back by the respective number:
1-20,16-35,31-40,36-55,51-70,...

Thus, 5 sectors are always read "overlapping".
As the overlapping part is identical, Feurio! can decide where it has to merge the parts by comparing the read data; it checks where the data last read in the previous read operation appear in the new data and thus knows which position the new data has to be appended to.

Settings for jittter correction

Feurio!® offers an integrated jitter correction.
The jitter correction can be configured for each CD-ROM in Feurio! CD-Manager under "Program" -> "Program Parameters" -> "Device Settings".

These settings are possible:

• Off (not required):
  Feurio!®'s jitter correction is turned off: Should be selected if the drive is already carrying out jitter correction internally because jitter correction per software would only mean an unnecessary overhead in this case!
  
• Only if device reports "Lost Stream":
  Some CD-ROMs are at least "intelligent" enough to inform the computer if there is a possible jitter error. (i.e. if they have had to "start again" during a read operation). In this mode sectors are normally read continuously and without jitter correction (i.e. for example: Sector 1-20, then 21-40, then 41-60 etc.). If the computer has not been able to read out the data in time and the CD-ROM has had to re-position the read-head, it will send a "lost stream" error message to the computer at the next read command. Thus Feurio!® "knows" that the data from the next read command may have jitter errors and therefore performs jitter correction at the next read command.
  
• On:
  Feurio!® performs jitter correction at each read operation.
  Because this means that the "limiting sectors" have to be read again on the next read operation, the reading rate will usually decrease considerably! (Unless the CD-ROM manages this new read operation in the cache; but "cheap" CD-ROMs usually don't cache audio data).

Jitter sectors:
Here you can set how many sectors are to be used for jitter correction (i.e. how many sectors Feurio!® shall "look back"). Of course this value should be as low as possible to reduce the overhead.
The exact value depends on the positioning accuracy of the drive; I have seen drives that "wrongly estimated" by more than one sector when reading audio data! As a general rule, 1 or 2 sectors should be sufficient, if there is an error a few more can be tried.

Probleme bei der Jitter-Korrektur

As explained above, jitter correction is based on reading the "limit areas" between two read operations (which causes the jitter errors occur) twice and then performing a bit comparison between the two read "sectors".

In particular "cheap" IDE CD-ROM-drives are not capable of reading audio data correctly; for each read operation they provide different data (the error correction of audio CD has probably not been implemented correctly).

Because the jitter correction depends on finding the same data in a new read operation, of course the jitter-correction doesn't work - the read operation will stop with an error.

As the data from such a drive is not to be trusted, you would really be best to buy a new CD-ROM.

Otherwise the problem is oftenly solved by reducing the read speed - with a lower read speed, the probability of jitter errors also reduces as the computer has less data to process and also the drive reads the data "better" at a lower speed as a general rule.

Furthermore no intensive access to the hard disk should be carried out while reading!

Rückmeldungen

After a read operation the following things are reported (if jitter correction is activated):

• Jitter warnings:
  The number of jitter warnings ("Lost Stream") the device has sent to the computer.
  (see "3: Settings for jitter correction").
  
• Jitter corrections:
  The number of jitter corrections actually carried out, i.e. the cases in which Feurio!® has found that the data does not match.

An Example:

With activated jitter correction Feurio!® for example reads sectors 1-20 during the first read operation, then on the 2nd operation the sectors 18-37 (read block size: 20 sectors, jitter sectors: .
Normally the data in the last two sectors of the 1st read operation should be the same as the data in the first two sectors of the 2nd read operation.
If this is the case, no jitter error occured, the data from the first two sections of the 2nd read operation are discarded and the rest is appended to the data from the first read operation. No jitter correction was necessary and therefore the counter is not increased.
If however the data is not the same, then a jitter error did occur and Feurio!® tries to find a correlation between the data as described above and append the data correctly.

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