Transcoder

Abstract

If the number of frames in a GOP of an input stream is not less than 15, the GOP is determined as a control unit time. If the number of frames in the GOP is less than 15, the following GOP is connected thereto until the number of frames becomes not less than 15 and the connected GOPs are determined as a control unit time. After correcting the control unit time, the average input bit rate S n in each control unit time is obtained, and by using information on the input stream including the average input bit rate S n , a quantization step value of an output stream is calculated.

1 . A transcoder for converting a first stream into a second stream, comprising:
 a period determination part for determining a control unit time;   a part for acquiring a bit rate of a first stream per determined control unit time; and   a quantization step value calculation part for calculating a quantization step value of a second stream by using information on a first stream including an acquired bit rate of a first stream per control unit time,   wherein said period determination part determines each control unit time so that each control unit time has frames of which number is not less than a predetermined value.   
   
   
       2 . The transcoder according to  claim 1 , wherein
 said period determination part determines a GOP as a control unit time if the number of frames in the GOP is not less than a predetermined value, and said period determination part connects a plurality of GOPs until the number of frames becomes not less than said predetermined value and determines the connected GOPs as a control unit time if the number of frames in a GOP is less than said predetermined value.   
   
   
       3 . The transcoder according to  claim 1 , further comprising:
 a period conversion factor calculation part for calculating a period conversion factor in the (N+1) period on the basis of a ratio between a target bit rate of a second stream in the (N+1) period and a bit rate of a first stream before the N period,   wherein said quantization step value calculation part includes   a part for calculating a quantization step value of a second stream of the (N+1) period by multiplying a quantization step value of a first stream in the (N+1) period by said period conversion factor.   
   
   
       4 . The transcoder according to  claim 3 , further comprising
 a target bit rate determining part for determining a target bit rate of a second stream in the (N+1) period on the basis of a target bit rate of a whole second stream, a bit rate of a second stream converted before the N period and a target bit rate of a second stream in the N period.   
   
   
       5 . The transcoder according to  claim 4 , wherein
 assuming that a ratio between a bit rate of a second stream converted before the N period and a target bit rate of a second stream in the N period is determined as a target ratio, said target bit rate determining part calculates a target bit rate of a second stream in the (N+1) period on the basis of a ratio between a target bit rate of a whole second stream and said target ratio.   
   
   
       6 . The transcoder according to  claim 5 , wherein
 assuming that a ratio between a bit rate of a first stream in the (N−1) period and a bit rate of a first stream in the N period is determined as a period ratio, said target bit rate determining part adjusts said target ratio by multiplying said target ratio by said period ratio.   
   
   
       7 . The transcoder according to  claim 5 , wherein
 assuming that a ratio between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N+1) period is determined as a period ratio, said target bit rate determining part adjusts said target ratio by multiplying said target ratio by said period ratio.   
   
   
       8 . The transcoder according to  claim 4 , wherein
 assuming that a value calculated on the basis of a difference between a target bit rate of a second stream in the N period and a bit rate of a second stream converted before the N period is determined as a target difference, said target bit rate determining part calculates a target bit rate of a second stream in the (N+1) period by adding said target difference to a target bit rate of a whole second stream.   
   
   
       9 . The transcoder according to  claim 8 , wherein
 assuming that a ratio between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N−1) period is determined as a period ratio, said target bit rate determining part adjusts said target difference by multiplying said target difference by said period ratio.   
   
   
       10 . The transcoder according to  claim 8 , wherein
 assuming that a value calculated on the basis of a difference between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N−1) period is determined as a period difference, said target bit rate determining part adjusts said target difference by multiplying said target difference by said period difference.   
   
   
       11 . The transcoder according to  claim 8 , wherein
 assuming that a value calculated on the basis of a difference between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N−1) period is determined as a period difference, said target bit rate determining part adjusts said target difference by adding said period difference to said target difference.   
   
   
       12 . A transcoder for converting a first stream into a second stream, comprising:
 a period determination part for determining a control unit time;   a part for acquiring a bit rate of a first stream per determined control unit time; and   a quantization step value calculation part for calculating a quantization step value of a second stream by using information on a first stream including an acquired bit rate of a first stream per control unit time,   wherein said period determination part determines each control unit time so that the ratio of I picture frames included in each control unit time becomes not more than a predetermined ratio.   
   
   
       13 . The transcoder according to  claim 12 , wherein
 said period determination part determines a GOP as a control unit time if the ratio of I picture frames in the GOP is not more than a predetermined ratio, and said period determination part connects a plurality of GOPs until the ratio of I picture frames becomes not more than said predetermined ratio and determines the connected GOPs as a control unit time if the ratio of I picture frames in a GOP exceeds said predetermined ratio.   
   
   
       14 . The transcoder according to  claim 13 , wherein
 said period determination part sets predetermined weights for I, P and B picture frames, respectively, and with weighting, it is judged whether the ratio of I picture frames in the GOP is not more than the predetermined ratio or not.   
   
   
       15 . The transcoder according to  claim 12 , further comprising:
 a period conversion factor calculation part for calculating a period conversion factor in the (N+1) period on the basis of a ratio between a target bit rate of a second stream in the (N+1) period and a bit rate of a first stream before the N period,   wherein said quantization step value calculation part includes   a part for calculating a quantization step value of a second stream of the (N+1) period by multiplying a quantization step value of a first stream in the (N+1) period by said period conversion factor.   
   
   
       16 . The transcoder according to  claim 15 , further comprising
 a target bit rate determining part for determining a target bit rate of a second stream in the (N+1) period on the basis of a target bit rate of a whole second stream, a bit rate of a second stream converted before the N period and a target bit rate of a second stream in the N period.   
   
   
       17 . The transcoder according to  claim 16 , wherein
 assuming that a ratio between a bit rate of a second stream converted before the N period and a target bit rate of a second stream in the N period is determined as a target ratio, said target bit rate determining part calculates a target bit rate of a second stream in the (N+1) period on the basis of a ratio between a target bit rate of a whole second stream and said target ratio.   
   
   
       18 . The transcoder according to  claim 17 , wherein
 assuming that a ratio between a bit rate of a first stream in the (N−1) period and a bit rate of a first stream in the N period is determined as a period ratio, said target bit rate determining part adjusts said target ratio by multiplying said target ratio by said period ratio.   
   
   
       19 . The transcoder according to  claim 17 , wherein
 assuming that a ratio between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N+1) period is determined as a period ratio, said target bit rate determining part adjusts said target ratio by multiplying said target ratio by said period ratio.   
   
   
       20 . The transcoder according to  claim 16 , wherein
 assuming that a value calculated on the basis of a difference between a target bit rate of a second stream in the N period and a bit rate of a second stream converted before the N period is determined as a target difference, said target bit rate determining part calculates a target bit rate of a second stream in the (N+1) period by adding said target difference to a target bit rate of a whole second stream.   
   
   
       21 . The transcoder according to  claim 20 , wherein
 assuming that a ratio between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N−1) period is determined as a period ratio, said target bit rate determining part adjusts said target difference by multiplying said target difference by said period ratio.   
   
   
       22 . The transcoder according to  claim 20 , wherein
 assuming that a value calculated on the basis of a difference between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N−1) period is determined as a period difference, said target bit rate determining part adjusts said target difference by multiplying said target difference by said period difference.   
   
   
       23 . The transcoder according to  claim 20 , wherein
 assuming that a value calculated on the basis of a difference between a bit rate of a first stream in the N period and a bit rate of a first stream in the (N−1) period is determined as a period difference, said target bit rate determining part adjusts said target difference by adding said period difference to said target difference.