| 摘要 |
The invention provides a method and apparatus for generating delays to shift edges of an EFM data stream from an EFM modulator for a CD-R and CD-RW write encoder. EFM data pulse edges may be delayed in increments of about {fraction (1/32 Tefm where Tefm is the code rate clock of an EFM data stream output. The delays are generated by a synthesizer running at four times the EFM code rate. The synthesizer may be built using a four-stage ring oscillator. Delays are selectable based upon the write strategy matrix in coarse increments of ¼ T and fine increments of {fraction (1/32 T. For the coarse delay, the EFM data may be passed through a four stage shift register running at Fsynth, where Tefm=4xFsynth, allowing for a coarse delay selection of ¼ Tefm. The four stages of the ring oscillator provides eight phases of the synthesizer, each phase shifted by 1/8 Tsynth thereby providing a delay selection of 1/8 within each Tsynth period, translating into {fraction (1/32 Tefm resolution. The method and apparatus of the present invention may be extended to even finer delays by increasing the number of stages in the ring oscillator. Resulting delay increments would be Tefm/(2*N*M) wherein N is defined as the number of ring oscillator stages and M is the coarse delay.
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