Offset correction
- Don Bosco 20:24, December 31, 2011 (CET)
Offset correction is a feature of some CD reading and writing programs. It describes the possibility of correcting shifts in the audio material, called sample offset, that can occur when reading or writing an audio CD . The sample offsets depend on the one hand on the individual CD drive type (or its firmware ), but can also be variable ( jitter ).
Technical description
When reading out (or burning) audio CDs, the exact assignment of the track or index marks to the relevant positions in the read out (or burned) audio data stream is not precisely defined, but varies from drive type to drive type or even Firmware version of drives. The shift is usually in the millisecond range, both positive and negative are possible. With some drive types, in addition to the fixed time offset, there is also variable, temporal noise , called jitter , which makes corrections difficult or impossible.
With the CD ripper software Exact Audio Copy, for example, you can set that the audio material is shifted relative to the positions of the track or index markings both when reading ( DAE ) and when writing ( CD burning ). In this way, the shifts caused by the CD drives can be compensated for. If necessary, the values can be set individually for each existing drive.
Positive values of the read offset correction mean that a corresponding number of samples are left out at the beginning of the resulting file. Negative values mean that a corresponding number of samples of silence is inserted or - in the case of drives that support this - data that comes from the supposed lead-in area immediately before the start position (which has been shifted too far towards the end by the firmware).
Since these shifts are very small, disregarding them usually does not cause any problems. However, this method makes it possible to compare audio files from one and the same CD but from different drives. In addition, files can be pieced together from the data from different drives, which is sometimes helpful with difficult to read (e.g. scratched) CDs.
List of CD writing or reading software that offers offset correction
- dBpoweramp (reading offset correction)
- Exact Audio Copy (both write and read offset correction);
- foobar2000 (reading offset correction)
- Rubyripper
Web links
www.daefeatures.co.uk/ , database on optical drives and their DAE properties, including the sample offset