I'd like to point out why extracting audio data from RedBook CDs is difficult. The CD has a long, spiral track like an LP. Although the CD is divided into frames, unlike sectors on a hard drive the CD is not designed for frame-accurate seeks. This is fine when the disk is being played in a CD player, because the player plays it back just like an LP, tracking the spiral groove from beginning to end. However, the extraction program usually reads the disc in a "bursty" manner, just as it would with a hard disk. The extraction program must use overlapping reads and sophisicated patter matching to ensure a continuous data stream since the drive may or may not seek to where it was asked. Newer drives (with so-called "Accurate Seek") try to hide all of this in the drive buffer and firmware, but the problem is the redbook standard, not the drive.
The measure of how accurately a drive seeks to the requested frame is called "jitter". I called it DAE (digital audio extraction) jitter, to distinguish it from the two other forms of jitter. Another form of jitter is the timing uncertainty of the bit transitions in the RF data from the CD read head. These are caused mainly by slight imperfections in the CD replication process. This "RF jitter", if it is severe enough, causes data errors at the circuit which decodes the EFM (eight to fourteen modulated) signal back to eight bits. These two forms of jitter certainly affect the extraction process and could affect the accuracy of the bits extracted to the hard drive. The third form of jitter is the one people are often refereing to: DAC clock jitter, timing uncertainty of the supposedly stable clock signal presented to the digital to analog converter. This has nothing to do with the other forms of jitter or digital audio extraction, since the DAC is not involved in the process. In a CD player or computer sound card, the audio is clocked to the DAC from a buffer by a stable clock. Exactly how stable depends on the source and power supplies. Crystal oscillators, used in DC players and devices like the Sqeezebox and Transporter tend to be more stable and jitter free. Phase locked loops, used in USB soundcards and DACs connected by S/PDIF tend to be more jitter prone since they must follow the speed of the clock signal embedded into the data. The Transporter lets you get around this problem if used with an external DAC which can generate its own clock by slaving to the DAC's clock. -- Timothy Stockman ------------------------------------------------------------------------ Timothy Stockman's Profile: http://forums.slimdevices.com/member.php?userid=8867 View this thread: http://forums.slimdevices.com/showthread.php?t=42435 _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
