fig3 is a diagram showing a general structure of an embodiment of an optical disk unit according to the present invention employing the cav recording system . in fig3 an optical disk 10 a is rotated at a constant angular velocity by a known means ( not shown ). a laser beam 14 is emitted from a laser light source ( not shown ) such as a laser diode , and is irradiated on a surface of the optical disk 10 a via a lens 12 which converges the laser beam 14 on the optical disk 10 a . as shown in the right portion of fig3 on an enlarge scale , the laser beam 14 is emitted in response to write pulses generated in a write pulse generator 16 according to the present invention . hence , as shown in the left portion of fig3 the laser beam 16 is made up of pulses . since the optical disk 10 a is rotated at the constant angular velocity , pits are formed on the surface of the optical disk 10 a by the pulse laser beam 14 . fig4 is a system block diagram showing a first embodiment of a write pulse generator according to the present invention . a write pulse generator 16 - 1 includes a first register 22 , a second register 24 , a switching timing generator 26 , a pattern length detector 28 , a data read signal generator 30 , a data reading section 32 , and a write pulse generating section 34 which are connected as shown in fig4 . the first and second registers 22 and 24 are used to hold set values of a pulse width control ( write strategy ). the first register 22 reads a content of a set value of the pulse width control ( write strategy ) in response to an external write enable signal ( register write enable signal ). before a write operation ( recording operation ) starts , the content of the set value to the first register 22 is unconditionally read by the second register 24 . after the write operation starts , the second register 24 can no longer read the content held in the first register 22 due to the operation of the switching timing generator 26 , that is , under control of a register write enable signal ( which indicates a register write disable ) from the switching timing generator 26 . the content of the set value held in the second register 24 is supplied to the write pulse generating section 34 via the data reading section 32 , in response to a data read enable signal generated from the data read signal generator 30 . the data read signal generator 30 generates the data read enable signal based on an eight - to - fourteen modulation ( 8 - 14 modulation or simply efm ) signal on which the write pulses are based . the write pulse generating section 34 generates and outputs the write pulses , based on the content of the set value held in the second register 24 . when changing the set value of the pulse width control ( write strategy ) during the write operation of the optical disk unit , the first register 22 is first made to read the set value of the pulse width control ( write strategy ) which optimizes the write pulse width . then , in order to reflect the content held in the first register 22 to the second register 24 , the switching timing generator 26 generates a register write enable signal with respect to the second register 24 . the switching timing generator 26 generates the register write enable signal with respect to the second register 24 only when all of the following conditions c1 through c3 are satisfied . c2 : the write pulse generating section 34 is not reading data from the second register 24 ; and c3 : the second register 24 can secure a time required for reading data from the first register 22 . it is possible to judge whether or not the condition c 3 is satisfied , in the following manner . in other words , a space pattern indicating an end of a frame appears for every frame length , that is , for every predetermined data length of 588 bits , for example . normally , the space pattern has a length of 11t or 14t . in this embodiment , the pattern length detector 28 detects the space pattern . the detection of the space pattern by the pattern length detector 28 is notified to the switching timing generator 26 which uses this information as a condition for generating the register write enable signal with respect to the second register 24 . the content of the setting value of the pulse width control ( write strategy ) with respect to the second register 24 is updated to the content of the setting value of the pulse width control held by the first register 22 , every time the space pattern of 11t or 14t appears . the setting value of the optimum pulse width control ( write strategy ) in this first embodiment can be calculated from time information or the like prerecorded on the optical disk 10 a . in addition , the setting value of the optimum value of the pulse width control ( write strategy ) may be calculated by other methods . by reading the setting value of the optimum pulse width control ( write strategy ) into the first register 22 , the write pulse generator 16 - 1 can always output optimum write pulses . therefore , according to this first embodiment , it is possible to regularly change a set value of an adjusting width of a pulse width control ( write strategy ) with respect to a pit pattern on the optical disk depending on a radial position where a recording operation is carried out . fig5 is a system block diagram showing a second embodiment of the write pulse generator according to the present invention . in fig5 those parts which are the same as those corresponding parts in fig4 are designated by the same reference numerals , and a description thereof will be omitted . in a write pulse generator 16 - 2 shown in fig5 a switching timing generator 26 a is provided in place of the switching timing generator 26 shown in fig4 . in addition , the write pulse generator 16 - 2 is additionally provided with a channel clock frequency comparator 36 and a selector 38 which are connected as shown in fig5 . the selector 38 is switched depending on whether or not the efm signal is output . in other words , the selector 38 is switched to immediately reflect the content of the first register 22 to the second register 24 when no efm signal is output , that is , when not during the recording operation . this second embodiment compares the frequencies of a channel clock which has a varying frequency and a main clock which has a fixed frequency , so as to enable setting of the setting value of the pulse width control ( write strategy ) in advance . the first register 22 reads the setting value of the pulse width control ( write strategy ) used after the write operation starts . on the other hand , the second register 24 reads the setting value of the pulse width control ( write strategy ), which is to be read by the data reading section 32 . when the write operation starts , the first register 22 is set to a state where changing of the content thereof from the outside is disabled , in response to the register write enable signal ( which indicates a register write disable ). the channel clock is compared in the channel clock frequency comparator 36 with the main clock which has the fixed frequency . if the channel clock frequency becomes lower than the fixed frequency , the channel clock frequency comparator 36 outputs to the switching timing generator 26 a a switching signal for switching the setting value of the pulse width control ( write strategy ). when the switching signal for switching the setting value of the pulse width control ( write strategy ) is output from the channel clock frequency comparator 36 and the space pattern is then output by the efm signal , the pattern length detector 28 detects the space pattern and notifies this information to the switching timing generator 26 a . the switching timing generator 26 a outputs a signal when the switching signal is received from the channel clock frequency comparator 36 and the detection of the space pattern is notified from the pattern length detector 28 . the setting value of the pulse width control ( write strategy ) held in the first register 22 is loaded into the second register 24 in response to the signal output from the switching timing generator 26 a via the selector 38 . when a next data read enable signal is generated from the data read signal generator 30 , setting value of the pulse width control ( write strategy ) held in the second register 24 is read by the write pulse generating section 34 via the data reading section 32 . except for the time ( timing ) when the setting value is being loaded into the second register 24 , the first register 22 is enabled of reading the data from the outside . accordingly , it is possible to read into the first register 22 in advance a setting value of the pulse width control ( write strategy ) which is to be used when the channel clock frequency becomes even lower . hence , when the reference frequency is again set by the main clock and the channel clock frequency becomes lower than this reference frequency , an operation similar to that described above is carried out . by repeating the above described operation , it becomes possible to continuously switch the setting value of the pulse width control ( write strategy ) depending on the channel clock frequency . as a result , it is possible to always output optimum write pulses . therefore , according to this second embodiment , it is possible to regularly change a set value of an adjusting width of a pulse width control ( write strategy ) with respect to a pit pattern on the optical disk depending on a radial position where a recording operation is carried out . in addition , it is possible to change the setting value of the pulse width control ( write strategy ) at a desired timing . the present invention is of course applicable to various kinds or types of optical disks , including cd - r / rw , dvd - r / rw and dvd + rw . further , the present invention is not limited to these embodiments , but various variations and modifications may be made without departing from the scope of the present invention .