Talk:Absolute Time in Pregroove
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Article fails to mention where the title fits in
[edit]The article is entitled "Absolute time in pregroove". Although the article discusses some of the uses of the data encoded in the pregrove, it completely fails to mention how the absolute time is encoded or even to what the absolute time refers. 109.153.242.10 (talk) 12:52, 19 December 2011 (UTC)
FWIW -- ATIP and what it is used for
[edit]I worked on one of the first consumer recordable CD systems from 1990 through 1991, a joint venture between Philips, Sony, and Tandy, developed at Tandy's Electronics Research Center in San Jose, CA. So what I have to say is firsthand engineering knowledge rather than documentation from external sources. Anyone who is interested in chasing this stuff down and incorporating it into the article in a manner acceptable to Wikipedia is advised to start here: http://www.google.com/patents/US5363360 or here: http://www.google.com/patents/US5506824
A recordable CD-R or CD-RW disc has a depressed recordable section (the Pre-Groove) into which the data will eventually be written. This pre-groove allows the laser head servos to track where the data will be written, so that the spiral track of data will be packed as tightly and spaced as evenly as possible. On a normal CD the data itself is what the servos track, but a CD-R or CD-RW disc begins with no data recorded on it and so the pre-groove is required.
There is a physical "wobble" that is molded into the sides of the pre-groove. It is a much higher frequency than the tracking servos can follow, and as such it appears as as a high frequency component in the tracking error signal. This high-frequency component is an FM signal which carries a digital data stream.
The servo processing electronics split the tracking error signal into three parts: The lower frequency components are amplified and fed back into the tracking servo motors so that the laser head will remain centered on the pre-groove; The average frequency of the high-frequency component is used to control the CLV (Constant Linear Velocity) servo in those devices which use CLV; And the rapid variations in the high-frequency component are decoded using an FM demodulator in order to extract a continuous series of time-marks showing how far in from the start of the recordable area the laser head is, measured in hours, minutes, seconds, and milliseconds -- this is the Absolute Time in the Pre-groove.
The ATIP information is used to locate and position the laser head over the desired section of a CD-R or CD-RW disc before there is any data recorded in that section, so that the new information may be written at the desired location on the disc.
The term ATIP is also often used to describe other information which is encoded into the wobbles in the the pre-groove, including disc manufacturer ID, dye type, recordable time, etc. Technically speaking though this is not ATIP infromation as it does not have anything to do with the Absolute Time or the Pre-groove -- it just happens to be communicated by the same means and signal which carries the ATIP information. 38.126.115.194 (talk) 01:23, 22 November 2012 (UTC)TagFerret